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Updating the evidence that physical activity is good for health: an epidemiological review 2000–2003
Updating the evidence that physical activity is good for health: an epidemiological review 2 0 0 0 - 2 0 0 3 AE Bauman NSW Centre for Physical Activity and Health and School of Public Health, The University of Sydney, Australia Bauman, AE. Updating the evidence that physical activity is good for health - an epidemiological review 2000-2003. Journal of Science and Medicine in Sport 7 {1)."Supplement: 6-19. Developing policy and strategic initiatives to increase population levels of physical activity (PA) requires constant referral to the epidemiological evidence base. This paper updates the evidence that PA confers a positive benefit on health, using research studies in the peer-reviewed scientific literature published between 2000-2003. Areas covered include updates in all-cause mortality and in cardiovascular disease prevention, diabetes, stroke, mental health, falls and injuries, and in obesity prevention. Recent evidence on PA and all-cause mortality replicates previous findings, and is consistent with current Australian moderate PA recommendations. Recent papers have reinforced our understanding of the cardiovascular protective effects of moderate PA, with new evidence that walking reduces the risk of CVD and, in two studies, at least as much as vigorous activity. The evidence base for protective effects of activity for women, older adults and for special populations has strengthened. Cancer prevention studies have proliferated during this period but the best evidence remains for colon cancer, with better evidence accumulating for breast cancer prevention, and uncertain or mixed evidence for the primary prevention of other cancers. Important new controlled-trial evidence has accumulated in the area of type 2 diabetes: moderate PA combined with weight loss, and a balanced diet can confer a 50-60% reduction in risk of developing diabetes among those already at high risk. Limited new evidence has accumulated for the role of PA in promoting mental health and preventing falls.
Introduction P h y s i c a l a c t i v i t y (PA) is r e c o g n i s e d a s b e i n g i m p o r t a n t for r e d u c i n g t h e o v e r a l l b u r d e n of d i s e a s e (1,2). B a s e d o n s o u n d e p i d e m i o l o g i c a l e v i d e n c e , A u s t r a l i a n P h y s i c a l A c t i v i t y g u i d e l i n e s w e r e p u b l i s h e d in 1999, w h i c h r e c o m m e n d e d t h a t e v e r y a d u l t s h o u l d a c c u m u l a t e m o d e r a t e i n t e n s i t y a c t i v i t y for h a l f a n h o u r o n m o s t d a y s of t h e w e e k {3). A d d i t i o n a l h e a l t h b e n e f i t s w o u l d a c c r u e for t h o s e u n d e r t a k i n g s o m e a d d i t i o n a l v i g o r o u s a c t i v i t i e s , a n d for t h o s e i n c l u d i n g m o r e o p p o r t u n i t i e s to b e p h y s i c a l l y a c t i v e i n t o all a s p e c t s o f e v e r y d a y life. It is i m p o r t a n t c o n t i n u a l l y to u p d a t e t h e evidence on w h i c h t h e s e r e c o m m e n d a t i o n s a r e b a s e d . This review f o c u s e s on a d u l t s , a n d u p d a t e s t h e evidence o n t h e r e l a t i o n s h i p b e t w e e n PA a n d a l l - c a u s e mortality, c a r d i o v a s c u l a r d i s e a s e , d i a b e t e s , obesity, cancer, mel~tal a n d m u s c u l o s k e l e t a l health. This d o c u m e n t u p d a t e s t h e evidence s i n c e t h e p u b l i c a t i o n of G e t t i n g A u s t r a l i a Active (4).
Methods T h i s r e v i e w u s e d m u l t i p l e e l e c t r o n i c d a t a b a s e s , i n c l u d i n g NIH P u b M e d , Medline, Current Contents, Cinhal, Psychlit, Embase and the evidenceb a s e d d i r e c t o r i e s ( C o c h r a n e , DARE). S t u d i e s p u b l i s h e d b e t w e e n 2 0 0 0 - 2 0 0 3 w e r e s o u g h t u s i n g p r e v i o u s l y d e f i n e d s e a r c h a p p r o a c h e s (4). P o p u l a t i o n - b a s e d
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studies, r a t h e r t h a n evidence from clinical or small scale trials, are the p r i m a r y focus for this review. I n s t e a d of providing a detailed description of the more t h a n 200 studies t h a t have been p u b l i s h e d in this period, this review focuses on those studies t h a t provide new perspectives on the evidence.
Epidemiological developments in the conceptualisation of physical activity measures There are challenges in the epidemiological m e a s u r e m e n t of exposure (PA a n d fitness a n d related attributes) in relation to health outcomes. Measures have r a n g e d from self-report surveys a n d interviews to objective m e a s u r e s of cardiorespiratory fitness (CRF). There are still debates a b o u t the relative benefits of PA and cardiorespiratory fitness in t e r m s of which confers health benefits 15J, a n d the differences are of policy relevance. Both a p p e a r to have health benefits b u t the health o u t c o m e s and biological m e c h a n i s m s m a y be different. Furthermore, the current national PA recommendations are based on epidemiological studies of leisure time physical activity (LTPA), but this represents only a small fraction of total daily energy expenditure (EE). In recent years, a broader conceptualisation of EE is now thought relevant to health outcomes and obesity prevention. This includes the accumulation of PA through 'activities of daily living', active transport, and in occupational and domestic settings. Some studies have focused on sedentary time, sitting time or measures of 'inactivity' as being of interest in relation to health outcomes. New technologies for assessing step counts and all movement (pedometers and accelerometers) may be used for more accurate exposure measurements in population studies.
Physical activity and All Cause Mortality (ACM) Recent s t u d i e s reaffirm the dose r e s p o n s e relationship between PA a n d allc a u s e mortality (ACM). There is typically a risk reduction of a r o u n d 30% for those achieving the r e c o m m e n d e d levels of at least moderate intensity PA on m o s t days of the week, c o m p a r e d with t h o s e who are inactive 16). F u r t h e r m o r e , the m a x i m u m benefits on ACM a p p e a r to be in moving people from the m o s t s e d e n t a r y group to the middle of the PA or cardio-respiratory fitness distribution curve [at least 'moderately active']. Other p a p e r s illustrate some n e w elements of this association. Andersen (7) described data from a Danish cohort, showing that moderate levels of LTPA conferred a 35% risk reduction compared with the sedentary. This study also showed a benefit of regular 'active commuting', with those who cycled to work for 3 h o u r s per week having a 30% lower ACM risk. Data from Puerto Rico show a dose response reduction in ACM risk vcith increasing PA levels; those in the middle of the distribution of PA in the population had a 32-37% reduction in risk ts). Compared with the most sedentary quartile, the next most active quartile of the population had an accumulated survival that was around three years longer. Gregg(9) examined changes in PA and its impact on ACM amongst a cohort of 7500 older women in the USA who were followed for six years. Those who became active or maintained activity levels, had around 60% the risk of ACM compared with those who stayed sedentary. Wannamethee ~1°) reported data from a cohort of older British males, followed for 12-14 years. There was a dose response reduction in ACM risk across PA categories, observed for all ages, among those with cardiac symptoms. The relationships were similar for all-cause mortality and for cardiovascular deaths. 7
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Walking was protective ff more t h a n 40 minutes a day (relative risk (RR) for ACM =0.48). Moderate or heavy gardening was also protective, reducing ACM by 41%, b u t with no significant impact for light gardening. Those who were inactive and became moderately active had a 42% risk reduction of all-cause mortality (RR 0.58, 95% CI 0.33 to 1.03, p=0.06). Lee (hI reported on further data from the Harvard Alumni male cohort. Light PA showed a non significant relationship with ACM, while vigorous PA was clearly protective. With respect to walking, only those achieving > 20 kflometres a week of walking showed a decrease in risk, and only those climbing >20 flights of stairs per week showed a reduced risk of ACM.
