Secular Change in Cardiorespiratory Fitness and Body Composition of Women

ORIGINAL ARTICLE

Secular Change in Cardiorespiratory Fitness and Body Composition of Women: The Aerobics Center Longitudinal Study Haiwei Li, PhD; Xuemei Sui, MD, MPH, PhD; Shouqing Huang, MD; Carl J. Lavie, MD; Zhengzhen Wang, PhD; and Steven N. Blair, PED Abstract Objective: To investigate secular change of cardiorespiratory fitness (CRF) and body composition during 35 years in a large sample of women enrolled in the Aerobics Center Longitudinal Study. Patients and Methods: A cross-sectional analysis of baseline fitness data collected during preventive medical examination of 13,037 women aged 20 to 64 years evaluated at the Cooper Clinic in Dallas, Texas, from January 1, 1970, through December 30, 2004, who underwent a body composition assessment and a maximal treadmill exercise test. Women were stratified by examination year (5 years for each group) and age. Analysis of covariance was used to ascertain secular change of CRF and body composition. Results: Adjusted CRF levels, as indicated by maximal metabolic equivalent or relative maximum oxygen consumption, among women in the cohort increased significantly during a 35-year period for both age groups (P<.001). The greatest change occurred during the 1970s to 1980s, with a small decrease in 2000 through 2004 in both age groups. Adjusted body mass index increased 9.05% during the past 35 years (P<.001), but adjusted percentage of body fat was significantly higher in 1980 through 1984 than in the other groups (P<.001). Conclusions: In a large cohort of women, the mean CRF has improved during the past 35 years, with a slight decrease in 2000 through 2004. From 1980 through 2004, the increase in body weight was mainly attributable to the increase in fat-free mass. ª 2015 Mayo Foundation for Medical Education and Research

O

besity and physical inactivity are notable public health challenges in the United States and most of the Western world. In the United States, the prevalence of obesity was recently reported as 32.2% in men and 35.5% in women.1 In addition, recent statistics suggest that less than 5% of adults met the recommendation to obtain 30 min/d of physical activity estimated with an accelerometer.2 Low cardiorespiratory fitness (CRF), an objective indicator of recent habitual physical activity, and high levels of adiposity have been established as strong risk factors for cardiovascular disease (CVD)3 and predictors of all-cause and CVD mortality in young and older groups.4-7 Examination of long-term trends of CRF and body composition is useful to determine changes in the direction and magnitude of these parameters and to evaluate the effect of individual behaviors and environmental factors, including policy, culture, and economy, on predicting future trends. Previous cross-sectional studies have examined the secular trend of CRF,8-11 fatness,1,12-16

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or both17-19 in children, adolescents, and male adult populations. Most of these analyses have found an increased trend in fatness and a decreased trend in CRF. Of these studies, only 2 observed the secular trend of CRF and fatness in female populations.1,19 When examining the secular trend of fatness, most of these studies have used body mass index (BMI; calculated as the weight in kilograms divided by the height in meters squared) as a crude measure of adiposity.1,13-18 A major limitation of the use of BMI as an index of adiposity is that one cannot determine whether the secular trend observed in the prevalence of BMI-defined obesity is an indication of increase in fat mass (FM), fat-free mass (FFM), or both. As the largest database available with objective measurements of CRF and body composition, the Aerobics Center Longitudinal Study (ACLS) offers a unique opportunity to examine trends in CRF and body composition during an extended period. The aim of this study, therefore, was to investigate secular change of CRF and body composition

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From the School of Physical Education, Shanxi Normal University, Linfen, China (H.L.); Department of Exercise Rehabilitation, Beijing Sport University, Beijing, China (H.L., Z.W.); Department of Exercise Science (X.S., S.N.B.) and Department of Epidemiology and Biostatistics (S.N.B.), University of South Carolina, Columbia, SC; The Second People’s Hospital Affiliated to Fujian University of Traditional Chinese Medicine, Fuzhou, China (S.H.); and Department of Cardiovascular Diseases, John Ochsner Heart and Vascular Institute, Ochsner Clinical School, The University of Queensland School of Medicine, New Orleans, LA (C.J.L.).

