Moderate Physical Activity and Bone
Density among Perimenopausal Women Jun Zhang MB, Paul J. Feldblwn MSPH, and Judith A. Fotnwey, PhD
Inftdudion It has been suggested that exercise increases bone density' and prevents or decelerates age- and menopause-related bone loss.2 Studies comparing highly trained athletes and sedentary individuals concluded that exercise increases bone density.3-6 However, conflicting findings were reported from both cross-sectional and prospective studies with nonathlete subjects.7 8 Block et al. suggested that inaccurate or subjective measurement of physical activity by questionnaires may, in part, account for the conflicting findings.9 Small study sizes, resulting in low statistical power, may be another important factor. The present study used personal activity computers to measure physical activity objectively. These devices are considered by some researchers to provide superior direct measurements of physical activity.10,11 In addition to its large size, our study also adjusted for other determinants of bone density.
Matrals and Methods A group of 352 White women 40 to 54 years of age were recruited in central North Carolina by means of newspaper advertisement and flyers in workplaces, stores, and doctors' offices. Volunteers were required to have at least one ovary and no physical impairment inhibiting daily activities. Women who were pregnant, had smoked within the past 5 years, or were using estrogen replacement therapy were ineligible. Also excluded were women with a history of acromegaly, parathyroid disease, insulin-dependent diabetes mellitus, inflammatory bowel disease, liver disease (except hepatitis), or renal diseases (except pyelonephritis). Participants completed a self-administered questionnaire that covered health history, reproductive and contraceptive histories, past and current exercise, and past and current diet. A simple life calendar was provided to aid recall. Menopausal status was defined on the basis of the combination of answers to items on
menstrual changes, recent menses, and last menses. A woman was designated postmenopausal if she (1) reported menstrual changes, (2) reported no menstrual period in the previous 6 months, and (3) descnbed herself as being in menopause. Others were designated premenopausal. A series of questions concerning the respondents' exercise in high school were asked. Adolescent exercise was coded as follows: high level-played sports on a school team; moderate level-exercised regularly; and low level-did not exercise
regularly.
A current food intake frequency chart was included in the questionnaire, from which dietary calcium was calculated.12 We also queried the women about nondietary calcium in the form of antacids and dietary supplements. Based on the criterion of the recommended dietary allowance, subjects were dichotomized as above or below recommended total daily calcium intake.
Current physical activity was meaby a personal activity computer (Caltrac, Hemokinetics Inc, Madison, sured
Wis), which is a portable accelerometer estimating daily energy expenditure in kilocalories.11 The subjects were instructed to wear the deviceJor 3 continuous days (all day long), inUluding at least 1 weekend day and 1 weekday. Daily energy expenditure on physical activity was derived from total daily energy expenditure minus basal metabolic rate (BMR): BMR = [(331 x weight) + (351 x height) - (352 x age) + 49 8541/100 000 (kcall min), where weight is measured in pounds, height in inches, and age in years. The authors are with the Division of Reproductive Epidemiology and Sexually Transmitted Diseases, Family Health International, Research Triangle Park Branch, Durham, NC. Requests for reprints should be sent to Paul J. Feldblum, MSPH, Division of Reproductive Epidemiology and Sexually Transmitted Diseases, Family Health International, PO Box 13950, Research Triangle Park Branch, Durham, NC 27709. This paper was submitted to the Journal May 17, 1991, and accepted with revisions November 12, 1991.
May 1992, Vol. 82, No. 5
Public Health Briefs
Bone mineral density (BMD) was measured by dual photon absorptiometiy at the second to fourth lumbar vertebrae, and single photon absorptiometry measured BMD at the midradius and the distal radius (5-mm site).
Resufts The spine BMD slightly decreased with age, and postmenopausalwomen had substantially lower lumbar BMD than did premenopausal women (Table 1). Women who had ever breast-fed had significantly higher lumbar BMD than did nonbreastfeeding women. There was a modest dose-response relationship between exercise in high school and BMD at all sites that did not reach statistical significance. Women with more physical activity as measured by the personal activity computer had significantly higher bone density at the spine and distal radius. This assoMay 1992, Vol. 82, No.
ciation also prevailed at the midradius but was not statistically significant. Multiple linear regression, controlling for age, body mass, breast-feeding, oral contraceptive use, exercise in high school, and menopausal status, indicated that current physical activity is directly and significantly associated with bone density at the spine, distal, and midradius sites (Table 2). Based on these coefficients, each additional 100 kcallday energy expenditure in physical activity adds an estimated 12 mg/cm2, 3.8 mg/cm2, and 3.5 mg/cm2 higher bone density at the spine, distal, and midradius sites, respec-
tively.