Physical Activity and Cardiovascular Disease Prevention The relationship b e t w e e n PA a n d incident a n d fatal c a r d i o v a s c u l a r disease (CVD) h a s b e e n recognised since the s y s t e m a t i c review of Powell tl~) a n d the m e t a - a n a l y s i s by Berlin (13). An interesting policy challenge h a s b e e n the slow u p t a k e of this evidence b y clinicians a n d health d e c i s i o n - m a k e r s . The p o p u l a t i o n risk r e d u c t i o n c o n s i s t e n t l y a p p e a r s g r e a t e s t for i n c r e a s i n g PA or fitness a m o n g those who are s e d e n t a r y [or unfit] a n d moving t h e m to the r e c o m m e n d e d PA levels (L3). Recent reviews support previous Australian and US recommendations (~4'15).The relationships appears to be valid for m e n and women in middle ages and a m o n g older adults. For tertiary prevention, which is rehabilitation or post heart attack, PA m a y be effective, b u t there are too few exercise-only studies to m a k e definitive statements here. Dissenting reviews have been published by Williams, who h a s suggested that the cardiovascular benefits for fitness are clearly greater t h a n for self-reported PA(5). A further s t u d y h a s challenged the relationship between changes in fitness and s u b s e q u e n t CVD (16)by suggesting that m e a s u r e m e n t error could account for the observed cardioprotective associations. The consistency of epidemiological research across populations and the diversity of m e a s u r e s of self-report and objective PA a n d fitness m a k e it unlikely t h a t the consistent cardioprotective relationships are spurious, as Williams contends (16). The methods used in this CVD review are descriptive. A s u m m a r y of the evidence relating PA or fitness to cardiovascular disease or coronary heart disease risk, from studies published in 2000-2003, is shown in Figure 1. Data are described compared with the least active or least fit segment of the population, which is assigned a relative risk of 1.0. The next increments, by Fifths or quarters of PA in the population, are c o m p a r e d to the most inactive. Where estimates are not given in the papers, linear interpolation is used to estimate intermediate risks. All studies had at least three actual data points to contribute to the figure. The data show the generally consistent pattern of reduction in risk, maximal by about the midpoint of the distribution [median reduction of 31% in the middle distribution of PA for CVD risk across studies shown in the figure]. This level of activity approximates to the recommendations of half a n hour, of moderate intensity activity on m o s t days. There is further risk reduction for those expending greater a m o u n t s of energy [levels 4 a n d 5 in the figure]. New studies have explored the dose-response relationship for at least moderate intensity walking (3.5-4 k m per hour). For example, Manson ~7) observed a dose response relationship for walking quintiles and CVD outcomes (RRs 1.0, .91, .82, .75, .68), and similar findings were reported across walking quartiles by LeetIS),(RRs 1.0, .86, .49, .48). These data support the notion of health benefits for regular walking.
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Figure 1: Risk reducti•n estimates •f the re•ati•nship between physica• acuvity and cardi•vascu•ar disease• from epidemiologicalstudies published 2000-2003.
Similar relationships were shown for general population samples and a m o n g those with diabetes (shown as dotted lines in the figure) a9-22). The first two of these studies also showed significantly lower CHD rates a m o n g those who were in the highest quintiles of walking hours per week. The Whitehall British civil servants study showed no protective association with moderate activity (23), b u t this study did show more significant relationships across tertiles of PA for all-cause mortality (RRs across tei~Jles 1.0, .92, .66), and for the relationship between walking pace and CHD (RRs 1.0, .76, .69). Data from the Caerphilly s t u d y in W a l e s (241 observed a cardioprotective relationship across total PA levels (as shown in the figure), b u t observed a n independent relationship only for vigorous activity, not for combined light and moderate activity alone. There have been a few studies furthering the evidence that PA improves other cardiovascular risk factors. Systematic reviews of the relationships between PA and hypertension have suggested a reduction of around 3 m m Hg for systolic BP, and 2 m m Hg for diastolic BP (2a 271. These reductions were particularly evident for moderate levels of PA, including walking. Favourable effects on other risk factors, including regular walking influencing lipid levels, have also been reported (2s). There is evidence that even short.bouts of PA, s u c h as stair climbing, m a y impact on cardiovascular risk factors in a favourable m a n n e r (291. Biological m e c h a n i s m s are being explored, with effects of exercise a p p a r e n t on cardiac endothelial function(30.31). There is little new evidence concerning PA and the risk of stroke. There is a n ongoing suggestion that PA m a y protect against ischaemic stroke, b u t this evidence h a s been mostly a m o n g males. Hu (a2) reported data from a cohort of 72,488 n u r s e s and identified a dose response reduction in stroke risk for all PA 9
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(quintiles RR: 1, .98, .82, .74, .66) and for brisk walking (RR 1,.76,.78,.70,.66). Lee and Blair (a3) reported data from the Dallas cohort of middle-aged males. The results showed t h a t the middle and u p p e r third of cardiorespiratory fitness demonstrated a two-thirds reduction in the risk of stroke death, compared with the least fit third of the population, b u t the study was limited b y small n u m b e r s of stroke outcomes. A study by Ivey(34) demonstrated that a single exercise session could increase levels of tissue Plasminogen Activator (tPA) and reduce risks of blood clotting, which is a possible biologic m e c h a n i s m for reducing atherosclerosis risk, and preventing ischaemic stroke.
Physical activity and obesity prevention I n c r e a s i n g t r e n d s in obesity h a v e o c c u r r e d globally over the p a s t two decades, especially in developed c o u n t r i e s (aS,36). These i n c r e a s e s are poorly u n d e r s t o o d , b u t pose challenges to p o p u l a t i o n health. Recently, efforts h a v e b e e n m a d e to quantify the role of PA in the genesis of the obesity epidemic 137). Several a u t h o r s h a v e s u g g e s t e d t h a t t h e r e have b e e n only small i n c r e a s e s in energy intake d u r i n g the last two d e c a d e s (3s,39). Although no d a t a h a v e m o n i t o r e d energy e x p e n d i t u r e (EE) in a c o m p r e h e n s i v e fashion, it is s u g g e s t e d t h a t EE h a s declined. Although the leisure time PA c o m p o n e n t h a s r e m a i n e d m o s t l y u n c h a n g e d or only declined slightly, other m o d e s of EE are t h o u g h t to h a v e declined markedly. The r e a s o n s for this include technological a d v a n c e s which require less EE in the d o m e s t i c a n d o c c u p a t i o n a l settings, a n d g r e a t e r u s e of m o t o r i s e d (and therefore sedentary) m o d e s of t r a n s p o r t (ag). Even small c o n s i s t e n t declines in EE, a v e r a g e d over a year, could r e s u l t in p o p u l a t i o n weight gain. This s u g g e s t s t h a t i n c r e a s i n g total inactivity h a s b e e n a n i m p o r t a n t c o n t r i b u t o r to the obesity epidemic. Current debates rage around the a m o u n t of PA which might be required to [i] prevent weight gain in populations, and [ill induce and maintain weight loss in populations already obese or overweight (1~). There are no clear answers to these questions, so the results of consensus discussions are presented. The International Association for the Study of Obesity (IASO) c o n s e n s u s group reported on the deliberations of a meeting held in 2002 (4°/. This review concluded that there were m a n y health benefits to be gained from the current 30 minutes moderate-intensity PA recommendations in preventing hypertension, diabetes and heart disease. However, "for the prevention of weight regain in formerly obese individuals, at least 60-90 m i n u t e s o f moderate intensity PA or lesser a m o u n t s o f vigorous PA are required". This is slightly more t h a n the a m o u n t r e c o m m e n d e d to prevent the transition to overweight or obesity in the general population [weight maintenance], where "moderate intensity activity o f at least 45-60 m i n u t e s p e r day, or i. 7 PAL is required '(4°). The latter measure, PAL, is a n estimate of total daily average PA level, where a PAL of 1.0 is basal resting metabolic rate. This a m o u n t of activity is around twice the current recommendations for CVD and diabetes prevention. The new concept tfere is energy balance, and the total a m o u n t of PA expended across the whole day is what a p p e a r s necessary for population level obesity prevention. The only strategies likely to succeed in increasing population levels of total EE are related to policy, regulatory mad environmental changes to reengineer PA into everyday life(15).
