Letter by McAuley et al Regarding Article, “Impact of Body Mass Index and the Metabolic Syndrome on the Risk of Cardiovascular Disease and Death in Middle-Aged Men”
To the Editor:
We read with interest in the article by Arnlov et al1 that middle-aged men defined as overweight and obese by Body Mass Index (BMI), but without metabolic abnormalities, were at increased risk for death. Epidemiological studies are limited by the adjustments they make for potential confounders. The study in question included only 3 confounders: age, smoking, and low-density lipoprotein cholesterol. Yet many other factors confound the BMI–mortality relationship, including but not limited to diet, weight (cycling) history, weight loss medication use, psychosocial stress, socioeconomic status, physical activity, and fitness level. Unless these factors are taken into account, it is uncertain whether being overweight and obese independently increases mortality risk.
By far the most studied and influential confounder of the association of BMI with mortality is cardiorespiratory fitness, measured objectively by a treadmill exercise test. Low fitness is a well-established independent mortality predictor.2 Failure to account for the influences of fitness may confound the relationship between BMI and mortality. Decades of research with simultaneous measurements of BMI and fitness from the Aerobics Center Longitudinal Study (ACLS) clearly demonstrate that fitness attenuates or eliminates mortality risk associated with obesity.3–5 In other words, fat but fit adults are no more likely to die than normal-weight individuals of a similar age and fitness level, and fat but fit adults are far less likely to die than unfit normal-weight people.
Important research questions about physical activity, fitness, fatness, and mortality remain unanswered. What are the separate and combined influences of physical activity and fitness on mortality risk? Why is obesity protective in some instances (obesity paradox)? Are the benefits of fitness independent of weight loss? How do these variables interact with age, sex, ethnicity, and health status? If the effects of BMI on health outcomes are to have any meaning, investigators must include simultaneous objective measures of fitness or physical activity. Nevertheless, we commend this otherwise well-conducted study for focusing more attention on this important research area.
Arnlov J, Ingelsson E, Sundstrom J, Lind L. Impact of body mass index and the metabolic syndrome on the risk of cardiovascular disease and death in middle-aged men. Circulation. 2010; 121: 230–236.
McAuley PA, Sui X, Church T, Hardin J, Myers J, Blair SN. The joint effects of cardiorespiratory fitness and adiposity on mortality risk in men with hypertension. Am J Hypertens. 2009; 22: 1062–1069.
Lee CD, Blair SN, Jackson AS. Cardiorespiratory fitness, body composition, and all-cause and cardiovascular disease mortality in men. Am J Clin Nutr. 1999; 69: 373–380.