The Obesity Paradox

A paper in the February Mayo Clinic Proceeding gives food for thought: Obesity Paradox and Cardiorespiratory Fitness in 12,417 Male Veterans Aged 40 to 70 Years. Everyone is aware of the negative health consequences of obesity. Less familiar is the consistent inverse correlation (the so-called obesity paradox) between mortality and body mass index (BMI) among “patients with heart failure, coronary heart disease, hypertension, peripheral artery disease, type 2 diabetes, and chronic kidney disease. An obesity paradox has been observed in healthier populations as diverse as San Francisco longshoremen, Native American women of the Pima tribe, men from rural Scotland, Nauruan men, and the elderly.” – quotation is from the above article.

The current study sought to examine the obesity paradox further by factoring in fitness. The study built on previous observations that both fitness and a higher BMI were associated with reduced mortality in patients referred for exercise training. The new work looks at both fitness and BMI in the same population of 12,417 men (all veterans age 40 to 70) with known or suspected cardiovascular disease. The men were stratified by BMI. They were also divided into three fitness groups low, moderate, and high. Fitness was defined by exercise ability – METs (metabolic equivalents tasks).

Table four from the study shows the pertinent findings. Fitness was associated with decreased all cause mortality, but so was increased BMI. Thus the best mortality outcomes were in subjects with both increased BMIs and increased exercise tolerance. A hazard ratio (HR) is statistically significant if its 95% confidence interval does not straddle 1. A HR greater than 1 signifies an increased risk, while an HR lower than 1 indicates reduced risk. The reference group (the group to which all others were compared) was the normal BMI high fitness group. A BMI greater than 25 was classified as overweight. A BMI greater than 30 was classified as obese.  High fitness was defined as the ability do more than 10 METs. This is about equivalent to walking up two flights of stairs at a “normal” pace without stopping.

Thus mortality rates were lower in fit overweight and obese male veterans than in equally fit male veterans who were at normal weight. We’re back to the obesity paradox. The authors of this study consider a number of possible reasons for this paradox none of which satisfies. Nobody argues that you should intersperse trips to the gym with frequent stops at Hamburger Heaven so as to be fat and fit, but there seem to be so many examples of the obesity paradox that one must continue to look for an explanation. Maybe the Duchess of Windsor was wrong when she said you could never be too thin. It would be a cause of schadenfreude if all those skinny joggers were running themselves into a premature grave. Regardless, we don’t know as much about what ideal weight should be as we think we do.