The "Fit But Fat" Concept Revisited: Population-based Estimates Using Nhanes

Abstract

Low cardiovascular fitness is an independent risk factor for developing type 2 diabetes and of all-cause and cardiovascular disease mortality in adults. The so-called "fit but fat" concept suggests that cardiovascular fitness attenuates risk of metabolic and cardiovascular disease independent of body mass index (BMI), even among individuals classified as obese. However, the proportion of the U.S. population that is both fit and obese is unknown. PURPOSE: To estimate the proportion of U.S. adults who are clinically obese yet have a high cardiovascular fitness level, and determine the independent effect of obesity on cardiovascular fitness. METHODS: Analysis of data on 4,675 adults (20-49 years) who had completed a submaximal exercise test, from the National Health and Nutrition Examination Survey (1999-2002), a cross-sectional health survey of a nationally representative sample of the non-institutionalized U.S. population. Cardiovascular fitness and body weight were expressed as continuous (estimated VO2max [ml·kg-1·min-1] and BMI [kg/m2]) and categorical variables (low, moderate, and high cardiovascular fitness level; and normal weight, overweight, and obese), the latter using sex and age-specific criteria established in the Aerobic Center Longitudinal Study and standard BMI cut-points, respectively. RESULTS: The prevalence of meeting the "fit but fat" definition among U.S. adults aged 20-49 was 8.9% (95% C.I. = 6.9-10.9%). The proportion of low, moderate, and high cardiovascular fitness differed significantly (P < 0.05) by body weight category. Overall, 8.9% were obese and had a high cardiovascular fitness level, 17.4% were overweight and high fit, and 30% were normal weight and high fit. In multiple regression, obesity was associated with a 9.2% lower estimated VO2max compared to a normal weight adult, controlling for age, sex, race, and income. CONCLUSION: A small proportion of U.S. adults can be considered "fit but fat", and obesity is significantly and independently associated with reduced cardiovascular fitness. These results suggest that obese adults in the U.S. population may be at increased health risk due to low cardiovascular fitness. Analyses Supported by U01CA114642.