The mean age- and BMI-adjusted values for WC, total abdominal, visceral, and abdominal subcutaneous AT in the HIGH and LOW CRF groups are provided in Figure 4.
The results based on AT areas measured at L4–L5 and L3–L4 were similar to those for the corresponding AT mass values. There were no significant BMI × CRF group interactions, indicating that the slopes were not different in the HIGH versus LOW CRF groups (P > 0.10). Significant main effects (P < 0.001) were obtained for total abdominal, subcutaneous, and visceral AT area (e.g., cm2) at both the L3–L4 and L4–L5 levels. The mean visceral AT areas at L4–L5, adjusted for age and BMI, were 157.1 cm2 for the LOW CRF group and 138.6 cm2 for the HIGH CRF group.
All general linear models included age as a covariate. Significant main effects for age were observed for WC, total abdominal and visceral AT mass, and area values (P < 0.001) but not for subcutaneous AT. The increase in WC, and total abdominal and visceral AT with age was observed independent of BMI and CRF group.
The primary finding of this study is that for a given BMI, men with high CRF levels have significantly lower levels of abdominal AT, by comparison with those with low CRF. The differences in abdominal adiposity between CRF groups provide insight into the plausible mechanisms by which CRF attenuates the health risks attributed to obesity. Further, they reinforce the importance of regular physical activity in the prevention and treatment of obesity-related disease independent of a reduction in body weight. Accordingly, dependence on BMI alone to determine health risk may be misleading because potential differences in abdominal adiposity as a consequence of increased CRF is ignored. Although the measurement of abdominal AT is not practical in clinical practice, the results of this and previous studies (28) show that WC is also lower in association with high CRF. Consequently, it is suggested that WC and CRF be routinely measured to assess health risk.
The results of this study support the hypothesis that men with moderate to high CRF levels have lower WC than men with low CRF independent of BMI. Although our sample consisted of Caucasian men from middle to upper socioeconomic class, the results agree with Ross and Katzmarzyk (28), who report similar observations for WC in a large sample of the Canadian population, which included men and women varying in socioeconomic class. However, whether this observation holds true independent of race is unknown.
The novel finding of this study was that men with high CRF had significantly less total abdominal, subcutaneous, and visceral AT by comparison with men in the low CRF group. This finding agrees with our observations based on WC and is consistent with numerous reports indicating that WC is a strong correlate of radiographically measured visceral and subcutaneous AT (11,15,23). Others report that exercise per se is associated with substantial reduction in subcutaneous (26) and visceral AT (17,26) in the absence of any change in BMI. Given the established association between both abdominal subcutaneous AT (13,29) and visceral AT (7,19,27) with metabolic risk, these observations suggest that a robust mobilization of abdominal fat may be a mechanism by which CRF attenuates health risk without a concomitant decrease in BMI. Because our findings are derived from a sample of Caucasian men, extrapolation to other races and women requires confirmation by further study.
Whether the combination of higher CRF and lower abdominal AT conveys a metabolic benefit by comparison with those with lower CRF and higher visceral AT for a given BMI is unknown. It has been reported that visceral AT remains a significant predictor of Type 2 diabetes after controlling for the disease risk predicted by BMI (5), and visceral AT has been implicated in the etiology of numerous metabolic risk factors for Type 2 diabetes and CVD (19,25). Further, it is consistently reported that lower visceral AT is associated with corresponding reduction in metabolic risk factors independent of obesity (6,17). Therefore, it seems reasonable to suggest that the combination of high CRF and low abdominal AT, in particular visceral AT, would be associated with reductions in metabolic risk compared with those with the same BMI, but low CRF and high visceral AT.
The observation that total abdominal and visceral AT, but not subcutaneous AT, increased with age independent of BMI and CRF extends previous reports wherein the age-associated increase in abdominal adipose tissue for a given fat mass in women is reportedly a consequence of an increase in visceral and not abdominal subcutaneous AT (8,20). Of particular interest in this study was that the CRF effect persisted throughout the age range. In other words, for a given age those in the HIGH CRF group had lower levels of abdominal and visceral fat than those in the LOW CRF group. The lower abdominal and visceral fat in the HIGH CRF group in the absence of a difference in BMI is likely explained by correspondingly lower lean mass. This notion is consistent with Gallagher et al. (9), who report that aging is associated with a progressive decrease in lean mass, and increase in fat mass, despite no change in BMI. Together, these observations reinforce once more the limitations inherent to the use of BMI alone to detect important age- and health-related changes in body composition.
The findings of this study should not be interpreted to suggest that improvement in CRF requires a concomitant change in abdominal obesity to observe a reduction in health risk. To the contrary, it is well established that regular physical activity is associated with numerous physiological effects that are beneficial to health independent of reduction in abdominal AT (3). Indeed, a single exercise session has been shown to reduce triglyceride levels, increase high-density lipoprotein levels, reduce resting blood pressure, increase glucose tolerance, and reduce insulin resistance (31). Thus, it is clear that physical activity (acute and/or chronic) conveys health benefits independent of concomitant improvement in either CRF and reductions in body weight and/or adiposity.
In summary, moderate to high CRF was associated with lower levels of abdominal AT, both subcutaneous and visceral AT depots, for a given BMI by comparison with those with low CRF in men. To the extent that these AT depots convey an increased health risk, this finding suggests a mechanism by which CRF attenuates the health risks attributed to obesity as measured by BMI. Accordingly, they suggest that assessment of health risk by BMI alone may be misleading and that the measurement of WC and CRF would substantially improve the ability to identify those at increased health risk. Further, our findings reinforce the importance of CRF in the reduction of obesity-related health risk, independent of a reduction in body weight. Clinicians unfamiliar with the measurement of CRF are encouraged to seek the expertise of appropriately certified health and fitness professionals for whom the measurement of CRF is routine. Finally, additional research is required to determine whether CRF is associated with lower abdominal AT, independent of BMI, in populations that vary by gender, ethnicity, and socioeconomic class.
Sincere appreciation is extended to Thanh-Binh Nguyen-Duy and Elisa Priest for their contributions to this research.
This research was supported in part by research grants from the National Institutes of Health to Steven N. Blair (AG06945) and Milton Z. Nichaman (HL62508), and from the Canadian Institutes of Health Research to Robert Ross (MT13448). Suzy L. Wong was supported by an Ontario Graduate Scholarship.
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