In this cross- sectional study associations between two obesity indexes (BMI and waist circumference) and between cardiorespiratory fitness, as measured by calculated VO2max, were both stronger in women than in men. For men, the correlation between waist circumference and VO2max was stronger, with statistical significance, than the correlation between BMI and VO2max. For women, while both correlations were higher than those for men, the correlation between BMI and VO2max was stronger than the correlation between waist circumference and VO2max, a difference not statistically significant.
Other studies have reported a wide range of values for correlations between BMI and VO2max [16–18], and between waist circumference and VO2max [16, 17, 19], with differences reported between the sexes. In a Finnish cross sectional study of 807 men and 633 women (age: 18–75 years) individuals within the same BMI category with normal waist circumference were more physically active and had better cardiorespiratory fitness than did those with high waist circumference . Cardiorespiratory fitness was negatively associated with obesity, a relationship that remained after adjustment for level of physical activity. Physical activity levels were negatively associated with obesity in men, but not in women. In a cross sectional study that included young men only, the level of fitness was more closely associated with waist circumference than with BMI .
Similar to other findings , the correlations observed in the current study between waist circumference and BMI were high and statistically significant in both sexes. Nevertheless, the differences found between BMI and waist circumference concur with the distinction between these two measures of obesity, as highlighted by the United States National Health and Nutrition Examination Survey 1999–2004. There, when BMI was the measure of adiposity, 51.3% of overweight adults were metabolically healthy; conversely, 23.5% of normal weight adults were metabolically abnormal. In contrast, when waist circumference was the measure of adiposity, only 36.4% of abdominally obese adults were metabolically healthy, yet 28.3% of normal weight adults were metabolically abnormal . Evidently, both adipose measures associate only partially, and differently, with metabolic normalcy.
In the current study, age, hours of training per week and caloric expenditure in sport activity per week, were highly associated with cardiorespiratory fitness in both men and women. Nevertheless, the linear regression models presented suggest that the preferred obesity index is different in men and women, and that BMI may better indicate cardiorespiratory fitness for women, and waist circumference for men. The R2 values for the statistical models of approximately 0.4 (Table 2) indicate that from simple and readily available characteristics (BMI, waist circumference, age, smoking status, number of weekly hours spent training, and caloric expenditure per week on physical exercise) cardiorespiratory fitness can be estimated quite well in a healthy population.
Analysis of the regression models showed VO2max to be associated positively with weekly caloric expenditure, yet negatively with the weekly number of hours of physical activity for the same caloric expenditure. The upshot is that, for the same caloric expenditure, engagement in more hours per week of physical activity is associated with a lower fitness level than engagement in fewer hours (i.e. at greater intensity). This applies to men and women, highlighting the importance of intensity of physical activity for maintenance of cardiorespiratory fitness for both sexes. We assume that similar investigation of athletes would reveal positive coefficients for both caloric expenditure and hours of activity, since their activity level is usually intense, and the level of intensity is not generally related to the duration of activity. According to a literature review, when total energy expenditure of exercise is held constant, exercise performed at vigorous intensity conveys greater cardioprotective benefit than exercise at moderate intensity .
The lack of a statistically significant association between smoking and cardiorespiratory fitness observed in the current study contrasts with a previous report of a negative association between smoking and physical fitness, as assessed by VO2max . However, our lack of information on smoking history is a limitation that raises the possibility of reverse causality. Past smokers, classified here as nonsmokers, may have quit smoking due to a poorer health profile, and may thus show poorer physical fitness. Conversely, the current smokers may represent a selected subsample of the ‘healthier’ and more resilient study participants. Moreover, it is conceivable that had we divided the participants by the number of cigarettes smoked per day, we would have found an association between smoking and physical fitness. In addition, the relatively low proportion of smokers in the study sample, 18% for both men and women (compared with 28% and 13%, respectively, for the over age 20 year population estimated from nationwide surveys in Israel) , suggests that the lack of association could be due to small sample size or to reporting bias resulting from participants denying their smoking habits.
The differences we found between the sexes in associations of waist circumference with VO2max support the clinical use of this measure, in addition to BMI, for assessment of cardiorespiratory fitness.
