Overweight, obesity, and all-cause mortality.

To the Editor: Dr Flegal and colleagues concluded that grade 1 obesity was not associated with higher all-cause mortality and that overweight was associated with significantly lower all-cause mortality. Other studies have shown that obesity in different populations, such as elderly people and patients with cardiovascular diseases, is also paradoxically not associated with a higher but rather with a lower mortality risk. This has been termed the obesity paradox. The study by Flegal et al extends these findings to the general population. The apparent paradox may be due to the use of the body mass index (BMI; calculated as weight in kilograms divided by height in meters squared) 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. Flegal et al found an association between all-cause mortality and overweight and obesity by using an inaccurate method—BMI—for their classification. Villareal et al proposed a definition of obesity as “an unhealthy excess of body fat, which increases the risk of medical illness and premature mortality.” Direct estimates of total fat mass should provide a more accurate body assessment. It has been shown that, for the general population, in addition to BMI, waist circumference and waist-to-hip ratio are of importance for assessing mortality risk. Consequently, even though it is widely accepted, classifications of obesity based on BMI are inadequate.