Physical activity and diabetes prevention, Epidemiological r e s e a r c h in the 1980s a n d 1990s explored the r e l a t i o n s h i p 10
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between PA a n d diabetes. Initial s t u d i e s were cross sectional or c o h o r t studies, showing high rates of diabetes in s e d e n t a r y populations, suggesting t h a t d e c r e a s e d PA was as i m p o r t a n t a risk factor for developing diabetes as was i n c r e a s e d body m a s s index t41,42). Other observational studies have provided new ideas about PA and diabetes. Hu (4a described a cohort of male health professionals, and showed an increased risk of diabetes among those who watched >40 hours per week of television, which was independent of the protective benefits of PA participation. Fulton-Kehoe t44) carried out a case-control study among Hispanics in Colorado, and showed a 40% decrease in the odds ratio of being diabetic in the most active third of the population, whether leisure time or occupational PA was measured. A study of middle-aged Finnish adults showed that at least moderate intensity occupational activity, active commuting or LTPA were each associated with reduced diabetes incidence t45). Finally, early evidence is appearing that suggests that contributors to the Metabolic Syndrome might be reduced substantially in those exposed to prolonged vigorous exercise programs (46). In the last few years, even stronger evidence has become available from randomised controlled trials (RCTs), which have explored the concept of diabetes prevention in high risk populations. This review summarises three trials and their impact on diabetes prevention. The Da Qing study t47)was an RCT of 577 people with impaired glucose tolerance in China. Randomisation was by outpatient clinics, and patients were allocated to four intervention groups: [i] control, [ii] diet, [iii] an exercise group, [iv] exercise and diet groups. The behavioural outcomes showed increases in exercise in groups [iii] and [iv], but no differences in weight were noted for groups [i], [ill and [iii]. Diabetes incidence showed a graded reduction across groups i-iv, with rates of 15.7, 10, 8.3 and 9.6 per 100 person years. A greater risk reduction effect seemed to be attributable to exercise t h a n to diet, although both were significantly different from the control group. The Finnish Diabetes Prevention Study (48) was an RCT of 522 people with impaired glucose tolerance (IGT). The intervention included intensive nutritional counselling and some endurance exercise advice. The aims were to reduce weight by about 5% (achieved by 43% intervention, 13% controls), to reduce total fat intake to a r o u n d 30% or less (47% intervention, 26% controls), and to achieve the moderate PA recommendations of 30 minutes per day (increased PA 36% intervention, 16% controls). Intervention subjects lost around 4.2 kg (compared with 0.8kg in controls) at 12 months, and also showed greater reductions in waist circumference and in blood pressure t h a n controls. Diabetes incidence was reduced by 58% more among the intervention group than controls, and was related to the a m o u n t of lifestyle change. It was concluded that for every 22 people with IGT who received the intervention, one more case of diabetes might be prevented. The Diabetes Prevention Project (49) (DPP group, 2002) was a multi-centre RCT in the USA with 3234 adults with IGT. There were three trial arms: an intensive 16-session lifestyle intervention, a usual care arm, and a pharmacological arm (Mefformin). The behavioural outcomes showed that 74% achieved their PA goal at one year, and also achieved their weight loss goal of around 7% (weight loss of about 7 kg). There was a 58% reduction in the incidence of diabetes in the intensive lifestyle intervention group and a 31% reduction in the Metformin group, compared with controls. Given the strength of the evidence, the study was 11
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stopped early, with the behavioural intervention significantly more effective in preventing diabetes t h a n Mefformin. These three trials provide evidence that diabetes can be prevented in those at high risk. One caveat is the expense of these intensive interventions, with the DPP behavioral intervention trial costing around $3,000 per participant tS°) (DPP 2003). There is a n urgent need now to translate this evidence into more cost-effective population-level interventions.
Physical activity and mental health There are few new studies in the a r e a of PA a n d m e n t a l health, b u t m u c h r e m a i n s to be clarified with r e s p e c t to evidence of PA a n d diverse m e n t a l health outcomes. For example, clinical studies h a v e explored PA a n d anxiety or d e p r e s s i o n in small s a m p l e s (al,a2), b u t few h a v e u s e d r e p r e s e n t a t i v e or large p o p u l a t i o n samples. F u r t h e r m o r e , even less w o r k h a s b e e n carried out to explore the r e l a t i o n s h i p b e t w e e n PA and p s y c h o s o c i a l wellbeing or positive m e n t a l health, other t h a n positive a s s o c i a t i o n s in c r o s s - s e c t i o n a l analytic surveys t53~. R e s e a r c h evidence on psychosocial wellbeing is least clear, p r o b a b l y reflecting its c o n c e p t u a l i s a t i o n variously as 'self esteem', cognitive function, sleep quality a n d m o r e generalised m e n t a l h e a l t h states. D u n n (54/has documented the need for further clarification and for standardised m e a s u r e s of mental health outcomes. Although reviews of earlier studies have shown cross sectional associations between PA and exercise a n d s y m p t o m s of depression, cohort studies have shown mixed relationships, with some studies showing a decreased risk of depression in those who are physically active and other studies showing no clear associations. T h u s the evidence is mixed, especially a m o n g the better-designed longitudinal research studies. Lawlor (Saconducted a systematic review of 14 small clinical trials, which explored exercise as therapy for the m a n a g e m e n t of depression. Exercise, compared with no intervention, seemed to have a significant effect across these studies, b u t again there were substantial methodological concerns. For example, the dose or type of PA required was not stated, few trials used 'intention to treat' m e t h o d s (which should be standard practice in controlled trial research) and m o s t were unblinded trials, suggesting the possibility of social desirability bias. Some excellent trials are in progress (DOSE study (55)) b u t Final results have not been reported. In spite of all this research the evidence b a s e is still relatively m o d e s t ..... ',especially for whole populations "(al,52). The widely held attributed mental health benefits of being active cannot be substantiated at this stage. The research to date is suggestive, b u t definitive policy decisions and investments in this a r e a require a m u c h stronger evidence base. The research needs include better prospective observational studies to examine the dose response relationship, and larger RCTs to identify the dose and type of activity required. There is also a need to explore the biological m e c h a n i s m s for observed mental health benefits.