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during 35 years in a large sample of women enrolled in the ACLS. PATIENTS AND METHODS Study Population The ACLS is an observational study of individuals who received extensive preventive examinations at the Cooper Clinic in Dallas, Texas. The Cooper Clinic serves anyone who elects to come for a preventive examination and for counseling regarding diet, exercise, and other lifestyle factors associated with increased risk of chronic disease. Therefore, patients came from all 50 states and were self-referred or referred by their employers or physicians. More than 95% of participants are non-Hispanic whites from middle to upper socioeconomic strata.20 All patients signed informed consent forms and approved the use of their data for research. The data collection protocols and informed consent were reviewed and approved annually by The Cooper Institute’s Institutional Review Board. Our current analyses included women aged 20 through 64 years who had examination data necessary for the planned analyses during 1970 through 2004. If patients had more than one evaluation, only data from the earliest visit were included in the analysis. Clinical Examination All participants completed a comprehensive clinical examination by a physician. Blood chemical analyses were performed after an at least 12-hour overnight fast with automated bioassays in the Cooper Clinic laboratory. Data on smoking status (current smoker or not), high alcohol consumption (>7 drinks per week or not), marriage status (currently married or not), fertility status (
1 children or not), and physician-diagnosed CVD (myocardial infarction or stroke) and cancer were obtained from a standardized medical history questionnaire. Hypertension, diabetes mellitus, and hypercholesterolemia were diagnosed by clinical test or a history of physician diagnosis. Leisure-Time Physical Activity Leisure-time physical activity (LTPA) was assessed by self-reported leisure time or recreational activities during the past 3 months. To calculate the total volume of LTPA, the metabolic equivalent (MET) level for a given speed 44

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or activity was multiplied by the frequency and the duration and then summed over all activities, resulting in total MET minutes per week of LTPA, which is the principal metric used in the 2008 Physical Activity Guidelines. Detailed measurement methods were described in a previous published article.21 Cardiorespiratory Fitness Maximal treadmill testing using a modified Balke protocol was used to assess CRF, as previously described.22-24 The test protocol and its administration have remained constant during the past 35 years. Participants were encouraged to give maximal effort, and the test was terminated on volitional exhaustion or medical reasons determined by a physician. Maximal MET levels (1 MET ¼ 3.5 mL$kg1$min1 of oxygen uptake) were derived from relative maximum oxygen consumption (V_ O2max) estimated by regression from the final treadmill speed and grade. Except for MET levels, the following variables were also estimated from the duration of the Balke test: V_ O2max in both relative V_ O2max and absolute V_ O2max values achieved. Time on the treadmill with this protocol has been found to be highly correlated (r¼0.92) with measured V_ O2max in both men and women.25,26 Body Composition Briefly, height and weight were measured using a stadiometer and a standard scale. The BMI was calculated as weight in kilograms divided by height in meters squared. The percentage of body fat (BF) was assessed by hydrostatic weighing or the sum of 7 skinfold measures, following standardized protocols.24,27 Some participants chose to go through the underwater weighing assessment for hydrostatically estimated body density with a mathematical conversion to percentage of BF, whereas other participants received a skinfold estimate of percentage of BF. Standardized protocols used and specific procedures for the ACLS assessment of percentage of BF have been published elsewhere.4,24,28,29 The correlation between hydrostatically estimated percentage of BF and skinfold estimated percentage of BF was 0.90 for participants who had both measurements.28,29 When available, hydrostatically estimated percentage of BF was always used in the analysis. The FM was calculated as body weight  percentage of BF, and

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CARDIORESPIRATORY FITNESS AND BODY COMPOSITION

FFM was calculated as body weight  (1  percentage of BF). Statistical Analyses All statistical analyses were performed using PASW SPSS statistical software, version 18.0 (SPSS Inc). The level of significance was set at P<.05 for all the analyses. Tests for linear trend were performed using ordinal coding. We used 1-way analysis of covariance (ANCOVA) with Tukey-adjusted post hoc pairwise comparisons to examine CRF and body composition secular change among the examination time groups of 1970 through 1974, 1975 through 1979, 1980 through 1984, 1985 through 1989, 1990 through 1994, 1995 through 1999, and 2000 through 2004. According to the Provisional Guidelines on Standard International Age Classifications, we divided our participants into 2 age groups (20-44 and 45-64 years of age). In addition, ANCOVA was used to test the main effects and interaction effect of age and time as well as marriage status and time on CRF and body composition, and the Tukeyadjusted post hoc analysis was used to compute pairwise comparisons to determine where differences lie. Finally, we conducted sensitivity analyses to examine the influence of more than 2 age groups on the main results. On the basis of the National Center for Health Statistics, we classified women into 4 age groups: 20 through 34, 35 through 44, 45 through 54, and 55 through 64 years. We repeated these main analyses and did not find major differences regarding the results. Because the direction of the associations from the sensitivity analyses was similar to the initially proposed analyses, we do not report them in this article (Supplemental Table 1 and Supplemental Figures 1 and 2; available online at http:// www.mayoclinicproceedings.org). RESULTS A total of 13,037 women were examined and included in the current study during 1970 through 2004. The population characteristics across examination dates are given in Table 1. During the 35 years, the mean age increased by approximately 4 years, body weight increased by 6.3 kg, there was almost no change in height, and the BMI increased by 2.1. The percentage of current smokers decreased, but the percentage of Mayo Clin Proc. n January 2015;90(1):43-52 www.mayoclinicproceedings.org