Discussion It has been established that highly trained athletes have significantly higher BMD than do sedentary persons.3-6 Although research on this highly selected
group suggests that intensive exercise increases BMD, the finding has lesser practical and public health value for perimenopausal women, for whom the issue of moderate physical activity may be of greater interest. The present study shows that perimenopausal women with more physical activity have significantly higher BMD when other determinants of BMD are taken into account. Some limitations of our study should be noted. First, this was a cross-sectional study in which limited information on past exercise, body weight, and diet was collected. Second, misclassification may exist for some variables such as exercise in high school. Behavior change after adolescence may further lessen the value of this indicator. Third, although we improved the measurement of physical activity by using personal activity computers, it can be argued that 3 days of physical activity are not truly representative of the A American Joumal of Pubfic Health 737
Public Heath Brief
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women's life-styles. Some women might have increased their exercise in the 3 measured days because of the study. Furthermore, since the device could onlymeasure acceleration and deceleration in a vertical direction, some types of exercise, such as bicycling, may accumulate too few kilocalories. If this were true, however, the bias would be in the direction of the null because of increased variance. Thus, our significant findings suggest either that there is no strong bias in the physical activity results in our study or that the relationship between activity and BMD was underestimated. Nonetheless, the advantages of the present study are noteworthy. The study size was substantially larger than most previous studies of exercise and BMD. Use of personal activity computers provided both objectivity and precision. In addition, the present study measured and adjusted for factors that were often overlooked in previous studies, such as breast-
738 American Journal of Public Health
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feeding, oral contraceptive use, and calcium intake. These advantages, alongwith the significant findings of this study and their biological plausilbility and consistencywith previous studies, lead us to conclude that moderate physical activity is associated with higher bone density among perimenopausal women. O
Acknowledgments Partial support for this work was provided by Family Health Intemational (FWI) with funds from the US Agency for Intemational Development (USAID) and the National Institute for Child Health and Human Development (NICHD), although theviews expressed in this article do not necessarily reflect those of USAID or NICHD. FHI is a not-for-profit organization that conducts research and provides technical assistance in reproductive health, family planning, and sexually transmitted diseases.
References 1. Dalsky GP, Stocke KS, Ehsani AA, Slatopolsky E, Lee WC, Birge SJ. Weight-
bearing exercise training and lumbar bone mineral content in postmenopausal women. Ann Intern Med. 1988;108:824828. 2. Chow R, Harrison JE, Notarius C. Effect of two randomised exercise programmes on bone mass of healthy postmenopausal women. Br Med J. 1987;295:1441-1444. 3. Jacobson PC, Beaver W, Grubb SA, et al. Bone density in women: college athletes and older athletic women. J Orthop Res. 1984;2:328-332. 4. Wolman RL, Clark P, McNally E, Harries M, Reeve J. Menstrual state and exercise as determinants of spinal trabecular bone density in female athletes. Br Med J. 1990;301:516-518. 5. Lane NE, Bloch D, Jones HH, et al. Longdistance running, bone density, and osteoarthritis. JAMA 1986;255:1147-1151. 6. BlockJE, Genant HK, Black DM. Greater vertebral bone mineral mass in exercising young men. West JMed. 1986;145:39-42. 7. Simkin A, Ayalon J, Leichter I. Increased trabecular bone density due to bone-loading exercises in postmenopausal osteoporotic women. Cakf TLssue Int. 1986;39:8. 8. Aloia JF, Cohn SH, Ostuni JA, et al. Prevention of involutional bone loss by exercise. Ann Intern Med. 1978;89:356-358. 9. Block JE, Smith R, Black D, Genant UK Does exercise prevent osteoporosis? JAMA. 1987;257:3115-3117. 10. Washburn RA, Montoye HJ. The assessment of physical activity by questionnaire. Am JEpidendoL 1986;123:563-576. 11. Montoye HJ, Washburn R, Senrais S, Ertl A, Webster JG, Nagle FJ. Estimation of energy expenditure by a portable accelerometer. Med Sci Sports Exerc. 1983;15: 40347. 12. US Department of Agriculture, Agricultural Research Service. Composition of Foods, Daiy and EggPmducts:Raw, Processed, Prpatre Washington, DC: US Government Printing Office; 1976. Agriculture Handbook No. 8-1.
