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Making an impact on pediatric bone health
PEDIATRICS THE JOURNAL OF
February 2000
Volume 136
Number 2
EDITORIALS
M
Making an impact on pediatric bone health Bone mineral acquired during childhood and adolescence is a key determinant of adult skeletal health.1 Peak bone mass is reached by early adulthood and serves as the “bone bank” for the remainder of life. A replete bone “account” diminishes the chances that osteoporosis will develop. Peak bone mass is strongly influenced by genetic factors, but full genetic potential is attained only if nutrition, activity, endocrine function, and other lifestyle factors are optimized. Osteoporosis prevention can begin in childhood by encouraging habits that foster bone health. To date, research efforts have focused primarily on the relationship between calcium intake and bone acquisition. Bone mineral increases directly with calcium intake until a threshold is reached.2 Age-adjusted recommendations for daily calcium intake have been revised to optimize bone health. For children aged 9 to 18 years, the goal has been set at 1300 mg a day, equivalent to about 4 glasses of milk.2 The importance of calcium has been reinforced in several ways. The “Got milk?” advertisement campaign makes a highly visible and appealing pitch for more dairy
J Pediatr 2000;136:137-9. Copyright © 2000 by Mosby, Inc. 0022-3476/2000/$12.00 + 0 9/18/104024
consumption. Calcium-fortified juices, cereals, and other foods are marketed as alternatives to dairy products. Even calcium supplements have been made more palatable, with calcium-laced chocolates as the latest option! BMD
Bone mineral density
However, the road to bone health requires more than adequate calcium intake. Several studies indicate that increased consumption of calcium boosts bone mineral only when combined with adequate weight-bearing physical activity.3 However, there is no consensus on the optimal amount of activity needed to foster early gains in bone mineral. Bone acquisition and remodeling are modulated by mechanical forces applied to the skeleton; thus, the pull of muscle on bone or strains in bone elicited by moderate or high impacts (such as jumping) may stimulate bone metabolism.4 It follows that changes in bone mineral vary across a spectrum of activity. Bone mineral is lost with immobilization (bed rest) or weightlessness (space travel) because there is no strain on bone. At the other end of the spectrum, bone mineral and even bone size increase in elite athletes involved in very high impact activities. Gymnasts have greater bone mass (corrected for bone size) than distance
runners, presumably because repeated jumps and dismounts create greater impact on bone than running.5 Conversely, elite college swimmers have no greater bone mineral than nonathletic control subjects, perhaps because they exercise in a weightless state.6 The effect of moderate exercise is less certain, but several studies suggest that active children and adults may gain 5% to 10% more bone mineral than their sedentary peers.4,7
See related articles, p. 149 and p. 156. The study by McKay et al8 in this issue of The Journal examines the effect of a randomized, controlled activity intervention on bone mineral acquisition in grade school youth. The goal was to determine whether incorporating more weight-bearing activity within the school curriculum could stimulate greater gains in bone mineral. Third- and fourth-grade students were randomly assigned by classroom to their usual physical education programs (control group) or to an intervention program consisting of 10 or more tuck jumps 3 times week and increased jumping activity during twice weekly physical education classes. Bone mineral density was measured by using dual-energy x-ray absorptiome137
EDITORIALS
try, a precise noninvasive technique. All children had increased BMD at the end of the 8-month study, but the activity group gained 1.2% more bone mineral than the control group at a region of the hip that is prone to osteoporotic fracture; this difference remained significant after controlling for other variables that might have affected outcome. The difference was small and limited to this one region of the skeleton, but the finding is encouraging for several reasons. The time devoted to the intervention was modest, the program was administered in a regular school setting by classroom teachers, and gains were seen after only 8 months of intervention. Gains in bone density of this magnitude (similar to those achieved with 1 year of calcium supplementation) could be clinically important if sustained because the risk of osteoporotic fracture decreases an estimated 40% for each 5% increase in peak bone mass.1 This intervention study,8 two others in children,9,10 and one in young adults11 suggest that even brief periods of specific weight-bearing activity can stimulate gains in bone mineral. Peripubertal children who exercised for 30 minutes 2 to 3 times per week gained 2.5% to 10% more BMD at the hip, spine, or whole body than did control subjects.8-10 Young women performing 50 vertical jumps a day 6 days a week for 5 months gained 2% to 3% in BMD at the proximal femur, whereas control subjects were unchanged.11 One of these studies showed that increased activity also influenced bone geometry, which could affect bone strength, independent of BMD.10 This research leaves several unanswered questions about the effects of activity. Because these studies have been short term (5-10 months), it is not known whether gains in bone mineral in response to activity will continue or will reach a plateau over time. Will the gains in BMD be sustained after the intervention is stopped? The increase in bone mineral seen in calcium sup138