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high alcohol consumers increased from 1970 through 2004. Table 2 lists the secular changes of body composition, LTPA, and CRF after adjustment for age, smoking, alcohol consumption, and chronic conditions (including any of the following conditions: myocardial infarction, stroke, cancer, hypercholesterolemia, diabetes, and hypertension). The BMI increased 9.05% during the past 35 years, and the 2000 through 2004 group was higher than other groups (P<.001), except for the 1995 through 1999 group (P¼.81). The percentage of BF was significantly higher in the 1980 through 1984 group than in the other groups (P<.001). The FM in the 1995 through 1999 group was higher than in the other groups (P<.05), except for the 1990 through 1994 group (P¼.14). The FFM was highest in the 2000 through 2004 group during the 35 years (P<.001). The LTPA was higher in the 1990 through 1994 group than in any of the other groups (P<.001). The CRF, treadmill duration, and relative V_ O2max had similar secular changes, with the highest in the 1995 through 1999 group (P<.001). Absolute V_ O2max was higher in the 2000 through 2004 group than in the other groups (P<.001), except for the 1995 through 1999 group (P¼.71). Table 3 and Supplemental Table 2 (available online at http://www.mayoclinicproceedings. org) give the adjusted body compositions, LTPAs, and CRFs by different marriage statuses during 20 years (marriage status data were only available for 1985 through 2004). Currently unmarried women had significantly higher BMIs, percentages of BF, FMs, and FFMs than currently married women (P<.05), and a significant difference was also found across the time groups (P<.001). The LTPA was significantly different between the marriage groups (P<.05), time groups (P<.001), and interaction (P<.001). In addition, the CRF was statistically different between the marriage groups (P<.01) and the interaction was significant (P<.05). Because BMI, percentage of BF, FM, FFM, and LTPA had no significant differences among women with different fertility statuses, these results were provided in Supplemental Table 3 (available online at http://www.mayoclinicproceedings.org). Finally, we evaluated the secular change of body composition (Figure 1), CRF, and LTPA (Figure 2) in different age groups from 1970

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TABLE 1. Baseline Characteristics of the 13,073 Women in the Aerobics Center Longitudinal Studya,b,c Study period Parameter

1970-1974 (n¼388)

1975-1979 (n¼1293)

1980-1984 (n¼2191)

Age (y) Height (cm) Weight (kg) BMI BF (%) FM (kg) FFM (kg) Triglycerides (mg/dL1)e Total cholesterol (mg/dL1)f Glucose (mg/dL1)g LTPA (MET min/wk) HRmax (beats/min) CRF (MET) Treadmill duration (min) Relative V_ O2max (mL$min1$kg1) Absolute V_ O2max (L$min1) Current smoker Heavy drinkerh Chronic conditionsi Currently marriedj Have childj

41.59.3 164.55.7 59.88.5 22.12.8 23.38.7 14.36.9 45.55.8 97.379.6 212.438.6 100.912.6 29.4215.6 17913 8.41.7 10.83.6 30.55.2 1.80.4 62 (16.0) 4 (1.0) 154 (39.7) . .

40.59.7 164.55.9 59.710.1 22.03.5 24.48.1 14.96.9 44.86.5 79.653.1 200.838.6 91.912.6 195.4452.8 17914 8.91.9 11.94.1 32.15.9 1.90.4 196 (15.2) 24 (1.9) 365 (28.2) . .

40.69.9 164.35.8 59.79.8 22.13.3 26.96.5 16.56.3 43.25.0 88.561.9 196.938.6 93.712.6 248.3471.6 17813 9.42.1 13.24.5 34.06.4 2.00.4 307 (14.0) 43 (2.0) 530 (24.2) . .