Physical activity and ,cancer prevention PA a n d c a n c e r is a relatively n e w a r e a of epidemiological r e s e a r c h , with i n c r e a s i n g evidence of PA c o n t r i b u t i n g to r e d u c i n g the r i s k of a l l - c a u s e c a n c e r s (56,57). A review b y T h u n e (58~ n o t e d t h a t t h e r e w a s a n overall r i s k r e d u c t i o n r e l a t i o n s h i p b e t w e e n activity a n d all c a n c e r d e a t h s , with a doser e s p o n s e relationship, b u t t h a t this w a s s t r o n g e s t for colon c a n c e r a n d b r e a s t cancer, which are i m p o r t a n t c o n t r i b u t o r s to all c a n c e r d e a t h s . Recent consensus statements b y the International Agency for R e s e a r c h into Cancer (IARC)(591have concluded t h a t "there was sufficient evidence for the role of 12
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PA in preventing colon and breast cancers ... Some of these effects appeared to be independent of weight (control) .... the working group considered t h a t excess body weight and physical inactivity account for approximately a quarter to one third of cancers of the colon and breast .... T h u s adiposity a n d inactivity a p p e a r to be the m o s t important avoidable causes of these cancers..."te°). Similar statements were m a d e a b o u t the American Cancer Society report by Willetttel): "that, after avoiding tobacco, staying lean and active provides the greatest potential for minimizing cancer risks". The most recent epidemiological review (77) concluded "there was a 30-40% risk reduction for colon cancer, and the evidence for breast cancer was also moderately strong....but the evidence, for other cancers (prostate, endometrial , ovarian, lung or, renal cancer) was weaker or inconsistent". Specific details of the relationship between PA and breast cancer remain unclear. The target groups at risk and types of PA required are not yet defined, and recent studies have generally added to the confusion. Some studies have reported protective associations with b r e a s t cancer and some have notl6a); while some have reported 'risk reduction', these were not statistically significant (6a). Other studies have shown a protective relationship, b u t only for more vigorous activity or only a m o n g post-menopausal w o m e n (64,65~. Implications of these conclusions are t h a t PA h a s a defined role in the primary prevention of colon and probably breast cancers. Considerations of the population health b u r d e n of these two cancers should consider physical inactivity as an independent risk factor. Finally, evidence around tertiary prevention studies is now growing, with exercise programs demonstrating quality of life and psychosocial benefits for those with established cancer (66). This is extending the range of cancer-related PA research and evidence generation.
Physical activity and musculoskeletal health PA is t h o u g h t to h a v e benefits for m u s c u l o s k e l e t a l health, t h r o u g h the p r e v e n t i o n of o s t e o p o r o s i s a n d in r e d u c i n g r i s k s or c o n s e q u e n c e s of arthritis. No m a j o r or n e w b r e a k t h r o u g h p a p e r s or reviews have b e e n p u b l i s h e d since the p r e v i o u s review (4). It is recognised that bone mineralisation p e a k s by the end of the second decade and that gradual bone loss thereafter contributes to osteoporosis and the risk of falls and fractures (67). Therefore, one prevention focus should be to encourage vigorous weight-bearing PA a m o n g children a n d adolescents, during which period bone deposition occurs (67). In middle-aged and older people, resistance training and balance activities are encouraged to help maintain strength and balance and prevent falls and fractures (67). The evidence on PA and arthritis h a s not progressed in the period reviev~ed. There is some support for possible benefits of moderate intensity PA, and it is unlikely to do harm; on the other hand, vigorous or prolonged activity m a y exacerbate or worsen the severity of knee a n d other large joint osteoarthritis and m a y be associated with higher injury rates. Increasing evidence suggests that older patients who have osteoarthritis m a y benefit from PA and exercise programs in t e r m s of improved functional status a n d independent living (68-7I). A few epidemiological studies have examined aspects of musculoskeletal health and all c a u s e mortality or other outcomes. Katzmarzyk and Craig (72}examined the Canadian Fitness Survey cohort (n=8116, followed since 1981), and showed that aspects of musculoskeletal fitness (sit ups, grip strength) were associated with all-cause mortality. Lowered grip strength was also associated with functional limitations a m o n g older adults (7a. 13
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Finally, the net s u m of evidence on falls prevention remains promising, although the relative contributions of different types of activity (including strength training, balance and gait training) remain uncertain. Review papers have shown inactivity to be a consistent risk factor for hip fractures (74). Another systematic review h a s shown that moving from being sedentary to at least moderately active can reduce the risk of hip fractures b y 20-40% (75). These reviews support public health recommendations regarding PA and falls prevention.
Conclusions This u p d a t e of the epidemiological evidence r e a f f i r m s the evidence u n d e r p i n n i n g the National Physical Activity Guidelines for Australia(a): m o d e r a t e i n t e n s i t y PA, on m o s t d a y s of the week, for a b o u t half a n hour, provides the m a x i m a l p o p u l a t i o n h e a l t h benefit. Recent s t u d i e s r e p o r t e d h e r e h a v e s u g g e s t e d t h a t walking, especially b r i s k walking (at l e a s t 3.2-4.8 k m per hour), itself confers a benefit which is i n d e p e n d e n t of o t h e r m o d e s of PA. Active c o m m u t i n g is s u p p o r t e d b y a few studies (7), b u t evidence in this a r e a r e m a i n s s p a r s e . The benefits of b e i n g active a c c r u e to different p o p u l a t i o n s u b - g r o u p s ; evidence is n o w clear for w o m e n as well as men, a n d for people with diabetes or those who are overweight. T h u s , it is a n i m p o r t a n t public health s t r a t e g y to e n c o u r a g e activity a m o n g the overweight a n d t h o s e with diabetes, irrespective of the p o t e n t i a l for PA to i m p a c t directly on weight loss. One confusing area remains in the q u a n t u m of activity for weight loss and weight maintenance. Each of these areas have different c o n s e n s u s statements, b u t both recommend more t h a n the 30 minutes per day; at least 45-60 minutes of additional activity per day a p p e a r s to be a m i n i m u m for obesity prevention. The m o s t exciting new information h a s been in the area of diabetes prevention. Several controlled trials have demonstrated that lifestyle change can reduce the incidence of diabetes in at-risk populations. The challenge here will be translational research, which can be developed and disseminated in whole populations and population groups at risk of developing diabetes. The evidence for mental health and for musculoskeletal health h a s not changed m u c h in the period u n d e r review. Further research in both of these health outcome areas is required to clarify the existence and magnitude of PA-related health benefits. For cancer prevention, evidence is strong for colon cancer, moderate for breast cancer prevention and uncertain for other cancers. Current work around tertiary prevention will be a strong focus in the coming decade. Finally, a review of the population b u r d e n of disease attributable to inactivity is in progress, and will redefine the global b u r d e n of illness and disease attributable to physical inactivity. A global strategy for diet and PA is being prepared by the World Health Organisation, and will be submitted to the World Health Assembly in May 2004 (76). This will allow for policy development, using the existing epidemiological data," to develop global and national approaches to tacking the problem of physical inactivity.