THE JOURNAL OF PEDIATRICS FEBRUARY 2000 plementation trials is no longer apparent 2 years after stopping the added calcium.12 Will gains in bone mineral with activity also prove to be a “use it or lose it” phenomenon? Finally, the intensity, duration, and type of activity required to strengthen bone mass for each age group at multiple skeletal sites needs to be better defined. The discrepant findings from the studies to date indicate that the exercise “prescription” may be different for the spine versus hip and for prepubertal versus pubertal individuals. Concerns have been raised about the risks of intensive activity in childhood and adolescence. Low bone mineral develops in some elite female athletes as part of the “athletic triad” of marginal nutrition, amenorrhea, and osteoporosis.13 Whether intensive activity compromises bone mass if the other risk factors are not present is debated. Theintz et al14 proposed that gymnastics could stunt linear growth based on the observations that adolescent gymnasts had shortened leg length and reduced height. In this issue of The Journal, Bass et al15 present new data to challenge these conclusions. In their study, leg length was reduced in girls at the time they began gymnastics and did not worsen during participation in the sport, suggesting that this finding reflected a selection bias. Upper body growth was slower in active gymnasts than in control subjects, but there was evidence of catch-up in spinal growth after retirement from the sport. It is important to determine whether activity can become “too much of a good thing,” but in reality, elite young athletes make up a tiny fraction of the population. From a public health perspective, the perils for the vast majority of American youth who do too little far outweigh the risks of the few who may do too much. Hours of television viewing continue to increase while time spent in physical activity declines, trends that bode poorly for future bone health.16 The annual cost of caring for osteo-
porosis has already reached $13.8 billion, exceeding expenditures for asthma and lung disease combined. The incidence of osteoporosis is projected to triple by the year 2040, reflecting increased longevity and unhealthy lifestyles. To address this problem, we need to work harder at fostering childhood gains in bone mineral. Additional controlled trials are needed to define the optimal activity “prescription(s)” for skeletal health. Most youth, like their parents, would rather take something than do something, but there is no “exercise in a bottle.” For this reason, I believe activity intervention should be packaged and delivered as part of a mandated school curriculum to reach the largest number of children. As health care providers, we also need to consider the activity needs of our patients whose mobility is limited by cerebral palsy, rheumatoid arthritis, or other chronic diseases. For these children, even very modest skeletal loading, such as standing, may be of benefit.17 The time has come to think outside the box (milk carton) and to recognize the importance of exercise as an essential component of osteoporosis prevention. Let’s hop to it! Laura K. Bachrach, MD Division of Pediatric Endocrinology Stanford University School of Medicine Stanford, CA 94305-5208
REFERENCES 1. Hui SL, Slemenda CW, Johnston CC. The contribution of bone loss to post menopausal osteoporosis. Osteoporos Int 1990;1:30-4. 2. National Institutes of Health. Optimal calcium intake. NIH Consensus Statement. 1994;12:1-31. 3. Specker BJ. Evidence of an interaction between calcium intake and physical activity on changes in bone mineral density. J Bone Miner Res 1996;11:1539-44. 4. Bailey DA, Faulkner RA, McKay HA. Growth, physical activity, and bone mineral acquisition. Exerc Sport Sci Rev 1996;24:233-66. 5. Robinson TL, Snow-Harter C, Taaffe DR, Gillis D, Show J, Marcus R.
EDITORIALS
THE JOURNAL OF PEDIATRICS VOLUME 136, NUMBER 2
6.
7.
8.
9.
Gymnasts exhibit higher bone mass than runners despite similar prevalence of amenorrhea and oligomenorrhea. J Bone Miner Res 1995;10:26-35. Taaffe DR, Snow-Harter C, Connolly DA, Robinson TL, Brown MD, Marcus R. Differential effects of swimming versus weight-bearing activity on bone mineral status of eumenorrheic athletes. J Bone Miner Res 1995;10:586-93. Bailey DA, McKay HA, Mirwald RL, Crocker PRE, Faulkner RA. The University of Saskatchewan Bone Mineral Accrual Study: a six-year longitudinal study of the relationship of physical activity to bone mineral accrual in growing children. J Bone Miner Res 1999; 14:1672-9. McKay HA, Petit MA, Schutz RW, Prior JC, Barr SI, Khan KM. Augmented trochanteric bone mineral density after modified physical education classes: a randomized school-based exercise intervention study in prepubescent and early pubescent children. J Pediatr 2000;136:156-62. Morris FL, Naughton GA, Gibbs JL,
10.
