Previous literature has shown Caucasian race, younger age, male sex, and lower BMI as independent predictors of CRF between African American and Caucasian men and women (29). Our results are parallel with these findings, suggesting African Americans have increased risk of having reduced CRF. Our findings are consistent with previous evaluations of population-based estimated submaximal CRF levels using NHANES in which lower CRF was also seen in African Americans (12,15,34). Our study demonstrates that among those of higher SES, disparities still exist in CRF levels between African Americans and Caucasians. The results of this report further support race as an independent risk factor for lower fitness levels among African Americans. Furthermore, there was a small but significant age difference (<2 yr) in African Americans compared with Caucasians. The African American men and women were on average younger than their respective Caucasian comparison groups. As fitness decreases with age, this observation would suggest that our finding of lower fitness in African Americans is conservative.
Individual-level SES has been defined by others using income, occupation, and/or education and contributes to health- and fitness-related disparities (1,11). Although demographic information is shared between physician and patient, we do not collect income levels, as noted. Therefore, education and access to preventive care have been used to define higher SES status in this population (18). Furthermore, built environment has been identified as a strong predictor of physical activity resources and is associated with both individual- and neighborhood-level SES (16,33). Although our findings suggest that disparities in CRF between African Americans and Caucasians are independent of individual-level SES, further studies in this population can explore whether differences in built environment or neighborhood-level SES are potential explanatory factors in racial/ethnic differences in CRF. Comparable physical activity levels among men in this study make it unlikely that racial differences related to fitness were a result of leisure time physical activity alone. Although African American women had a higher education level than their Caucasian counterparts and comparable access to a preventive care examination, African American women participated in less self-reported physical activity, although statistically insignificant. Previous literature suggests that perceptions regarding the benefits of physical activity between African Americans and Caucasians are similar; therefore, other neighborhood environment factors may be a barrier to physical activity and fitness (30). Our study results are similar to previous heterogeneous studies documenting increased BMI and lower fitness levels in African American women compared with Caucasian women (34). Of note, some suggest that increased BMI in young adult African Americans may not reflect increased body fat; therefore, BMI may not have a significant metabolic impact (22). Although the fully adjusted fitness model did not reach significance in women, which may be due to power issues, a trend persisted.
Although lower CRF in African Americans has been consistently found in a variety of populations including children, adults, and athletes, the underlying mechanisms leading to this phenomenon are not fully understood (21,35,37). It has been suggested that physiologic differences lead to the different CRF levels seen in racial groups, although the exact underlying mechanisms are unknown. For example, a previous study provided evidence that African Americans appeared to have greater physiologic difficulty when attaining maximal fitness levels suggested by increased heart rate, ventilation, and perceived exertion during physical tasks compared with Caucasians (21). However, our analysis revealed no significant difference in mean age-adjusted maximal heart rates between sex-specific counterparts, suggesting comparable effort and difficulty. Furthermore, adjusting for variables age, BMI, education, PA MET-minutes, smoking, high blood pressure, and hemoglobin failed to remove the significant effect of race on CRF in African American men. Even in populations of highly trained athletes, a disparity exists (37). A hypothesis that could explain racial differences in CRF involves the concept of increased exercise economy in Africans compared with their Caucasian counterparts. Weston et al. investigated the concept of “running economy” and found that African runners operated at a reduced V˙O2max, compared with Caucasians, yet completed a 10-km run within a comparable time frame (37). “Running economy” refers to a greater ability to produce energy during movements, specifically stepping tasks, as defined in this small study of elite marathon runners (37). Inconsistent evidence has shown increased running economy (31,37,39) and higher fractional utilization of V˙O2peak (9,13) in African individuals versus their Caucasian counterparts; however, little is known concerning these physiological factors within the African American and Caucasian population.
Several potential underlying mechanisms should be considered. First, it is believed that a reasonable percentage of an individual’s ability to attain high maximal fitness levels is genetic in basis (10). Second, other investigators have found differences in skeletal muscle characteristics including oxidative capacity and lower percentage of slow twitch muscle fibers in African Americans (4,5). Third, lower hemoglobin levels are generally seen in African Americans but do not appear to affect the race–fitness relationship in this study (21). Finally, there are well-known differences in cardiovascular health, including greater prevalence of hypertension, left ventricular hypertrophy, obesity, insulin resistance, and diabetes (32), that could affect and be affected by CRF level. In particular, higher levels of insulin resistance among African Americans as compared with Caucasians, independent of body fat distribution (3), may contribute to abnormalities in skeletal muscle mitochondria, hampering exercise performance and worsening CRF (14). Nonphysiologic considerations could account for the disparate CRF levels seen here. First, eating behaviors may have varied, and deficient energy consumption could have failed to supply the energy required to attain maximum CRF level, although maximal predicted heart rate was similar. Another possible explanation for the results is inaccurate measurement of physical activity that is inherent in self-reports of physical activity. Therefore, it is possible that the disparate fitness may be a result of different lifestyle behaviors in our African American and Caucasian populations that are not measured.
A major strength of our study includes an African American population with higher, more homogeneous individual-level SES relative to other studies in fitness. The disproportionate social disadvantages experienced by African Americans are well documented (1); these high levels of inequality introduce unmeasured confounders related to SES in other studies (20). In addition, extensive phenotyping of the cohort removed the likelihood of lower CRF being related solely to underlying disease. Moreover, we had the opportunity to examine the association of maximal fitness levels in relation to race, whereas other large databases estimate fitness levels from submaximal tests. An additional strength includes the similar mean and median year of examination dates among African Americans (2002 and 2003) and Caucasians (2000 and 2001) in this 40-yr period; these similar dates point against secular differences that may have occurred over time (38) because of increased public policy regarding physical activity recommendations. A limitation to this study is the disproportionately smaller African American population. We addressed this by using multivariable regression to compare adjusted mean CRF between groups, and P values were calculated for an unbalanced sample size and unequal variance between groups. Second, although fat-free mass may better represent metabolically active tissue than BMI, it was not available in this data set. Lastly, there are limitations regarding our definition of SES, defined by the available variable of education years and having access to the Cooper Clinic’s preventive examination. This potentially could artificially increase the SES of the examined individuals.
Overall, we conclude that the African Americans in this cohort have a lower fitness level than their Caucasian counterparts and a significantly lower CRF level within the male population. Disparate fitness levels between African Americans and Caucasians did not seem to be significantly mitigated despite higher individual-level SES in this population as measured by education and access to health care resources. The elimination of unmeasured confounders present in other economically diverse populations did not entirely remove racial differences seen in CRF, further emphasizing the need for a multifaceted approach to health equality. Our results suggest that racial/ethnic differences regarding CRF do exist among African Americans and Caucasians across the socioeconomic spectrum. These findings may have both individual and policy level implications. First, this fitness disparity in conjunction with the known health disparities should remind health care providers of the critical need to screen and follow their African American patients for low fitness-related disease. Second, as some experts are beginning to suggest the need for considering fitness levels a vital sign (23) in this era of increasing obesity and sedentary behavior, it will be critical to understand the nuanced differences between races to inform cut points and policy. Additional research in this area is warranted to verify these findings in other homogenous, higher SES populations. Finally, research should be done to delineate the mechanisms for lower fitness and to determine its impact on chronic disease development and hence prevention.
The authors thank Kenneth H. Cooper, M.D., M.P.H., for establishing the Cooper Center Longitudinal Study and The Cooper Institute for data management.
There are no conflicts of interest to disclose.
There are no funding sources to disclose.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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