May 1992, Vol. 82, No. 5
Density among Perimenopausal Women Jun Zhang MB, Paul J. Feldblwn MSPH, and Judith A. Fotnwey, PhD
Inftdudion It has been suggested that exercise increases bone density' and prevents or decelerates age- and menopause-related bone loss.2 Studies comparing highly trained athletes and sedentary individuals concluded that exercise increases bone density.3-6 However, conflicting findings were reported from both cross-sectional and prospective studies with nonathlete subjects.7 8 Block et al. suggested that inaccurate or subjective measurement of physical activity by questionnaires may, in part, account for the conflicting findings.9 Small study sizes, resulting in low statistical power, may be another important factor. The present study used personal activity computers to measure physical activity objectively. These devices are considered by some researchers to provide superior direct measurements of physical activity.10,11 In addition to its large size, our study also adjusted for other determinants of bone density.
Matrals and Methods A group of 352 White women 40 to 54 years of age were recruited in central North Carolina by means of newspaper advertisement and flyers in workplaces, stores, and doctors' offices. Volunteers were required to have at least one ovary and no physical impairment inhibiting daily activities. Women who were pregnant, had smoked within the past 5 years, or were using estrogen replacement therapy were ineligible. Also excluded were women with a history of acromegaly, parathyroid disease, insulin-dependent diabetes mellitus, inflammatory bowel disease, liver disease (except hepatitis), or renal diseases (except pyelonephritis). Participants completed a self-administered questionnaire that covered health history, reproductive and contraceptive histories, past and current exercise, and past and current diet. A simple life calendar was provided to aid recall. Menopausal status was defined on the basis of the combination of answers to items on
menstrual changes, recent menses, and last menses. A woman was designated postmenopausal if she (1) reported menstrual changes, (2) reported no menstrual period in the previous 6 months, and (3) descnbed herself as being in menopause. Others were designated premenopausal. A series of questions concerning the respondents' exercise in high school were asked. Adolescent exercise was coded as follows: high level-played sports on a school team; moderate level-exercised regularly; and low level-did not exercise
regularly.
A current food intake frequency chart was included in the questionnaire, from which dietary calcium was calculated.12 We also queried the women about nondietary calcium in the form of antacids and dietary supplements. Based on the criterion of the recommended dietary allowance, subjects were dichotomized as above or below recommended total daily calcium intake.
Current physical activity was meaby a personal activity computer (Caltrac, Hemokinetics Inc, Madison, sured
Wis), which is a portable accelerometer estimating daily energy expenditure in kilocalories.11 The subjects were instructed to wear the deviceJor 3 continuous days (all day long), inUluding at least 1 weekend day and 1 weekday. Daily energy expenditure on physical activity was derived from total daily energy expenditure minus basal metabolic rate (BMR): BMR = [(331 x weight) + (351 x height) - (352 x age) + 49 8541/100 000 (kcall min), where weight is measured in pounds, height in inches, and age in years. The authors are with the Division of Reproductive Epidemiology and Sexually Transmitted Diseases, Family Health International, Research Triangle Park Branch, Durham, NC. Requests for reprints should be sent to Paul J. Feldblum, MSPH, Division of Reproductive Epidemiology and Sexually Transmitted Diseases, Family Health International, PO Box 13950, Research Triangle Park Branch, Durham, NC 27709. This paper was submitted to the Journal May 17, 1991, and accepted with revisions November 12, 1991.
May 1992, Vol. 82, No. 5
Public Health Briefs
Bone mineral density (BMD) was measured by dual photon absorptiometiy at the second to fourth lumbar vertebrae, and single photon absorptiometry measured BMD at the midradius and the distal radius (5-mm site).
Resufts The spine BMD slightly decreased with age, and postmenopausalwomen had substantially lower lumbar BMD than did premenopausal women (Table 1). Women who had ever breast-fed had significantly higher lumbar BMD than did nonbreastfeeding women. There was a modest dose-response relationship between exercise in high school and BMD at all sites that did not reach statistical significance. Women with more physical activity as measured by the personal activity computer had significantly higher bone density at the spine and distal radius. This assoMay 1992, Vol. 82, No.
ciation also prevailed at the midradius but was not statistically significant. Multiple linear regression, controlling for age, body mass, breast-feeding, oral contraceptive use, exercise in high school, and menopausal status, indicated that current physical activity is directly and significantly associated with bone density at the spine, distal, and midradius sites (Table 2). Based on these coefficients, each additional 100 kcallday energy expenditure in physical activity adds an estimated 12 mg/cm2, 3.8 mg/cm2, and 3.5 mg/cm2 higher bone density at the spine, distal, and midradius sites, respec-
tively.