11.
12.
13.
Carlson JS, Wark JD. Prospective 10month exercise intervention in premenarcheal girls: positive effects on bone and lean mass. J Bone Miner Res 1997;12:1453-62. Bradney M, Pearce G, Naughton G, Sullivan C, Bass S, Beck T, et al. Moderate exercise during growth in prepubertal boys: changes in bone mass, size, volumetric density, and bone strength: a controlled prospective study. J Bone Miner Res 1998;13: 1814-21. Bassey EJ, Rothwell MC, Littlewood JJ, Pye DW. Pre- and postmenopausal women have different bone mineral density responses to the same high-impact exercise. J Bone Miner Res 1998;13:1805-13. Slemenda CW, Peacock M, Hui S, Zhou L, Johnston CC. Reduced rates of skeletal remodeling are associated with increased bone mineral density during the development of peak skeletal mass. J Bone Miner Res 1997;12: 676-82. Putukian M. The female triad: eating
14.
15.
16.
17.
disorders, amenorrhea and osteoporosis. Med Clin North Am 1994;778:34556. Theintz GE, Howald H, Weiss U, Sizonenko PC. Evidence for a reduction in growth potential in adolescent female gymnasts. J Pediatr 1993;122: 306-13. Bass S, Bradney M, Pearce G, Hendrich E, Inge K, Stuckey S, et al. Short stature and delayed puberty in gymnasts: influence of selection bias on leg length and the duration of training on trunk length. J Pediatr 2000;136:149-55. Gordon-Larsen P, McMurray RG, Popkin BM. Adolescent physical activity and inactivity vary by ethnicity: the National Longitudinal Study of Adolescent Health. J Pediatr 1999;135: 301-6. Chad KE, Bailey DA, McKay HA, Zello GA, Snyder RE. The effect of a weight-bearing physical activity program on bone mineral content and estimated volumetric density in children with spastic cerebral palsy. J Pediatr 1999;135:115-7.
139
February 2000
Volume 136
Number 2
EDITORIALS
M
Making an impact on pediatric bone health Bone mineral acquired during childhood and adolescence is a key determinant of adult skeletal health.1 Peak bone mass is reached by early adulthood and serves as the “bone bank” for the remainder of life. A replete bone “account” diminishes the chances that osteoporosis will develop. Peak bone mass is strongly influenced by genetic factors, but full genetic potential is attained only if nutrition, activity, endocrine function, and other lifestyle factors are optimized. Osteoporosis prevention can begin in childhood by encouraging habits that foster bone health. To date, research efforts have focused primarily on the relationship between calcium intake and bone acquisition. Bone mineral increases directly with calcium intake until a threshold is reached.2 Age-adjusted recommendations for daily calcium intake have been revised to optimize bone health. For children aged 9 to 18 years, the goal has been set at 1300 mg a day, equivalent to about 4 glasses of milk.2 The importance of calcium has been reinforced in several ways. The “Got milk?” advertisement campaign makes a highly visible and appealing pitch for more dairy
J Pediatr 2000;136:137-9. Copyright © 2000 by Mosby, Inc. 0022-3476/2000/$12.00 + 0 9/18/104024
consumption. Calcium-fortified juices, cereals, and other foods are marketed as alternatives to dairy products. Even calcium supplements have been made more palatable, with calcium-laced chocolates as the latest option! BMD
Bone mineral density
However, the road to bone health requires more than adequate calcium intake. Several studies indicate that increased consumption of calcium boosts bone mineral only when combined with adequate weight-bearing physical activity.3 However, there is no consensus on the optimal amount of activity needed to foster early gains in bone mineral. Bone acquisition and remodeling are modulated by mechanical forces applied to the skeleton; thus, the pull of muscle on bone or strains in bone elicited by moderate or high impacts (such as jumping) may stimulate bone metabolism.4 It follows that changes in bone mineral vary across a spectrum of activity. Bone mineral is lost with immobilization (bed rest) or weightlessness (space travel) because there is no strain on bone. At the other end of the spectrum, bone mineral and even bone size increase in elite athletes involved in very high impact activities. Gymnasts have greater bone mass (corrected for bone size) than distance
runners, presumably because repeated jumps and dismounts create greater impact on bone than running.5 Conversely, elite college swimmers have no greater bone mineral than nonathletic control subjects, perhaps because they exercise in a weightless state.6 The effect of moderate exercise is less certain, but several studies suggest that active children and adults may gain 5% to 10% more bone mineral than their sedentary peers.4,7