14
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References 1. United States Department of Health and H um an Services. PhysicalActivity and Health: A Report of The Surgeon General Atlanta, GA. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, The President's Council on Physical Fitness and Sports. 1996. 2. World Health Organisation. World Health Report, 2002. Geneva. World Health Organisation. Retrieved in J a n u a r y 2004 from http://www.who.int/ whr/2002/en/. 3. Commonwealth Department of Health and Aged Care (DHAC). National Physical Activity Guidelines For Australians. Canberra, Australia. Commonwealth Department of Health and Aged Care (DHAC). 1999. 4. B au man A, Bellew B, Vita P, et al. Getting Australia Active: Towards Better Practice for the Promotion of Physical Activity. Melbourne, Australia. National Public Health Partnership. 2002. 5. Williams PT. (2001) Physical fitness and activity as separate heart disease risk factors: a recta-analysis. Med Sci Sports Exert 2001;33:754-61. 6. Lee IM, Skerrett PJ. Physical activity and all-cause mortality: what is the dose-response relation? Med Sei Sports Exert 2001;33(6 Suppl):S459-71; discussion $493-454. 7. Andersen LB, Schnohr P, Schroll M, et al. (2000) All-cause mortality associated with physical activity during leisure time, work, sports, and cycling to work. Arch Intern ivied 2000; 160(11): 1621-8. 8. Crespo CJ, Palmieri MR, Perdomo RP, et al. The relationship of physical activity arid body weight with all-cause mortality: results from the Puerto Rico Heart Health Program. A nn Epidemiol 2002; 12{8):543-52 9. Gregg EW, Cauley JA, Stone K, et al. Relationship of changes in physical activity and mortality among older women, dAMA 2003;289(18):2379-86. 10. Wannamethee SG, Shaper AG, Walker M. (2000) Physical activity and mortality in older men with diagnosed coronary heart disease. Circulation 2000; 102{12): 1358-63. 11. Lee IM, Paffenbarger RS, Jr. Associations of light, moderate, and vigorous intensity physical activity with longevity. The Harvard Alumni Health Study. Am d Epidemiol 2000; 151 (3):293-9. 12. Powell KE, Thompson PD, Caspersen CJ, et al. Physical activity and the incidence of coronary heart disease. A n n u Rev Public Health 1987;8:25387. 13. Berlin J, Colditz GA. A meta analysis of physical activity in the prevention of coronary heart disease. Am d Epidemiol 1990; 132:612-28. 14. Wannamethee SG, Shaper AG. Physical activity in the prevention of cardiovascular disease: an epidemiological perspective. Sports ivied 2001;31(2): 101-14. 15. Erlichman J, Kerbey AL, J am e s WFI'. Physical activity and its impact on health outcomes. Paper 2: prevention of unheal t hy weight gain and obesity by physical activity: an analysis of the evidence. Obes Rev 2002;3:273-87. 16. Williams PT. The illusion of improved physical fitness and reduced mortality. Med Sci Sport Exert 2003;35(5):736-40. 17. Manson JE, Greenland P, LaCroix AZ, et al. Walking compared with vigorous exercise for the prevention of cardiovascular events in women. N 15
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71 American College of Sports Medicine Position Stand. Exercise and physical activity for older adults. Med Sci Sports Exerc 1998;30(6):992-1008. 72 Katzmarzyk PT, Craig CL. Musculoskeletal fitness and risk of mortality. Med Sci Sports Exerc 2002;34(5):740-4. 73 Rantanen T, Guralnik JM, Foley D, et al. Midlife hand grip strength as a predictor of old age disability. JAMA 1999;281 (6):558-60. 74 Gitlespie LD, Gillespie WJ, Robertson MC, et al. Interventions for preventing falls in elderly people (Cochrane Review). Cochrane Library, Issue 2, 2002. Oxford: Update Software. 75 Gregg EW, Pereira MA, Caspersen CJ. Physical activity, falls, and fractures among older adults: a review of the epidemiologic evidence. J A m Geriatr Soc 2000; 48(8):883-93. 76 World Health Organisation. Global Strategy on Diet, Physical Activity and Health. 2003. Geneva. World Health Organisation, Health Promotion Branch. Retrieved J a n a u r y 2004 from http://www.who.int/hpr/global.strategy. shtml. 77 Lee IM. Physical activity and cancer prevention--data from epidemiologic studies. Med Sci Sports Exerc 2003;35(11): 1823-7.
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Introduction P h y s i c a l a c t i v i t y (PA) is r e c o g n i s e d a s b e i n g i m p o r t a n t for r e d u c i n g t h e o v e r a l l b u r d e n of d i s e a s e (1,2). B a s e d o n s o u n d e p i d e m i o l o g i c a l e v i d e n c e , A u s t r a l i a n P h y s i c a l A c t i v i t y g u i d e l i n e s w e r e p u b l i s h e d in 1999, w h i c h r e c o m m e n d e d t h a t e v e r y a d u l t s h o u l d a c c u m u l a t e m o d e r a t e i n t e n s i t y a c t i v i t y for h a l f a n h o u r o n m o s t d a y s of t h e w e e k {3). A d d i t i o n a l h e a l t h b e n e f i t s w o u l d a c c r u e for t h o s e u n d e r t a k i n g s o m e a d d i t i o n a l v i g o r o u s a c t i v i t i e s , a n d for t h o s e i n c l u d i n g m o r e o p p o r t u n i t i e s to b e p h y s i c a l l y a c t i v e i n t o all a s p e c t s o f e v e r y d a y life. It is i m p o r t a n t c o n t i n u a l l y to u p d a t e t h e evidence on w h i c h t h e s e r e c o m m e n d a t i o n s a r e b a s e d . This review f o c u s e s on a d u l t s , a n d u p d a t e s t h e evidence o n t h e r e l a t i o n s h i p b e t w e e n PA a n d a l l - c a u s e mortality, c a r d i o v a s c u l a r d i s e a s e , d i a b e t e s , obesity, cancer, mel~tal a n d m u s c u l o s k e l e t a l health. This d o c u m e n t u p d a t e s t h e evidence s i n c e t h e p u b l i c a t i o n of G e t t i n g A u s t r a l i a Active (4).
Methods T h i s r e v i e w u s e d m u l t i p l e e l e c t r o n i c d a t a b a s e s , i n c l u d i n g NIH P u b M e d , Medline, Current Contents, Cinhal, Psychlit, Embase and the evidenceb a s e d d i r e c t o r i e s ( C o c h r a n e , DARE). S t u d i e s p u b l i s h e d b e t w e e n 2 0 0 0 - 2 0 0 3 w e r e s o u g h t u s i n g p r e v i o u s l y d e f i n e d s e a r c h a p p r o a c h e s (4). P o p u l a t i o n - b a s e d
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studies, r a t h e r t h a n evidence from clinical or small scale trials, are the p r i m a r y focus for this review. I n s t e a d of providing a detailed description of the more t h a n 200 studies t h a t have been p u b l i s h e d in this period, this review focuses on those studies t h a t provide new perspectives on the evidence.
Epidemiological developments in the conceptualisation of physical activity measures There are challenges in the epidemiological m e a s u r e m e n t of exposure (PA a n d fitness a n d related attributes) in relation to health outcomes. Measures have r a n g e d from self-report surveys a n d interviews to objective m e a s u r e s of cardiorespiratory fitness (CRF). There are still debates a b o u t the relative benefits of PA and cardiorespiratory fitness in t e r m s of which confers health benefits 15J, a n d the differences are of policy relevance. Both a p p e a r to have health benefits b u t the health o u t c o m e s and biological m e c h a n i s m s m a y be different. Furthermore, the current national PA recommendations are based on epidemiological studies of leisure time physical activity (LTPA), but this represents only a small fraction of total daily energy expenditure (EE). In recent years, a broader conceptualisation of EE is now thought relevant to health outcomes and obesity prevention. This includes the accumulation of PA through 'activities of daily living', active transport, and in occupational and domestic settings. Some studies have focused on sedentary time, sitting time or measures of 'inactivity' as being of interest in relation to health outcomes. New technologies for assessing step counts and all movement (pedometers and accelerometers) may be used for more accurate exposure measurements in population studies.
