Medical Studies

The Obesity Paradox: Do overweight people live longer?

According to a study by Flegal and co-authors, the answer is yes.  The study conducts a systematic review and meta-analysis and finds found that individuals who are overweight live longer than those who are healthy weighted.  On the other hand, severely obese individuals live less.  The hazard ratios (i.e., probability of death in period t conditional on living to the period t-1) for individuals relative to normal individuals were 0.94 for overweight and 1.18 for obesity.  The authors even found that mildly obese individuals (grade 1) have the same mortality rates as those who are healthy weighted and only the most severely obese live shorter than those of healthy weight.  In the authors own words:

Relative to normal weight, both obesity (all grades) and grades 2 and 3 obesity were associated with significantly higher all-cause mortality. Grade 1 obesity overall was not associated with higher mortality, and overweight was associated with significantly lower all-cause mortality. The use of predefined standard BMI groupings can facilitate between-study comparisons.

What can explain this conclusion? Is it really the case that adding some weight is good for you?  In a letter to the editor, Jose Viña, Consuelo Borras, and Mari Carmen Gomez-Cabrera propose an alternative explanation: measurement error.

…[the obesity] paradox may be due to the use of the body mass index [BMI] because it provides an inadequate definition of obesity. It does not take into consideration body composition (fat mass and fat-free mass) and can underestimate the degree of adiposity and its distribution. Although weight is correlated with body fat, it is also correlated with the amount of lean mass individuals have. Therefore, muscular individuals may be classified as overweight or even obese when BMI is used.

In aging and in conditions such as malignancy or rheumatoid arthritis, lean body mass may be lost while fat mass is preserved or even increased. Thus, the relationship between age-related reduction of muscle mass and strength is often independent of body mass. Moreover, spontaneous weight loss is an accepted criterion of age-associated frailty.

Rather than using BMI to measure overweight and obesity, the commenters wisely suggest using either direct estimates of body fat or waist-to-hip ratio; both of which may better measure whether a person is carrying an unhealthy amount of weight.

In short, don’t run out an start ordering double cheeseburgers in response to the Flegal study.


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