See related articles, p. 149 and p. 156. The study by McKay et al8 in this issue of The Journal examines the effect of a randomized, controlled activity intervention on bone mineral acquisition in grade school youth. The goal was to determine whether incorporating more weight-bearing activity within the school curriculum could stimulate greater gains in bone mineral. Third- and fourth-grade students were randomly assigned by classroom to their usual physical education programs (control group) or to an intervention program consisting of 10 or more tuck jumps 3 times week and increased jumping activity during twice weekly physical education classes. Bone mineral density was measured by using dual-energy x-ray absorptiome137
EDITORIALS
try, a precise noninvasive technique. All children had increased BMD at the end of the 8-month study, but the activity group gained 1.2% more bone mineral than the control group at a region of the hip that is prone to osteoporotic fracture; this difference remained significant after controlling for other variables that might have affected outcome. The difference was small and limited to this one region of the skeleton, but the finding is encouraging for several reasons. The time devoted to the intervention was modest, the program was administered in a regular school setting by classroom teachers, and gains were seen after only 8 months of intervention. Gains in bone density of this magnitude (similar to those achieved with 1 year of calcium supplementation) could be clinically important if sustained because the risk of osteoporotic fracture decreases an estimated 40% for each 5% increase in peak bone mass.1 This intervention study,8 two others in children,9,10 and one in young adults11 suggest that even brief periods of specific weight-bearing activity can stimulate gains in bone mineral. Peripubertal children who exercised for 30 minutes 2 to 3 times per week gained 2.5% to 10% more BMD at the hip, spine, or whole body than did control subjects.8-10 Young women performing 50 vertical jumps a day 6 days a week for 5 months gained 2% to 3% in BMD at the proximal femur, whereas control subjects were unchanged.11 One of these studies showed that increased activity also influenced bone geometry, which could affect bone strength, independent of BMD.10 This research leaves several unanswered questions about the effects of activity. Because these studies have been short term (5-10 months), it is not known whether gains in bone mineral in response to activity will continue or will reach a plateau over time. Will the gains in BMD be sustained after the intervention is stopped? The increase in bone mineral seen in calcium sup138
THE JOURNAL OF PEDIATRICS FEBRUARY 2000 plementation trials is no longer apparent 2 years after stopping the added calcium.12 Will gains in bone mineral with activity also prove to be a “use it or lose it” phenomenon? Finally, the intensity, duration, and type of activity required to strengthen bone mass for each age group at multiple skeletal sites needs to be better defined. The discrepant findings from the studies to date indicate that the exercise “prescription” may be different for the spine versus hip and for prepubertal versus pubertal individuals. Concerns have been raised about the risks of intensive activity in childhood and adolescence. Low bone mineral develops in some elite female athletes as part of the “athletic triad” of marginal nutrition, amenorrhea, and osteoporosis.13 Whether intensive activity compromises bone mass if the other risk factors are not present is debated. Theintz et al14 proposed that gymnastics could stunt linear growth based on the observations that adolescent gymnasts had shortened leg length and reduced height. In this issue of The Journal, Bass et al15 present new data to challenge these conclusions. In their study, leg length was reduced in girls at the time they began gymnastics and did not worsen during participation in the sport, suggesting that this finding reflected a selection bias. Upper body growth was slower in active gymnasts than in control subjects, but there was evidence of catch-up in spinal growth after retirement from the sport. It is important to determine whether activity can become “too much of a good thing,” but in reality, elite young athletes make up a tiny fraction of the population. From a public health perspective, the perils for the vast majority of American youth who do too little far outweigh the risks of the few who may do too much. Hours of television viewing continue to increase while time spent in physical activity declines, trends that bode poorly for future bone health.16 The annual cost of caring for osteo-
