Bone Density and Exercise

By Lynn Keegan, RN, PhD, HNC, FAAN

Bone is a living, growing tissue—a composite of minerals, with calcium being the most abundant. Bone is not a hard and lifeless structure; rather, it is a complex, living tissue. Throughout life new bone tissue is formed while the old tissue is reabsorbed. At puberty bones grow at an accelerated pace resulting in increasing of both length and density of the skeletal system.

Because bones are changing constantly, they can heal and may be affected by diet and exercise. Until the age of about 30, the body builds and stores bone efficiently. Then, as part of the natural aging process, bones begin to break down faster than new bone can be formed. The critical years for building bone mass are from prior to adolescence to about age 30. Some think that young women can increase their bone mass by as much as 20%, a critical factor in protecting against osteoporosis. In women, bone loss accelerates after menopause, when the ovaries stop producing estrogen.

Osteoporosis

Osteoporosis is a significant public health problem associated with increased mortality and morbidity.1 It is a common disease in the elderly, and the fractures that result from this disorder affect 40% of women and 14% of men older than age 50.2 Results from a Swedish study involving heel and finger ultrasound measurements found that virtually all institutionalized elderly women could be classified as osteoporotic.3 The risk of fracture relates to bone mineral density (BMD) and the risk of falling, among other factors. Low BMD in the elderly can result from either low peak bone mass or accelerated bone loss, or a combination of the two. Nutritional factors play a role in both the attainment of peak bone mass and in the rate of age-related bone loss. The main determinants of peak bone mass are genetic factors, early-life nutrition, diet, and exercise.2 According to one estimate, approximately 66% of British Columbia, Canadian women older than age 40 are considered to have osteoporosis or osteopenia when their bone density results are compared to those of young adults.

Roughly 340,000 people in the United States fracture a hip each year, and four out of five fractures occur in elderly women.4 Osteoporosis typically progresses without symptoms. The first sign of osteoporosis may be a decrease in height, rounded shoulders, or hip or back pain. A broken bone after a minor fall also may indicate osteoporosis.

Role of Exercise in Increasing Bone Density

Data supporting the notion that exercise during growth builds a stronger skeleton is compelling. Many active investigations give rise to an optimistic outlook. Exercise during growth, especially during the pre-pubertal years, increases BMD and perhaps also bone size, each independently conferring bone strength.5 The role of exercise in increasing bone density is under investigation. One less-than-optimistic Swedish investigator thinks that in adulthood, exercise at best halts bone loss or increases BMD by a few percentage points, an increase of questionable biological significance.

The Achilles heel of exercise is its cessation. Most BMD benefits achieved by exercise during growth are lost with cessation of exercise. Exercise at a lower level, after a period of high-intensity activity, may retain residual BMD benefits into old age. High lifelong workload and high leisure-time activity level are associated with high BMD. A reduced rate of fragility fractures in the population perhaps could be achieved by promoting a physically active lifestyle with lifelong high-activity level during work and leisure time, leading to high BMD and fewer fractures.6

Research Studies

The objective in one cross-sectional study was to investigate the relationship between lifetime physical activity and calcium intake and BMD and bone mineral content (BMC) in 42 regularly menstruating Caucasian women (age 21.26 ± 1.91 years, body mass index [BMI] 23.83 ± 5.85 mg/kg2).1 BMD and BMC at the lumbar spine (L2-L4), hip (femoral neck, trochanter, total), and total body were assessed by dual energy X-ray absorptiometry (DXA). Lifetime history of physical activity and calcium intake was obtained by a structured interview using valid and reliable instruments. Measures of both lifetime physical activity and calcium intake were highly correlated. In stepwise multiple regression analyses, lean mass was the most important and consistent factor for predicting BMD and BMC at all skeletal sites. Lifetime physical activity contributed to 3% of the variation in total body BMD, and lifetime weight-bearing physical activity explained 15.1% of variance in lumbar spine BMC. Current calcium intake predicted 6% of the variance in BMD at the femoral neck and trochanter. The researchers found lean body mass (LBM), which can be modified to some extent by physical activity, to be a powerful predictor of BMD and BMC in young women. These results also suggest that adequate calcium intake may help to enhance bone mass, thus decreasing the risk of osteoporotic fracture later in life.

