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Racial and Ethnic Differences in Physical Activity and BMD

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Racial and Ethnic Differences in Physical Activity and BMD

Discussion


We sought to determine whether current PA is related to bone mineral density in a racial/ethnically diverse sample of adults aged 40 to 80 after controlling for age, sex, BMI, poverty–income ratio, tobacco use, vitamin D and calcium intake, and use of osteoporosis medications. We found that participation in both high and moderate levels of PA was more prevalent among non-Hispanic whites than among non-Hispanic blacks and Hispanics. These findings are consistent with data showing that in a sample of adults aged 60 years or older, participants from minority groups reported lower levels of PA. Furthermore, our study showed that participants who engaged in low and moderate levels of PA had lower bone mineral density than those who engaged high levels of PA. A study published in 2001 indicated that women who engaged in low to moderate levels of PA had lower bone mineral density than those who engaged in high levels PA. However, our study also found that, among non-Hispanic blacks, rates of osteoporosis were low despite the low levels of PA. This paradoxical finding may suggest that either PA is not as strongly associated with bone mineral density among non-Hispanic blacks or that the high bone mineral density among non-Hispanic blacks is due to other factors not identified in this study.

In our study, a high bone mineral density was significant only for non-Hispanic blacks, which suggests multifactorial effects. Two studies examined the racial differences in bone mineral density in a diverse sample and similarly categorized participants as Hispanic, non-Hispanic white, and non-Hispanic black. In both studies, higher bone mineral density was found among non-Hispanic blacks than among non-Hispanic whites. However, neither of these studies included PA status.

Physical inactivity and poor nutrition are behavioral factors that increase BMI and perhaps adversely affect bone mineral density. These factors are also more prevalent in racial/ethnic minority and lower socioeconomic groups. For example, Hispanics and non-Hispanic blacks are reported to have higher BMI, consume less calcium and vitamin D, and engage in less PA than non-Hispanic whites. They are also reported to have lower levels of education, which can be associated with higher BMI, lower levels of income, and lower levels of fruit and vegetable consumption. Moreover, because poor bone health is reported to be enhanced by the cumulative effects of unhealthful behaviors occurring throughout life, it can be inferred that racial/ethnic minorities may be at increased risk for poor bone health as adults because of inadequate PA and nutrition during childhood, resulting in suboptimal achievement of peak bone mass earlier in life. Our findings also suggest that PA is not associated with bone mineral density as strongly among non-Hispanic blacks; other possible explanations for this association should be explored in future studies.

Our study has strengths and limitations. One strength was the use of a large community-dwelling sample representative of the US adult population. However, the NHANES data used in this study are subject to at least 2 limitations. First, a cross-sectional measurement of PA does not identify an individual's true average activity over a given interval. Moreover, the type and length of PA participation plays a role in bone health; non-weight–bearing, high-force activity (eg, progressive resistance strength training) may benefit neck femur bone mineral density, which was the area measured in this study, but no other bone sites. This finding is important because PA declines with age while functional limitations, which impair the ability to participate in regular activity such as walking and climbing a flight of stairs, are also often reported to increase with advanced age. This decline in PA may help to explain the racial/ethnic group differences in PA and its effect on bone health.

In addition, self-reported PA data may include reporting errors because some respondents could not recall their past month's level of PA or did not answer the question correctly. Furthermore, more participants reported high levels of PA than moderate levels, which may have resulted from self-report bias or misclassification. However, we calculated our categories using standard cutpoints provided by the measuring tool we used. Non-Hispanic blacks and Hispanics are more likely to report functional limitations and disability than non-Hispanic whites, which may explain the lower levels of PA among these 2 groups. Research also shows an increased prevalence of functional decline among those diagnosed with a chronic medical condition or conditions (eg, heart problems, fractures, arthritis, diabetes) than among those without these conditions, which were not measured in our study. Another potential limitation may be related to assessment of race/ethnicity. The Hispanic category includes whites, black Hispanics, and Mexican Americans, and we were not able to subclassify our Hispanic group into a more refined group of black and white Hispanics. Other limitations included the background confounders that were not evaluated, including menopause status, family history of osteoporosis or hip fracture, chronic diseases (eg, hyperparathyroidism, hypogonadism, malabsorption) and long-term use of medications (eg, steroids), predisposing participants to osteoporosis. To the extent that the associations studied here may change over time, these analysis should be replicated as more recent data becomes available. Despite these limitations, other studies corroborate our findings, showing that elderly minorities are disproportionally reported to reduce their ability to remain physically active, and this can be a contributing factor to the subsequent deterioration of bone health. However, the use of a validated PA scale that quantifies the degree of PA during the past month increases the reliability of our results.

The public health impact of inadequate levels of PA among older adults is significant, given the substantial increase in the percentage of racial/ethnic minority older adults expected in the next 40 years. With this increase, we need to gain a better understanding of the plausible explanations for the discrepancies in self-reported levels of PA among older adults and their effect on bone health data. However, additional research on the specific risk factors for osteoporosis among racial/ethnic minority populations continues to be limited, with lifestyle factors such as PA inconsistently reported to prevent bone loss and osteoporosis among minority populations.

This study concurs with previous literature showing that the prevalence of osteoporosis is higher among non-Hispanic whites despite their having higher levels of PA than their Hispanic and non-Hispanic black counterparts, that those with higher BMI are less likely to have osteoporosis, and that non-Hispanic blacks are least likely to have osteoporosis despite lower levels of PA. This study adds to the literature on the effect of PA in a racial/ethnically diverse sample of older adults, but longitudinal studies with larger numbers of racial/ethnic minorities are still needed to improve our understanding of these differences and to assess potential causes. Further longitudinal studies looking into the effect of PA and bone mineral density by race/ethnicity using a life-course approach are needed to better characterize the relationship between PA and bone mineral density.

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