Discussion It has been established that highly trained athletes have significantly higher BMD than do sedentary persons.3-6 Although research on this highly selected
group suggests that intensive exercise increases BMD, the finding has lesser practical and public health value for perimenopausal women, for whom the issue of moderate physical activity may be of greater interest. The present study shows that perimenopausal women with more physical activity have significantly higher BMD when other determinants of BMD are taken into account. Some limitations of our study should be noted. First, this was a cross-sectional study in which limited information on past exercise, body weight, and diet was collected. Second, misclassification may exist for some variables such as exercise in high school. Behavior change after adolescence may further lessen the value of this indicator. Third, although we improved the measurement of physical activity by using personal activity computers, it can be argued that 3 days of physical activity are not truly representative of the A American Joumal of Pubfic Health 737
Public Heath Brief
.............----.-.---.....*1*--.---.-.
::: '' f5SsSsyssS:
.....................SSSS: Ss::: fs;;
S;:sf
:s::
:sxe
S,?B':'....
e-S::'S::;f ;fS;f
women's life-styles. Some women might have increased their exercise in the 3 measured days because of the study. Furthermore, since the device could onlymeasure acceleration and deceleration in a vertical direction, some types of exercise, such as bicycling, may accumulate too few kilocalories. If this were true, however, the bias would be in the direction of the null because of increased variance. Thus, our significant findings suggest either that there is no strong bias in the physical activity results in our study or that the relationship between activity and BMD was underestimated. Nonetheless, the advantages of the present study are noteworthy. The study size was substantially larger than most previous studies of exercise and BMD. Use of personal activity computers provided both objectivity and precision. In addition, the present study measured and adjusted for factors that were often overlooked in previous studies, such as breast-
738 American Journal of Public Health
.......
.:.:............Sf.Sf. .......SS S-
geseSSf geeyi
SS'''::S: S::S':: ::'';; ::-'':S-
feeding, oral contraceptive use, and calcium intake. These advantages, alongwith the significant findings of this study and their biological plausilbility and consistencywith previous studies, lead us to conclude that moderate physical activity is associated with higher bone density among perimenopausal women. O
Acknowledgments Partial support for this work was provided by Family Health Intemational (FWI) with funds from the US Agency for Intemational Development (USAID) and the National Institute for Child Health and Human Development (NICHD), although theviews expressed in this article do not necessarily reflect those of USAID or NICHD. FHI is a not-for-profit organization that conducts research and provides technical assistance in reproductive health, family planning, and sexually transmitted diseases.
References 1. Dalsky GP, Stocke KS, Ehsani AA, Slatopolsky E, Lee WC, Birge SJ. Weight-
bearing exercise training and lumbar bone mineral content in postmenopausal women. Ann Intern Med. 1988;108:824828. 2. Chow R, Harrison JE, Notarius C. Effect of two randomised exercise programmes on bone mass of healthy postmenopausal women. Br Med J. 1987;295:1441-1444. 3. Jacobson PC, Beaver W, Grubb SA, et al. Bone density in women: college athletes and older athletic women. J Orthop Res. 1984;2:328-332. 4. Wolman RL, Clark P, McNally E, Harries M, Reeve J. Menstrual state and exercise as determinants of spinal trabecular bone density in female athletes. Br Med J. 1990;301:516-518. 5. Lane NE, Bloch D, Jones HH, et al. Longdistance running, bone density, and osteoarthritis. JAMA 1986;255:1147-1151. 6. BlockJE, Genant HK, Black DM. Greater vertebral bone mineral mass in exercising young men. West JMed. 1986;145:39-42. 7. Simkin A, Ayalon J, Leichter I. Increased trabecular bone density due to bone-loading exercises in postmenopausal osteoporotic women. Cakf TLssue Int. 1986;39:8. 8. Aloia JF, Cohn SH, Ostuni JA, et al. Prevention of involutional bone loss by exercise. Ann Intern Med. 1978;89:356-358. 9. Block JE, Smith R, Black D, Genant UK Does exercise prevent osteoporosis? JAMA. 1987;257:3115-3117. 10. Washburn RA, Montoye HJ. The assessment of physical activity by questionnaire. Am JEpidendoL 1986;123:563-576. 11. Montoye HJ, Washburn R, Senrais S, Ertl A, Webster JG, Nagle FJ. Estimation of energy expenditure by a portable accelerometer. Med Sci Sports Exerc. 1983;15: 40347. 12. US Department of Agriculture, Agricultural Research Service. Composition of Foods, Daiy and EggPmducts:Raw, Processed, Prpatre Washington, DC: US Government Printing Office; 1976. Agriculture Handbook No. 8-1.
May 1992, Vol. 82, No. 5