Another cross-sectional study of 113 healthy white women, 20-88 years of age, evaluated relationships between BMD, body composition, calcium intake, and physical activity.7 The analysis was performed in the entire cohort and in groups divided by reproductive/ menopausal status (premenopausal, perimenopausal, early postmenopausal, and late postmenopausal). BMD and body composition were measured with Lunar DPX-MD densitometer using specialized software for total body, spine, femur, and forearm. Calcium intake from food and supplements was assessed by a food frequency questionnaire. Past physical activity and past and present walking were assessed only in the older cohort using modified version of the Allied Dunbar National Fitness Survey for Older Adults.

The results showed significant reduction of both total body BMD and LBM of 13% and 12%, respectively, with age. LBM was the strongest determinant of BMD in various skeletal sites in the entire cohort and by groups. Calcium was associated positively with BMD of various regions of hip in the entire cohort and in the youngest and oldest subjects (r ranging from 0.32-0.56, P < 0.05, in simple regression), but not in perimenopausal and early postmenopausal women. Past activity (sports and recreation) was associated positively with BMD in total body, spine, hip, and forearm (r ranging from 0.26-0.37, P < 0.05). Various modes of walking were associated positively with BMD in regions of femur and forearm. These results reveal the importance of lean tissue acting independently on bone at different skeletal sites in women across age groups as well as the positive effects on BMD of calcium in the youngest and oldest women and life-long engagement in physical activity in older women.

A third study involved a meta-analysis of individual patient data (IPD) to examine the effects of exercise on lumbar spine BMD in postmenopausal women.8 IPD were requested from a previously developed database of summary means from randomized and nonrandomized trials dealing with the effects of exercise on BMD. The results cross 13 trials that included 699 subjects (355 exercise, 344 control) and reveal a statistically significant increase in final minus initial BMD for the exercise group and a statistically significant decrease in final minus initial BMD for the control group. Changes were equivalent to an approximate 2% benefit in lumbar spine BMD (exercise +1%, control -1%). The results of this IPD meta-analysis suggest that exercise helps improve and maintain lumbar spine BMD in postmenopausal women.

One study assessed the relationship of walking, leisure-time activity, and risk of hip fracture among postmenopausal women.4 Prospective analysis began in 1986 and included 12 years of follow-up in the Nurses’ Health Study cohort. A total of 61,200 postmenopausal women (ages 40-77 years and 98% white) without diagnosis of cancer, heart disease, stroke, or osteoporosis at baseline were included in the study. Incident hip fracture resulting from low or moderate trauma was analyzed by measuring the intensity and duration of leisure-time activity and time spent walking, sitting, and standing at baseline and updated throughout follow-up.

From 1986 to 1998, 415 incident hip fracture cases were identified. After controlling for age, BMI, use of postmenopausal hormones, smoking, and dietary intakes in proportional hazards models, risk of hip fracture was lowered by 6% for each increase of 3 metabolic equivalent (MET)-hours per week of activity (equivalent to 1 hr/wk of walking at an average pace). Active women with at least 24 MET-hr/wk had a 55% lower risk of hip fracture compared to sedentary women with less than 3 MET-hr/wk. Even women with a lower risk of hip fracture due to higher body weight experienced a further reduction in risk with higher levels of activity. Risk of hip fracture decreased linearly with increasing level of activity among women not taking postmenopausal hormones, but not among women taking hormones. Among women who did no other exercise, walking for at least 4 hr/wk was associated with a 41% lower risk of hip fracture compared with less than 1 hr/wk. More time spent standing also was associated independently with lower risks.

The purpose of another study was to determine the effect of intense exercise training on physical fitness, coronary heart disease, BMD, and parameters related to quality of life in early postmenopausal women with osteopenia.9 Fifty-nine fully compliant women without any medication or illness affecting bone metabolism took part in intensive exercise training (> two sessions per week); 41 women served as nontraining controls. Both groups received as much as 1,500 mg/d of calcium and 500 IU/d of vitamin D. Bone density of the lumbar spine and hip, maximum isometric and dynamic strength, and quality-of-life parameters including vasomotor symptoms related to menopause were measured at baseline and after 14 months. The results demonstrated significant differences in bone density, maximum isometric strength, and quality-of-life parameters (e.g., lower back pain) between the exercise and control groups. Dynamic strength at four exercises, which was assessed in the exercise group only, increased significantly (15-43%). The intense exercise training program in this study was effective in improving strength, endurance, quality-of-life parameters, and BMD in women in their critical early postmenopausal years.