1985-1989 (n¼2428)

1990-1994 (n¼1766)

41.79.9 42.79.7 164.66.2 164.76.4 61.511.1 63.311.7 22.73.9 23.34.1 26.37.1 26.67.2 16.77.2 17.47.7 44.85.7 45.95.8 79.6106.2 97.361.9 204.650.2 196.938.6 99.19.0 93.714.4 673.81103.6 1089.31458.8 17814 17615 10.02.3 9.92.2 14.64.9 14.34.8 36.07.1 35.66.9 2.20.5 2.20.5 244 (10.1) 145 (8.2) 192 (7.9) 234 (13.3) 784 (32.3) 625 (35.4) 953 (39.3) 1381 (78.2) 817 (33.7) 1131 (64.0)

1995-1999 (n¼2193)

2000-2004 (n¼2814)

P for linear trendd

44.09.2 164.76.3 65.113.4 24.04.6 26.67.4 18.08.1 47.26.9 97.370.8 196.938.6 95.514.4 926.41231.1 17514 10.12.1 14.54.6 35.96.6 2.30.4 172 (7.8) 339 (15.5) 874 (39.9) 1729 (78.8) 1489 (67.9)

45.19.1 165.46.2 66.113.1 24.24.6 26.07.0 17.98.0 48.36.5 97.370.8 196.934.7 91.910.8 913.31089.9 17615 9.81.9 14.04.1 35.25.9 2.30.4 192 (6.8) 477 (17.0) 1313 (46.7) 2304 (81.9) 2088 (74.2)

<.001 .003 <.001 <.001 <.001 <.001 <.001 <.001 <.001 .20 <.001 <.001 <.001 <.001 <.001 <.001 <.001 <.001 <.001 <.001 <.001

BF ¼ body fat; BMI ¼ body mass index; CRF ¼ cardiorespiratory fitness; FFM ¼ fat free mass; FM ¼ fat mass; HRmax ¼ maximum heart rate; LTPA ¼ leisure-time physical activity; MET ¼ metabolic equivalent (1 MET ¼ 3.5 mL$min1$kg1 of oxygen uptake); V_ O2max ¼ maximum oxygen consumption. b SI conversion factors: To convert triglycerides to mmol/L, multiply by 0.0113; to convert total cholesterol to mmol/L, multiply by 0.0259; to convert glucose to mmol/L, multiply by 0.0555. c Categorical data are provided as No. (percentage) of patients and continuous data as mean  SD. d P value for linear trend was determined by general linear model. e Only available in 12,705 women. f Only available in 12,713 women. g Only available in 12,676 women. h Heavy drinker was defined as alcohol consumption of more than 7 drinks per week. i Chronic conditions including any of the following conditions: myocardial infarction, stroke, cancer, hypercholesterolemia, diabetes mellitus, and hypertension. Myocardial infarction, stroke, and cancer were defined by self-report questionnaire. Hypercholesterolemia was defined as a total cholesterol of 240 mg/dL or higher or a history of physician diagnosis. Diabetes was defined as fasting glucose level of 126 mg/dL or higher, insulin use, or a history of physician diagnosis. Hypertension was defined as systolic blood pressure of 140 mm Hg or higher, a diastolic blood pressure of 90 mm Hg or higher, or a history of physician diagnosis. j Data available only in 1985 through 2004. a

through 2004 after adjusting for smoking, alcohol consumption, and chronic conditions. The ANCOVA main effects for the mean differences of body composition expressed in BMI (Figure 1, A), percentage of BF (Figure 1, B), FM (Figure 1, C), and FFM (Figure 1, D) for both age group and time cohorts were significant (P<.001 for each). The interaction between these effects of percentage of BF and FFM was also significant (P<.007). Mean CRF expressed as METs $ (Figure 2,$A), relative V O2max (Figure 2, B), and absolute V O2max (Figure 2, C) also appeared significantly different in age group, time groups, 46

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and their interaction (P<.001). The LTPA (Figure 2, D) was significantly different between the age groups (P<.001), time groups (P<.002), and their interaction (P<.006). DISCUSSION The aim of this study was to investigate the secular change in CRF and body composition during the last 35 years in women in the ACLS. In analyses that adjusted for age, smoking, alcohol consumption, and chronic conditions, the CRF level, as indicated by maximal METs or relative V_ O2max, among women in

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BF ¼ body fat; BMI ¼ body mass index; FFM ¼ fat-free mass; FM ¼ fat mass; LTPA ¼ leisure-time physical activity; MET ¼ metabolic equivalent (1 MET ¼ 3.5 mL$min1$kg1 oxygen uptake); V_ O2max ¼ maximum oxygen consumption. b Data are presented as mean  SE. c The model (1-way analysis of covariance) was adjusted for age, smoking, alcohol consumption, and chronic conditions. d P value refers to the omnibus test (F test). e Significantly different from 1970 through 1974. f Significantly different from 1975 through 1979. g Significantly different from 1980 through 1984. h Significantly different from 1985 through 1989. i Significantly different from 1990 through 1994. j Significantly different from 1995 through 1999. a