Physical activity and All Cause Mortality (ACM) Recent s t u d i e s reaffirm the dose r e s p o n s e relationship between PA a n d allc a u s e mortality (ACM). There is typically a risk reduction of a r o u n d 30% for those achieving the r e c o m m e n d e d levels of at least moderate intensity PA on m o s t days of the week, c o m p a r e d with t h o s e who are inactive 16). F u r t h e r m o r e , the m a x i m u m benefits on ACM a p p e a r to be in moving people from the m o s t s e d e n t a r y group to the middle of the PA or cardio-respiratory fitness distribution curve [at least 'moderately active']. Other p a p e r s illustrate some n e w elements of this association. Andersen (7) described data from a Danish cohort, showing that moderate levels of LTPA conferred a 35% risk reduction compared with the sedentary. This study also showed a benefit of regular 'active commuting', with those who cycled to work for 3 h o u r s per week having a 30% lower ACM risk. Data from Puerto Rico show a dose response reduction in ACM risk vcith increasing PA levels; those in the middle of the distribution of PA in the population had a 32-37% reduction in risk ts). Compared with the most sedentary quartile, the next most active quartile of the population had an accumulated survival that was around three years longer. Gregg(9) examined changes in PA and its impact on ACM amongst a cohort of 7500 older women in the USA who were followed for six years. Those who became active or maintained activity levels, had around 60% the risk of ACM compared with those who stayed sedentary. Wannamethee ~1°) reported data from a cohort of older British males, followed for 12-14 years. There was a dose response reduction in ACM risk across PA categories, observed for all ages, among those with cardiac symptoms. The relationships were similar for all-cause mortality and for cardiovascular deaths. 7
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Walking was protective ff more t h a n 40 minutes a day (relative risk (RR) for ACM =0.48). Moderate or heavy gardening was also protective, reducing ACM by 41%, b u t with no significant impact for light gardening. Those who were inactive and became moderately active had a 42% risk reduction of all-cause mortality (RR 0.58, 95% CI 0.33 to 1.03, p=0.06). Lee (hI reported on further data from the Harvard Alumni male cohort. Light PA showed a non significant relationship with ACM, while vigorous PA was clearly protective. With respect to walking, only those achieving > 20 kflometres a week of walking showed a decrease in risk, and only those climbing >20 flights of stairs per week showed a reduced risk of ACM.
Physical Activity and Cardiovascular Disease Prevention The relationship b e t w e e n PA a n d incident a n d fatal c a r d i o v a s c u l a r disease (CVD) h a s b e e n recognised since the s y s t e m a t i c review of Powell tl~) a n d the m e t a - a n a l y s i s by Berlin (13). An interesting policy challenge h a s b e e n the slow u p t a k e of this evidence b y clinicians a n d health d e c i s i o n - m a k e r s . The p o p u l a t i o n risk r e d u c t i o n c o n s i s t e n t l y a p p e a r s g r e a t e s t for i n c r e a s i n g PA or fitness a m o n g those who are s e d e n t a r y [or unfit] a n d moving t h e m to the r e c o m m e n d e d PA levels (L3). Recent reviews support previous Australian and US recommendations (~4'15).The relationships appears to be valid for m e n and women in middle ages and a m o n g older adults. For tertiary prevention, which is rehabilitation or post heart attack, PA m a y be effective, b u t there are too few exercise-only studies to m a k e definitive statements here. Dissenting reviews have been published by Williams, who h a s suggested that the cardiovascular benefits for fitness are clearly greater t h a n for self-reported PA(5). A further s t u d y h a s challenged the relationship between changes in fitness and s u b s e q u e n t CVD (16)by suggesting that m e a s u r e m e n t error could account for the observed cardioprotective associations. The consistency of epidemiological research across populations and the diversity of m e a s u r e s of self-report and objective PA a n d fitness m a k e it unlikely t h a t the consistent cardioprotective relationships are spurious, as Williams contends (16). The methods used in this CVD review are descriptive. A s u m m a r y of the evidence relating PA or fitness to cardiovascular disease or coronary heart disease risk, from studies published in 2000-2003, is shown in Figure 1. Data are described compared with the least active or least fit segment of the population, which is assigned a relative risk of 1.0. The next increments, by Fifths or quarters of PA in the population, are c o m p a r e d to the most inactive. Where estimates are not given in the papers, linear interpolation is used to estimate intermediate risks. All studies had at least three actual data points to contribute to the figure. The data show the generally consistent pattern of reduction in risk, maximal by about the midpoint of the distribution [median reduction of 31% in the middle distribution of PA for CVD risk across studies shown in the figure]. This level of activity approximates to the recommendations of half a n hour, of moderate intensity activity on m o s t days. There is further risk reduction for those expending greater a m o u n t s of energy [levels 4 a n d 5 in the figure]. New studies have explored the dose-response relationship for at least moderate intensity walking (3.5-4 k m per hour). For example, Manson ~7) observed a dose response relationship for walking quintiles and CVD outcomes (RRs 1.0, .91, .82, .75, .68), and similar findings were reported across walking quartiles by LeetIS),(RRs 1.0, .86, .49, .48). These data support the notion of health benefits for regular walking.
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Figure 1: Risk reducti•n estimates •f the re•ati•nship between physica• acuvity and cardi•vascu•ar disease• from epidemiologicalstudies published 2000-2003.
Similar relationships were shown for general population samples and a m o n g those with diabetes (shown as dotted lines in the figure) a9-22). The first two of these studies also showed significantly lower CHD rates a m o n g those who were in the highest quintiles of walking hours per week. The Whitehall British civil servants study showed no protective association with moderate activity (23), b u t this study did show more significant relationships across tertiles of PA for all-cause mortality (RRs across tei~Jles 1.0, .92, .66), and for the relationship between walking pace and CHD (RRs 1.0, .76, .69). Data from the Caerphilly s t u d y in W a l e s (241 observed a cardioprotective relationship across total PA levels (as shown in the figure), b u t observed a n independent relationship only for vigorous activity, not for combined light and moderate activity alone. There have been a few studies furthering the evidence that PA improves other cardiovascular risk factors. Systematic reviews of the relationships between PA and hypertension have suggested a reduction of around 3 m m Hg for systolic BP, and 2 m m Hg for diastolic BP (2a 271. These reductions were particularly evident for moderate levels of PA, including walking. Favourable effects on other risk factors, including regular walking influencing lipid levels, have also been reported (2s). There is evidence that even short.bouts of PA, s u c h as stair climbing, m a y impact on cardiovascular risk factors in a favourable m a n n e r (291. Biological m e c h a n i s m s are being explored, with effects of exercise a p p a r e n t on cardiac endothelial function(30.31). There is little new evidence concerning PA and the risk of stroke. There is a n ongoing suggestion that PA m a y protect against ischaemic stroke, b u t this evidence h a s been mostly a m o n g males. Hu (a2) reported data from a cohort of 72,488 n u r s e s and identified a dose response reduction in stroke risk for all PA 9
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(quintiles RR: 1, .98, .82, .74, .66) and for brisk walking (RR 1,.76,.78,.70,.66). Lee and Blair (a3) reported data from the Dallas cohort of middle-aged males. The results showed t h a t the middle and u p p e r third of cardiorespiratory fitness demonstrated a two-thirds reduction in the risk of stroke death, compared with the least fit third of the population, b u t the study was limited b y small n u m b e r s of stroke outcomes. A study by Ivey(34) demonstrated that a single exercise session could increase levels of tissue Plasminogen Activator (tPA) and reduce risks of blood clotting, which is a possible biologic m e c h a n i s m for reducing atherosclerosis risk, and preventing ischaemic stroke.