Another study evaluated the potential benefits of regular Tai Chi Chuan exercise on the weight-bearing bones of postmenopausal women at a university medical school in Hong Kong.10 Participants were postmeno- pausal women (age range, 50-59 years), including 17 self-selected regular Tai Chi Chuan exercisers with more than four years of regular exercise, and 17 age- and gender-matched non-exercising controls. BMD in the lumbar spine and proximal femur was measured at baseline and at follow-up 12 months later using DXA and in the distal tibia using multislice peripheral quantitative computed tomography. Baseline results showed that the Tai Chi Chuan exerciser group had significantly higher BMD (10.1%-14.8%, all P < 0.05) than the control group in the lumbar spine, proximal femur, and the ultradistal tibia. The follow-up measurements showed generalized bone loss in both groups. This is the first case-control study to show that regular Tai Chi Chuan exercise may help retard bone loss in the weight-bearing bones of postmenopausal women.

Specific Exercises to Reduce Bone Loss and Fractures

Weight-bearing (impact) exercises appear to be the most important type of exercise to prevent bone loss. Specifically this includes: walking, jogging, running, stair climbing. Many aerobic exercises involve some aspect of weight-bearing movement. More entertaining activities, such as dancing, tennis, and skating, involve impact exercise, and thus, they too help to build bone.

Conclusion

Physical activity can reduce the risk of hip fractures in older women, although the required type and duration of activity have not been determined. Walking is the most common activity among older adults, and evidence suggests that it can increase femoral bone density and reduce fracture risk.

Most American practitioners now advocate exercise as a part of osteoporosis prevention. Canadian physicians recommend impact-type exercise, as well as age-appropriate calcium and vitamin D intake, for the prevention of osteoporosis.11

The bottom line about maintaining bone density is to get up off the couch or out of that office chair and begin walking, dancing, and skating toward maturity.

Dr. Lynn Keegan is the Director of Holistic Nursing Consultants in Port Angeles, WA.

References

1. Wallace LS, Ballard JE. Lifetime physical activity and calcium intake related to bone density in young women. J Womens Health Gend Based Med 2002;11:389-398.

2. Eastell R, Lambert H. Strategies for skeletal health in the elderly. Proc Nutr Soc 2002;61:173-180.

3. Ekman A, et al. Almost all institutionalized women are osteoporotic, when measured by heel and finger ultrasound. J Intern Med 2001;249:173-180.

4. Feskanich D, et al. Walking and leisure-time activity and risk of hip fracture in postmenopausal women. JAMA 2002;288:2300-2306.

5. Karlsson M. [Exercise increases bone mass in children but only insignificantly in adults.] Lakartidningen 2002;99:3400-3405.

6. Karlsson M. Is exercise of value in the prevention of fragility fractures in men? Scand J Med Sci Sports 2002;12:197-210.

7. Ilich-Ernst J, et al. Critical factors for bone health in women across the age span: How important is muscle mass? Medscape Womens Health 2002;7:2.

8. Kelley GA, et al. Exercise and lumbar spine bone mineral density in postmenopausal women: A meta-analysis of individual patient data. J Gerontol A Biol Sci Med Sci 2002;57:M599-M604.

9. Kemmler W, et al. Exercise effects on fitness and bone mineral density in early postmenopausal women: 1-year EFOPS results. Med Sci Sports Exerc 2002;34: 2115-2123.

10. Qin L, et al. Regular Tai Chi Chuan exercise may retard bone loss in postmenopausal women: A case-control study. Arch Phys Med Rehabil 2002;83: 1355-1359.

11. Brown JP, et al. 2002 clinical practice guidelines for the diagnosis and management of osteoporosis in Canada. CMAJ 2002;167(10 Suppl):S1-S34.