<.001 <.001 <.001 <.001 <.001 <.001 <.001 <.001 <.001 24.00.1e,f,g,h,i 25.40.1e,f,g,h,i,j 17.40.1e,f,g,h,j 48.30.1e,f,g,h,i,j 919.419.8e,f,g,h,i 10.00.04e,f,g,j 14.40.1e,f,g,j 35.80.1e,f,g,j 2.310.01e,f,g,h,i 23.40.1e,f,g,h 26.60.2e,f,g 17.40.2e,f,g,h 45.90.1e,f,g,h 1079.924.7e,f,g,h 9.90.04e,f,g 14.30.1e,f,g 35.50.1e,f,g 2.220.01e,f,g,h 22.00.2 23.30.4 14.20.4 45.40.3 53.052.7 8.40.1 10.80.2 30.60.3 1.810.02 BMI BF (%) FM (kg) FFM (kg) LTPA (MET min/wk) CRF (MET) Treadmill duration (min) Relative V_ O2max (mL$min1$kg1) Absolute V_ O2max (L$min1)

22.10.1 24.80.2e 15.20.2e 44.70.2e 200.630e 8.70.1e 11.60.1e 31.70.2e 1.880.01e

22.30.1 27.40.2e,f 16.90.2e,f 43.20.1e,f 248.522.4e 9.30.04e,f 12.80.1e,f 33.50.1e,f 1.990.01e,f

22.80.1e,f,g 26.50.1e,f,g 16.80.2e,f 44.80.1g 668.521.0e,f,g 10.00.04e,f,g 14.40.1e,f,g 35.70.1e,f,g 2.170.01e,f,g

23.90.1e,f,g,h,i 26.30.2e,f,g 17.80.2e,f,g,h 47.20.1e,f,g,h,i 924.422.2e,f,g,h,i 10.10.04e,f,g,h,i 14.70.1e,f,g,h,i 36.20.1e,f,g,h,i 2.310.01e,f,g,h,i

2000-2004 (n¼2814) 1995-1999 (n¼2193) 1990-1994 (n¼1766) 1985-1989 (n¼2428) 1980-1984 (n¼2191) 1975-1979 (n¼1293) 1970-1974 (n¼388) Parameter

Study period

TABLE 2. Secular Change in Cardiorespiratory Fitness and Body Composition of Women in the Aerobics Center Longitudinal Studya,b,c

the cohort increased rapidly from the 1970s to 1980s, with the trend leveling and revealing a small decrease in the years 2000 through 2004. The mean adjusted BMI and percentage of BF, however, revealed different change patterns, with the BMI increasing during the 35 years; however, during the first 15 years, the prominent change was the increase in FM, and in the last 20 years, the prominent change was the increase in FFM. Our findings on CRF of women in the ACLS reveal a secular change similar to that of men as reported by Willis et al11 and are consistent with data from cross-sectional studies for previously comparable periods.30,31 In the Coronary Artery Risk Development in Young Adults study, Sidney et al30 tested women aged 18 to 30 years with a graded exercise treadmill test protocol in 1985 through 1986 and found that the mean  SD CRF in white women was 11.10.1 METs. In our study during the comparable periods, the mean  SD adjusted CRF was 10.560.05 METs for women aged 20 to 44 years (data not shown). A CRF component was added to the National Health and Nutrition Examination Survey (NHANES) to provide an objective assessment of CRF level by using a submaximal treadmill test in 1999 through 2004. On the basis of data from NHANES 1999e2004, the estimated mean relative V_ O2max was 35.5 mL$kg1$min1 for women aged 20 to 49 years,31 which was again comparable to our result during the same period for the similar age group. Lifestyle changes leading to increased PA levels are possible explanations for the improved CRF. Data from the ACLS and NHANES revealed a positive association between levels of CRF and participation in LTPA.21,31 In the current study, we used selfreported leisure-time or recreational activities during the past 3 months to estimate the LTPA and calculate total MET minutes per week of LTPA. As indicated in Table 2 and Figure 2, D, the LTPA increased slowly from 1970 through 1984 and after that increased rapidly and reached a peak from 1985 through 1994. Then there was a slight decrease from 1994 through 2004 in both age groups. These changes were similar to previous studies.11,32 The amount of moderate- and vigorousintensity physical activity associated with markedly lower rates of disease or improvements in