Physical activity and obesity prevention I n c r e a s i n g t r e n d s in obesity h a v e o c c u r r e d globally over the p a s t two decades, especially in developed c o u n t r i e s (aS,36). These i n c r e a s e s are poorly u n d e r s t o o d , b u t pose challenges to p o p u l a t i o n health. Recently, efforts h a v e b e e n m a d e to quantify the role of PA in the genesis of the obesity epidemic 137). Several a u t h o r s h a v e s u g g e s t e d t h a t t h e r e have b e e n only small i n c r e a s e s in energy intake d u r i n g the last two d e c a d e s (3s,39). Although no d a t a h a v e m o n i t o r e d energy e x p e n d i t u r e (EE) in a c o m p r e h e n s i v e fashion, it is s u g g e s t e d t h a t EE h a s declined. Although the leisure time PA c o m p o n e n t h a s r e m a i n e d m o s t l y u n c h a n g e d or only declined slightly, other m o d e s of EE are t h o u g h t to h a v e declined markedly. The r e a s o n s for this include technological a d v a n c e s which require less EE in the d o m e s t i c a n d o c c u p a t i o n a l settings, a n d g r e a t e r u s e of m o t o r i s e d (and therefore sedentary) m o d e s of t r a n s p o r t (ag). Even small c o n s i s t e n t declines in EE, a v e r a g e d over a year, could r e s u l t in p o p u l a t i o n weight gain. This s u g g e s t s t h a t i n c r e a s i n g total inactivity h a s b e e n a n i m p o r t a n t c o n t r i b u t o r to the obesity epidemic. Current debates rage around the a m o u n t of PA which might be required to [i] prevent weight gain in populations, and [ill induce and maintain weight loss in populations already obese or overweight (1~). There are no clear answers to these questions, so the results of consensus discussions are presented. The International Association for the Study of Obesity (IASO) c o n s e n s u s group reported on the deliberations of a meeting held in 2002 (4°/. This review concluded that there were m a n y health benefits to be gained from the current 30 minutes moderate-intensity PA recommendations in preventing hypertension, diabetes and heart disease. However, "for the prevention of weight regain in formerly obese individuals, at least 60-90 m i n u t e s o f moderate intensity PA or lesser a m o u n t s o f vigorous PA are required". This is slightly more t h a n the a m o u n t r e c o m m e n d e d to prevent the transition to overweight or obesity in the general population [weight maintenance], where "moderate intensity activity o f at least 45-60 m i n u t e s p e r day, or i. 7 PAL is required '(4°). The latter measure, PAL, is a n estimate of total daily average PA level, where a PAL of 1.0 is basal resting metabolic rate. This a m o u n t of activity is around twice the current recommendations for CVD and diabetes prevention. The new concept tfere is energy balance, and the total a m o u n t of PA expended across the whole day is what a p p e a r s necessary for population level obesity prevention. The only strategies likely to succeed in increasing population levels of total EE are related to policy, regulatory mad environmental changes to reengineer PA into everyday life(15).
Physical activity and diabetes prevention, Epidemiological r e s e a r c h in the 1980s a n d 1990s explored the r e l a t i o n s h i p 10
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between PA a n d diabetes. Initial s t u d i e s were cross sectional or c o h o r t studies, showing high rates of diabetes in s e d e n t a r y populations, suggesting t h a t d e c r e a s e d PA was as i m p o r t a n t a risk factor for developing diabetes as was i n c r e a s e d body m a s s index t41,42). Other observational studies have provided new ideas about PA and diabetes. Hu (4a described a cohort of male health professionals, and showed an increased risk of diabetes among those who watched >40 hours per week of television, which was independent of the protective benefits of PA participation. Fulton-Kehoe t44) carried out a case-control study among Hispanics in Colorado, and showed a 40% decrease in the odds ratio of being diabetic in the most active third of the population, whether leisure time or occupational PA was measured. A study of middle-aged Finnish adults showed that at least moderate intensity occupational activity, active commuting or LTPA were each associated with reduced diabetes incidence t45). Finally, early evidence is appearing that suggests that contributors to the Metabolic Syndrome might be reduced substantially in those exposed to prolonged vigorous exercise programs (46). In the last few years, even stronger evidence has become available from randomised controlled trials (RCTs), which have explored the concept of diabetes prevention in high risk populations. This review summarises three trials and their impact on diabetes prevention. The Da Qing study t47)was an RCT of 577 people with impaired glucose tolerance in China. Randomisation was by outpatient clinics, and patients were allocated to four intervention groups: [i] control, [ii] diet, [iii] an exercise group, [iv] exercise and diet groups. The behavioural outcomes showed increases in exercise in groups [iii] and [iv], but no differences in weight were noted for groups [i], [ill and [iii]. Diabetes incidence showed a graded reduction across groups i-iv, with rates of 15.7, 10, 8.3 and 9.6 per 100 person years. A greater risk reduction effect seemed to be attributable to exercise t h a n to diet, although both were significantly different from the control group. The Finnish Diabetes Prevention Study (48) was an RCT of 522 people with impaired glucose tolerance (IGT). The intervention included intensive nutritional counselling and some endurance exercise advice. The aims were to reduce weight by about 5% (achieved by 43% intervention, 13% controls), to reduce total fat intake to a r o u n d 30% or less (47% intervention, 26% controls), and to achieve the moderate PA recommendations of 30 minutes per day (increased PA 36% intervention, 16% controls). Intervention subjects lost around 4.2 kg (compared with 0.8kg in controls) at 12 months, and also showed greater reductions in waist circumference and in blood pressure t h a n controls. Diabetes incidence was reduced by 58% more among the intervention group than controls, and was related to the a m o u n t of lifestyle change. It was concluded that for every 22 people with IGT who received the intervention, one more case of diabetes might be prevented. The Diabetes Prevention Project (49) (DPP group, 2002) was a multi-centre RCT in the USA with 3234 adults with IGT. There were three trial arms: an intensive 16-session lifestyle intervention, a usual care arm, and a pharmacological arm (Mefformin). The behavioural outcomes showed that 74% achieved their PA goal at one year, and also achieved their weight loss goal of around 7% (weight loss of about 7 kg). There was a 58% reduction in the incidence of diabetes in the intensive lifestyle intervention group and a 31% reduction in the Metformin group, compared with controls. Given the strength of the evidence, the study was 11
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stopped early, with the behavioural intervention significantly more effective in preventing diabetes t h a n Mefformin. These three trials provide evidence that diabetes can be prevented in those at high risk. One caveat is the expense of these intensive interventions, with the DPP behavioral intervention trial costing around $3,000 per participant tS°) (DPP 2003). There is a n urgent need now to translate this evidence into more cost-effective population-level interventions.
Physical activity and mental health There are few new studies in the a r e a of PA a n d m e n t a l health, b u t m u c h r e m a i n s to be clarified with r e s p e c t to evidence of PA a n d diverse m e n t a l health outcomes. For example, clinical studies h a v e explored PA a n d anxiety or d e p r e s s i o n in small s a m p l e s (al,a2), b u t few h a v e u s e d r e p r e s e n t a t i v e or large p o p u l a t i o n samples. F u r t h e r m o r e , even less w o r k h a s b e e n carried out to explore the r e l a t i o n s h i p b e t w e e n PA and p s y c h o s o c i a l wellbeing or positive m e n t a l health, other t h a n positive a s s o c i a t i o n s in c r o s s - s e c t i o n a l analytic surveys t53~. R e s e a r c h evidence on psychosocial wellbeing is least clear, p r o b a b l y reflecting its c o n c e p t u a l i s a t i o n variously as 'self esteem', cognitive function, sleep quality a n d m o r e generalised m e n t a l h e a l t h states. D u n n (54/has documented the need for further clarification and for standardised m e a s u r e s of mental health outcomes. Although reviews of earlier studies have shown cross sectional associations between PA and exercise a n d s y m p t o m s of depression, cohort studies have shown mixed relationships, with some studies showing a decreased risk of depression in those who are physically active and other studies showing no clear associations. T h u s the evidence is mixed, especially a m o n g the better-designed longitudinal research studies. Lawlor (Saconducted a systematic review of 14 small clinical trials, which explored exercise as therapy for the m a n a g e m e n t of depression. Exercise, compared with no intervention, seemed to have a significant effect across these studies, b u t again there were substantial methodological concerns. For example, the dose or type of PA required was not stated, few trials used 'intention to treat' m e t h o d s (which should be standard practice in controlled trial research) and m o s t were unblinded trials, suggesting the possibility of social desirability bias. Some excellent trials are in progress (DOSE study (55)) b u t Final results have not been reported. In spite of all this research the evidence b a s e is still relatively m o d e s t ..... ',especially for whole populations "(al,52). The widely held attributed mental health benefits of being active cannot be substantiated at this stage. The research to date is suggestive, b u t definitive policy decisions and investments in this a r e a require a m u c h stronger evidence base. The research needs include better prospective observational studies to examine the dose response relationship, and larger RCTs to identify the dose and type of activity required. There is also a need to explore the biological m e c h a n i s m s for observed mental health benefits.