Our findings support the stronger negative association observed in young adult men, between cardiorespiratory fitness and waist circumference, compared to BMI, in the Finnish Defense Force . Encouraging ‘waist loss’ rather than weight loss alone has been shown to effectively motivate weight reduction . Waist circumference measurement is rapid, inexpensive, and easily performed. However, despite the inclusion of waist circumference as a key diagnostic criterion for the metabolic syndrome [27, 28], a uniformly accepted protocol for its measurement has not been established. Nevertheless, the high variability in location of measurement site  was not found to have considerable effect on associations of waist circumference with cardiovascular risk factors and mortality [29, 30].
While waist circumference, and not BMI, reflects fat distribution, neither waist circumference nor BMI measures body tissue composition. Men and women differ considerably in fat proportion, as well as distribution. Sex-related differences, which are readily apparent in normal-weight men and women, may predispose to a spectrum of fat distribution phenotypes with obesity . Higher prevalence of “apple” shaped obesity in men, ie. central obesity (“android obesity”) may explain the stronger relationship we observed between waist circumference and VO2max in men. In contrast, higher prevalence of “pear” shaped obesity, ie, more fat in subcutaneous areas, especially in the gluteal and femoral depots (“gynecoid obesity”), may explain the stronger relationship observed between BMI and VO2max in women. Sex-based differences in fat distribution may explain differences between the sexes in VO2max, as well as differences between obesity indexes. Proctor et al., found that when VO2max is normalized to kilograms of fat free mass, the sex difference in VO2max levels often disappears. They suggested expressing VO2max per unit of fat free mass when comparing the cardiorespiratory fitness of individuals with different body sizes and composition .
Participants of the current study are of Caucasian ethnicity. Thus, the current targets for BMI and waist circumference, which were derived from studies of predominantly white and European populations, were appropriate. However, the applicability of these targets to other populations has been questioned [33, 34]. Further, contrary to BMI, waist circumference is independent of height. Even though waist circumference tends to be greater in tall adults, it increases at a “slower” relative rate than does height; short individuals would have greater absolute waist circumference-to-height ratios than would tall individuals. At present, studies examining circumference-height associations are inconsistent in their conclusions .
In this study we showed that associations between obesity and cardiorespiratory fitness are dependent on sex, and on the anthropometric measure used. Though a meta-analysis found BMI and waist circumference to be associated similarly to incident diabetes, most of the studies included did not analyze men and women separately, and some did not even adjust for sex . A recent review reported a stronger association of measures of central obesity than BMI to diabetes, but similar associations to other cardiovascular risk factors, namely, hypertension and dyslipidemia . However, the authors concluded that the cross-sectional design of the studies, as well as the lack of analysis by sex, limit generalizability of their conclusions. Since the participants of the current study were healthy men and women, we do not know if our findings apply to people with chronic illnesses.
Temporality of the relationship between obesity and physical fitness cannot be determined in the current study, due to its cross-sectional design. Reverse causality can therefore not be excluded, ie. people with poor physical fitness may gain weight and become more obese. The complexity of the relationship between physical fitness and obesity is further highlighted by reports of their differential effects on different diseases. In a systematic review, Fogelholm found the risk for all-cause and cardiovascular mortality to be lower in those with high BMI and good aerobic fitness than in those with normal BMI and poor fitness. In contrast, the concomitance of a high BMI with high physical activity level was associated with a greater risk for the incidence of type 2 diabetes and the prevalence of cardiovascular and diabetes risk factors than a concomitant normal BMI and low physical activity level .
The use of a calculated value for VO2max, rather than a direct measure, is a limitation of this study. The gold standard for measuring VO2max is by gas analysis during a maximal fitness test. However, this is an expensive test that requires highly skilled operators and motivated subjects. Since this test is not usually practical, formulas that predict VO2max have been developed over the years. VO2max can be evaluated by means of a fitness test or by other methods . The Bruce protocol assumes that maximum oxygen consumption can be evaluated by the duration of time a subject is able to walk or run on a treadmill. The test score is the time taken for the test, in minutes, which can then be converted to an estimated VO2max score [13, 14].