P valued

CARDIORESPIRATORY FITNESS AND BODY COMPOSITION

47

48

BF ¼ body fat; BMI ¼ body mass index; FFM ¼ fat-free mass; FM ¼ fat mass; LTPA ¼ leisure-time physical activity; MET ¼ metabolic equivalent (1 MET ¼ 3.5 mL$min1$kg1 of oxygen uptake); V_ O2max ¼ maximum oxygen consumption. b Data are presented mean  SE. c The model (2-way analysis of covariance) was adjusted for age, smoking, alcohol consumption, and chronic conditions. a

Variable

BMI 22.80.1 22.80.1 23.60.2 23.40.1 24.60.2 23.80.1 24.40.2 23.90.1 13.09 (<.001) 47.55 (<.001) 3.20 (.02) BF (%) 26.70.2 26.70.2 26.80.4 26.80.2 23.70.3 26.30.2 26.10.3 25.40.1 5.32 (.02) 8.61 (<.001) 2.39 (.07) FM (kg) 17.00.2 17.00.2 17.80.4 17.50.2 19.00.4 17.60.2 18.20.3 17.30.2 10.81 (.001) 10.23 (<.001) 2.92 (.03) FFM (kg) 44.80.2 44.70.2 46.20.3 45.80.2 48.00.3 47.00.2 48.60.3 48.20.1 8.57 (.003) 127.83 (<.001) 2.05 (.10) LTPA (MET min/wk) 456.131.5 986.038.5 124361.1 1032.632.0 890.155.5 937.928.7 979.552.8 915.725.1 6.12 (.01) 34.37 (<.001) 34.09 (<.001) CRF (MET) 9.90.1 10.00.1 9.90.1 9.80.1 9.80.1 10.20.1 9.80.1 10.00.04 8.68 (.003) 0.95 (.42) 3.55 (.01) Relative V_ O2max (mL$min1$kg1) 35.60.2 35.60.2 35.50.3 35.30.2 35.10.3 36.20.1 35.10.3 35.80.1 8.73 (.003) 0.77 (.51) 3.32 (.02) Absolute V_ O2max (L$min1) 2.20.01 2.20.01 2.20.02 2.20.01 2.30.02 2.30.01 2.30.02 2.30.01 0.05 (.82) 49.68 (<.001) 1.22 (.30)

Group  time Time

F (P value)

Group Currently married (n¼2304)

2000-2004

Currently unmarried (n¼510) Currently married (n¼1729)

1994-1999

Currently unmarried (n¼464) Currently married (n¼1381)

1990-1994

Currently unmarried (n¼385) Currently married (n¼953)

1985-1989

Currently unmarried (n¼1475)

Study period

TABLE 3. Secular Change in Cardiorespiratory Fitness and Body Composition of Women in the Aerobics Center Longitudinal Study by Marriage Status and 5-Year Groupa,b,c

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biomarkers and CRF is in the range of 500 to 1000 MET min/wk.33 Individual health benefits gained from increased physical activity depend on the initial physical activity level because more sedentary individuals are expected to benefit most from increasing their physical activity to the recommended level. People who already meet the physical activity recommendations are also likely, however, to derive additional health and CRF benefits from becoming more physically active.34 Our study found that the CRF increased markedly when the LTPA increased from 53.0 to 668.5 MET min/wk during 1970 through 1989. However, there were few changes in the CRF when the LTPA met the 2008 Physical Activity Guidelines recommendation during 1985 through 2004. Extensive studies exist in relation to the determinants of physical activity, with most research focusing on individual factors, such as age, sex, educational level, smoking status, marital status, attitudes, and self-efficacy.35 Brownson et al36 found that smoking has a negative correlation with physical activity. Possible explanations are that smokers may not be aware of the health risks associated with smoking and physical inactivity, smoking may lead to a reduced desire for LTPA or reduce the ability to perform physical activity, and the behavioral characteristic that leads to smoking might overlap with decreasing physical activity. The smoking rate in the ACLS women decreased markedly during the 35 years, which may partially explain the change in the LTPA. Compared with women who remained unmarried, married women had positive changes in physical activity.37 In our study, we also found that the LTPA level among married women was markedly higher than the unmarried women. The percentage of currently married women increased markedly from the period of 1985 through 1989 to the period of 1990 through 1994, which may be another factor that influences the increase in LTPA. More recently, the study of potential determinants of physical activity has shifted toward a macrolevel environment. Cameron et al38 found strong and consistent positive correlations between country prevalence of LTPA and country gross domestic product per capita in adults. During the 35 years of the study, the gross domestic product per capita increased by 8