porosis has already reached $13.8 billion, exceeding expenditures for asthma and lung disease combined. The incidence of osteoporosis is projected to triple by the year 2040, reflecting increased longevity and unhealthy lifestyles. To address this problem, we need to work harder at fostering childhood gains in bone mineral. Additional controlled trials are needed to define the optimal activity “prescription(s)” for skeletal health. Most youth, like their parents, would rather take something than do something, but there is no “exercise in a bottle.” For this reason, I believe activity intervention should be packaged and delivered as part of a mandated school curriculum to reach the largest number of children. As health care providers, we also need to consider the activity needs of our patients whose mobility is limited by cerebral palsy, rheumatoid arthritis, or other chronic diseases. For these children, even very modest skeletal loading, such as standing, may be of benefit.17 The time has come to think outside the box (milk carton) and to recognize the importance of exercise as an essential component of osteoporosis prevention. Let’s hop to it! Laura K. Bachrach, MD Division of Pediatric Endocrinology Stanford University School of Medicine Stanford, CA 94305-5208
REFERENCES 1. Hui SL, Slemenda CW, Johnston CC. The contribution of bone loss to post menopausal osteoporosis. Osteoporos Int 1990;1:30-4. 2. National Institutes of Health. Optimal calcium intake. NIH Consensus Statement. 1994;12:1-31. 3. Specker BJ. Evidence of an interaction between calcium intake and physical activity on changes in bone mineral density. J Bone Miner Res 1996;11:1539-44. 4. Bailey DA, Faulkner RA, McKay HA. Growth, physical activity, and bone mineral acquisition. Exerc Sport Sci Rev 1996;24:233-66. 5. Robinson TL, Snow-Harter C, Taaffe DR, Gillis D, Show J, Marcus R.
EDITORIALS
THE JOURNAL OF PEDIATRICS VOLUME 136, NUMBER 2
6.
7.
8.
9.
Gymnasts exhibit higher bone mass than runners despite similar prevalence of amenorrhea and oligomenorrhea. J Bone Miner Res 1995;10:26-35. Taaffe DR, Snow-Harter C, Connolly DA, Robinson TL, Brown MD, Marcus R. Differential effects of swimming versus weight-bearing activity on bone mineral status of eumenorrheic athletes. J Bone Miner Res 1995;10:586-93. Bailey DA, McKay HA, Mirwald RL, Crocker PRE, Faulkner RA. The University of Saskatchewan Bone Mineral Accrual Study: a six-year longitudinal study of the relationship of physical activity to bone mineral accrual in growing children. J Bone Miner Res 1999; 14:1672-9. McKay HA, Petit MA, Schutz RW, Prior JC, Barr SI, Khan KM. Augmented trochanteric bone mineral density after modified physical education classes: a randomized school-based exercise intervention study in prepubescent and early pubescent children. J Pediatr 2000;136:156-62. Morris FL, Naughton GA, Gibbs JL,
10.
11.
12.
13.
Carlson JS, Wark JD. Prospective 10month exercise intervention in premenarcheal girls: positive effects on bone and lean mass. J Bone Miner Res 1997;12:1453-62. Bradney M, Pearce G, Naughton G, Sullivan C, Bass S, Beck T, et al. Moderate exercise during growth in prepubertal boys: changes in bone mass, size, volumetric density, and bone strength: a controlled prospective study. J Bone Miner Res 1998;13: 1814-21. Bassey EJ, Rothwell MC, Littlewood JJ, Pye DW. Pre- and postmenopausal women have different bone mineral density responses to the same high-impact exercise. J Bone Miner Res 1998;13:1805-13. Slemenda CW, Peacock M, Hui S, Zhou L, Johnston CC. Reduced rates of skeletal remodeling are associated with increased bone mineral density during the development of peak skeletal mass. J Bone Miner Res 1997;12: 676-82. Putukian M. The female triad: eating
14.
15.
16.
17.
disorders, amenorrhea and osteoporosis. Med Clin North Am 1994;778:34556. Theintz GE, Howald H, Weiss U, Sizonenko PC. Evidence for a reduction in growth potential in adolescent female gymnasts. J Pediatr 1993;122: 306-13. Bass S, Bradney M, Pearce G, Hendrich E, Inge K, Stuckey S, et al. Short stature and delayed puberty in gymnasts: influence of selection bias on leg length and the duration of training on trunk length. J Pediatr 2000;136:149-55. Gordon-Larsen P, McMurray RG, Popkin BM. Adolescent physical activity and inactivity vary by ethnicity: the National Longitudinal Study of Adolescent Health. J Pediatr 1999;135: 301-6. Chad KE, Bailey DA, McKay HA, Zello GA, Snyder RE. The effect of a weight-bearing physical activity program on bone mineral content and estimated volumetric density in children with spastic cerebral palsy. J Pediatr 1999;135:115-7.
139