Physical activity and ,cancer prevention PA a n d c a n c e r is a relatively n e w a r e a of epidemiological r e s e a r c h , with i n c r e a s i n g evidence of PA c o n t r i b u t i n g to r e d u c i n g the r i s k of a l l - c a u s e c a n c e r s (56,57). A review b y T h u n e (58~ n o t e d t h a t t h e r e w a s a n overall r i s k r e d u c t i o n r e l a t i o n s h i p b e t w e e n activity a n d all c a n c e r d e a t h s , with a doser e s p o n s e relationship, b u t t h a t this w a s s t r o n g e s t for colon c a n c e r a n d b r e a s t cancer, which are i m p o r t a n t c o n t r i b u t o r s to all c a n c e r d e a t h s . Recent consensus statements b y the International Agency for R e s e a r c h into Cancer (IARC)(591have concluded t h a t "there was sufficient evidence for the role of 12
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PA in preventing colon and breast cancers ... Some of these effects appeared to be independent of weight (control) .... the working group considered t h a t excess body weight and physical inactivity account for approximately a quarter to one third of cancers of the colon and breast .... T h u s adiposity a n d inactivity a p p e a r to be the m o s t important avoidable causes of these cancers..."te°). Similar statements were m a d e a b o u t the American Cancer Society report by Willetttel): "that, after avoiding tobacco, staying lean and active provides the greatest potential for minimizing cancer risks". The most recent epidemiological review (77) concluded "there was a 30-40% risk reduction for colon cancer, and the evidence for breast cancer was also moderately strong....but the evidence, for other cancers (prostate, endometrial , ovarian, lung or, renal cancer) was weaker or inconsistent". Specific details of the relationship between PA and breast cancer remain unclear. The target groups at risk and types of PA required are not yet defined, and recent studies have generally added to the confusion. Some studies have reported protective associations with b r e a s t cancer and some have notl6a); while some have reported 'risk reduction', these were not statistically significant (6a). Other studies have shown a protective relationship, b u t only for more vigorous activity or only a m o n g post-menopausal w o m e n (64,65~. Implications of these conclusions are t h a t PA h a s a defined role in the primary prevention of colon and probably breast cancers. Considerations of the population health b u r d e n of these two cancers should consider physical inactivity as an independent risk factor. Finally, evidence around tertiary prevention studies is now growing, with exercise programs demonstrating quality of life and psychosocial benefits for those with established cancer (66). This is extending the range of cancer-related PA research and evidence generation.
Physical activity and musculoskeletal health PA is t h o u g h t to h a v e benefits for m u s c u l o s k e l e t a l health, t h r o u g h the p r e v e n t i o n of o s t e o p o r o s i s a n d in r e d u c i n g r i s k s or c o n s e q u e n c e s of arthritis. No m a j o r or n e w b r e a k t h r o u g h p a p e r s or reviews have b e e n p u b l i s h e d since the p r e v i o u s review (4). It is recognised that bone mineralisation p e a k s by the end of the second decade and that gradual bone loss thereafter contributes to osteoporosis and the risk of falls and fractures (67). Therefore, one prevention focus should be to encourage vigorous weight-bearing PA a m o n g children a n d adolescents, during which period bone deposition occurs (67). In middle-aged and older people, resistance training and balance activities are encouraged to help maintain strength and balance and prevent falls and fractures (67). The evidence on PA and arthritis h a s not progressed in the period reviev~ed. There is some support for possible benefits of moderate intensity PA, and it is unlikely to do harm; on the other hand, vigorous or prolonged activity m a y exacerbate or worsen the severity of knee a n d other large joint osteoarthritis and m a y be associated with higher injury rates. Increasing evidence suggests that older patients who have osteoarthritis m a y benefit from PA and exercise programs in t e r m s of improved functional status a n d independent living (68-7I). A few epidemiological studies have examined aspects of musculoskeletal health and all c a u s e mortality or other outcomes. Katzmarzyk and Craig (72}examined the Canadian Fitness Survey cohort (n=8116, followed since 1981), and showed that aspects of musculoskeletal fitness (sit ups, grip strength) were associated with all-cause mortality. Lowered grip strength was also associated with functional limitations a m o n g older adults (7a. 13
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Finally, the net s u m of evidence on falls prevention remains promising, although the relative contributions of different types of activity (including strength training, balance and gait training) remain uncertain. Review papers have shown inactivity to be a consistent risk factor for hip fractures (74). Another systematic review h a s shown that moving from being sedentary to at least moderately active can reduce the risk of hip fractures b y 20-40% (75). These reviews support public health recommendations regarding PA and falls prevention.
Conclusions This u p d a t e of the epidemiological evidence r e a f f i r m s the evidence u n d e r p i n n i n g the National Physical Activity Guidelines for Australia(a): m o d e r a t e i n t e n s i t y PA, on m o s t d a y s of the week, for a b o u t half a n hour, provides the m a x i m a l p o p u l a t i o n h e a l t h benefit. Recent s t u d i e s r e p o r t e d h e r e h a v e s u g g e s t e d t h a t walking, especially b r i s k walking (at l e a s t 3.2-4.8 k m per hour), itself confers a benefit which is i n d e p e n d e n t of o t h e r m o d e s of PA. Active c o m m u t i n g is s u p p o r t e d b y a few studies (7), b u t evidence in this a r e a r e m a i n s s p a r s e . The benefits of b e i n g active a c c r u e to different p o p u l a t i o n s u b - g r o u p s ; evidence is n o w clear for w o m e n as well as men, a n d for people with diabetes or those who are overweight. T h u s , it is a n i m p o r t a n t public health s t r a t e g y to e n c o u r a g e activity a m o n g the overweight a n d t h o s e with diabetes, irrespective of the p o t e n t i a l for PA to i m p a c t directly on weight loss. One confusing area remains in the q u a n t u m of activity for weight loss and weight maintenance. Each of these areas have different c o n s e n s u s statements, b u t both recommend more t h a n the 30 minutes per day; at least 45-60 minutes of additional activity per day a p p e a r s to be a m i n i m u m for obesity prevention. The m o s t exciting new information h a s been in the area of diabetes prevention. Several controlled trials have demonstrated that lifestyle change can reduce the incidence of diabetes in at-risk populations. The challenge here will be translational research, which can be developed and disseminated in whole populations and population groups at risk of developing diabetes. The evidence for mental health and for musculoskeletal health h a s not changed m u c h in the period u n d e r review. Further research in both of these health outcome areas is required to clarify the existence and magnitude of PA-related health benefits. For cancer prevention, evidence is strong for colon cancer, moderate for breast cancer prevention and uncertain for other cancers. Current work around tertiary prevention will be a strong focus in the coming decade. Finally, a review of the population b u r d e n of disease attributable to inactivity is in progress, and will redefine the global b u r d e n of illness and disease attributable to physical inactivity. A global strategy for diet and PA is being prepared by the World Health Organisation, and will be submitted to the World Health Assembly in May 2004 (76). This will allow for policy development, using the existing epidemiological data," to develop global and national approaches to tacking the problem of physical inactivity.
14
Updating the evidence.,.
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