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25.0

Age (y) 20-44 45-64

24.0

Age (y) 20-44 45-64

30.0 28.0

BMI

BF (%)

23.0

22.0

26.0

-2 0 00

95

20

19

19

04

99 -1 9

94 -1 9 90

85 19

B

Study period

-1 9

84 -1 9 80 19

19

19

75

70

-1 9

-1 9

79

74

20 00 -2 00 4

99 -1 9 95 19

90 19

19

A

94 -1 9

-1 9 85

-1 9 19

80

-1 9 19

75

-1 9 70 19

89

20.0 84

20.0 79

22.0

74

21.0

89

24.0

Study period

20.0 Age (y) 20-44 45-64

19.0 18.0

50.00

48.00

17.0

47.00 FFM (kg)

FM (kg)

Age (y) 20-44 45-64

49.00

16.0 15.0

46.00 45.00 44.00

14.0

C

Study period

D

20 00 -2 00 4

9 95 19

90 19

-1

99 -1

-1 85 19

99

4

9 98

4 98 19

80

-1

97 -1 75 19

19

70

-1

97

4

20 00 -2 00 4

9 -1 95 19

19

19

90

85

-1

-1

99

99

4

9 98

4 98 19

80

-1

97 -1 75 19

19

70

-1

97

9

41.00 4

42.00

12.0

9

43.00

13.0

Study period

FIGURE 1. Adjusted means of body composition for women in the Aerobics Center Longitudinal Study by age group and 5-year group during 1970 through 2004. The model (2-way analysis of covariance) was adjusted for smoking, alcohol consumption, and chronic conditions, and the interaction effects (age  time group) were significant for percentage of body fat (BF) and fat-free mass (FFM), respectively. A, Body mass index (BMI); B, percentage of BF; C, fat mass (FM); and D, FFM. Error bars indicate 95% CI.

times. The rapid economic growth has increased the number of people with higher educational levels, and educational attainment is perhaps the strongest determinant of LTPA.39 In addition, the national educational and policy efforts aimed at promoting physical activity may also play an important role in increasing the LTPA.11,32 Therefore, it is likely that the combination of many of these factors influenced the increase in the LTPA level over time and contributed to the secular change of CRF. Our study found that the BMI was lower than in the NHANES data during the same period, but secular change and range are similar.16 Compared with NHANES I (19711974), the BMI in NHANES III (1988-1994) increased 5.98%, whereas in our study, the BMI increased 6.28% during the comparable Mayo Clin Proc. n January 2015;90(1):43-52 www.mayoclinicproceedings.org

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period. The BMI in NHANES 1999e2002 increased 12.35% compared with NHANES I (1971-1974), but during a comparable period in our study the BMI increased only 8.64%. There are 2 potential explanations for the difference in secular change between our study and NHANES data. Participants in NHANES were a nationally representative sample of US civilians, including different racial/ethnic groups and socioeconomic statuses, whereas in our study more than 95% of participants are nonHispanic whites from middle to upper socioeconomic strata. Researchers found that the BMI varied by racial/ethnic group and socioeconomic status for both men and women.40,41 In addition, in NHANES, physical activity levels of the participants from both objective and subjective measurements were lower than in the ACLS participants in the same periods.2,42

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39.0

11.0 10.5

CRF (METs)

10.0

Relative VO2 max (mL.kg–1.min–1)

Age (y) 20-44 45-64

9.5 9.0 8.5

˙

8.0

Age (y) 20-44 45-64

37.0 35.0 33.0 31.0 29.0

7.5

-2 0 00

95

20

19

19

04

99 -1 9

94 90

85 19

B

Study period

-1 9

-1 9

84 -1 9 80 19

19

19

75

70

-1 9

-1 9

79

74

20 00 -2 00 4

99 19

95

-1 9 90 19

19

-1 9

94

89 85

-1 9

84 80

-1 9

79 19

19

75

-1 9

74 -1 9 70 19

A

89

27.0

7.0

Study period

1400.0 2.5 Age (y) 20-44 45-64

2.2 2.1 2.0

1000.0 800.0 600.0 400.0

20 00 -2 00 4

9 99 19

95

-1

99 19

90

-1

-1 85 19

D

4

9 98

4 98 -1 80

19

75

-1

97

4 97 -1 70

-1 95 19

19

99

4 -1 90 19

-1 85 19

99

9 98

98 -1 80 19

-1 75 19

70

-1

97

97

9

Study period

19

0.0 20 00 -2 00 4

1.7 9

200.0

4

1.8

9

1.9

19

C

Age (y) 20-44 45-64

1200.0 LTPA (MET min/wk)

2.3

4

Absolute VO2 max (L.min–1)

2.4

Study period

FIGURE 2. Adjusted means of cardiorespiratory fitness (CRF) and leisure-time physical activity (LTPA) for women in the Aerobics Center Longitudinal Study by age group and 5-year groups during 1970 through 2004. The model (2-way analysis of covariance) was adjusted for smoking, alcohol consumption, and chronic conditions and the interaction effects (age  time group) were significant for _ 2max) metabolic equivalents (METs; 1 MET ¼ 3.5 mL$min1$kg1 of oxygen uptake) (A), relative maximum oxygen consumption (VO _ 2max (C), and LTPA (D), respectively. Error bars indicate 95% CI. (B), absolute VO

The BMI is a measure of general adiposity and does not measure body composition. Therefore, the percentage of BF has been recommended as a more accurate measurement of body fatness. Limited data exist on estimates of FFM, total FM, and percentage of BF for the US population.43-45 Compared with data from NHANES III (1988-1994), the level of BF in the current study was lower, but the age difference was consistent with previous reports.43,45 The secular change of percentage of BF was different from BMI. Percentage of BF and FM increased markedly from 1970 through 1984 in our study, and this change might be due to the eating patterns46,47 and low levels of LTPA. From 1985 through 2004, with the incremental 50

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level of LTPA, percentage of BF decreased and FFM increased markedly. Strong aspects of our study include a relatively large sample size of adult women and consistent objective measures of CRF during 35 years. In addition, we used densitometry and skinfold thickness to measure body composition, which were likely to be more accurate measures of body fatness than BMI or height-weight indexes.24 On the other hand, limitations include a sample of all white women in the middle and upper socioeconomic statuses, which may limit the external validity of the findings. The marriage and fertility status data were only available during 1985 through 2004, and definitions in the questionnaire on marital

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CARDIORESPIRATORY FITNESS AND BODY COMPOSITION

status and fertility were not strict. We estimated, rather than directly measured, residual lung volume during underwater weighing. Research has suggested48 that the prediction accuracy of body fatness measured by densitometry when residual lung volume was estimated was only slightly better than anthropometric assessments.48 The LTPA measurements were self-reported data that tend to have recall bias.2 In addition, we estimated CRF by time, speed, and incline on a treadmill stress test, which has been well validated, but we did not precisely measure gas exchange by cardiopulmonary stress testing. Finally, this is a cross-sectional study with only one assessment of the CRF and body composition. Therefore, we do not know the extent to which these characteristics might have changed during follow-up. CONCLUSION In summary, our study found that the CRF increased 19.35% during the 35 years in women in the ACLS, although there was a slight decrease during the 2000 through 2004 period. The percentage of BF increased from 1970 through 1984 and then decreased from 1985 through 2004. The increase in body weight was mainly due to the increase in FFM. This study expanded our previous knowledge about secular change of CRF and body composition in men to women. In addition, our data indicate that when LTPA reaches 668.5 MET min/wk, the CRF stays at a higher level. To improve the CRF for physically inactive women, we should encourage them to meet this LTPA level. The drift downward in the CRF among women indicates the need for continuing efforts to promote their physical activity and fitness. Despite these encouraging findings, the increasing prevalence of chronic diseases in the current study is disturbing, and future studies are warranted to examine how changes in chronic diseases affect the secular change in fitness and fatness. ACKNOWLEDGMENT We thank the Cooper Clinic physicians and technicians for collecting the baseline data and the staff at the Cooper Institute for data entry and data management. We appreciate Mayo Clin Proc. n January 2015;90(1):43-52 www.mayoclinicproceedings.org

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Gaye Christmus for her editorial contribution to the manuscript.

SUPPLEMENTAL ONLINE MATERIAL Supplemental material can be found online at http://www.mayoclinicproceedings.org. Grant Support: This work was supported by grants AG06945, HL62508, and R21DK088195 from the National Institutes of Health. Dr Li is supported in part by the China Scholarship Council under grant 201206520018. Role of Sponsors: The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health or the China Scholarship Council. The funding organizations played no role in the design and conduct of the study; the collection, management, analysis, and interpretation of data; or the preparation, review, or approval of the manuscript. Correspondence: Address to Xuemei Sui, MD, MPH, PhD, Department of Exercise Science, Division of Health Aspects of Physical Activity, Arnold School of Public Health, University of South Carolina 921 Assembly St, Room 226, Columbia, SC 29208 ([email protected]).

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