We’ve been awash in studies that allegedly show that obesity is either not as harmful as thought or that it isn’t really harmful at all. Most of these studies were done by examining mortality rates along with the person’s body mass index (BMI) at the time of death. But you can see the problem here. If someone were to have, say, a BMI of 36, which is morbidly obese, and then develop cancer or heart disease, they might easily lose weight, since weight loss is a feature of many illnesses, Then if they lost enough and died with a BMI of 24, which is normal, the study records a death at normal weight, not a death at a BMI of 36. Also, when someone is older, they are more likely to be both ill and to have lost weight.

One researcher found an ingenious way around this by looking at maximum lifetime weight to study mortality risks. The abstract of this paper follows.

Using maximum weight to redefine body mass index categories in studies of the mortality risks of obesity

The high prevalence of disease and associated weight loss at older ages limits the validity of prospective cohort studies examining the association between body mass index (BMI) and mortality.

I examined mortality associated with excess weight using maximum BMI—a measure that is robust to confounding by illness-induced weight loss. Analyses were carried out on US never-smoking adults ages 50-84 using data from the National Health and Nutrition Examination Surveys (1988-1994 and 1999-2004) linked to the National Death Index through 2006. Cox models were used to estimate hazard ratios for mortality according to BMI at time of survey and at maximum.

Using maximum BMI, hazard ratios for overweight (BMI, 25.0-29.9 kg/m2), obese class 1 (BMI, 30.0-34.9 kg/m2) and obese class 2 (BMI, 35.0 kg/m2 and above) relative to normal weight (BMI, 18.5-24.9 kg/m2) were 1.28 (95% confidence interval [CI], 0.89-1.84), 1.67 (95% CI, 1.15-2.40), and 2.15 (95% CI, 1.47-3.14), respectively. The corresponding hazard ratios using BMI at time of survey were 0.98 (95% CI, 0.77-1.24), 1.18 (95% CI, 0.91-1.54), and 1.31 (95% CI, 0.95-1.81). **The percentage of mortality attributable to overweight and obesity among never-smoking adults ages 50-84 was 33% when assessed using maximum BMI. The comparable figure obtained using BMI at time of survey was substantially smaller at 5%. The discrepancy in estimates is explained by the fact that when using BMI at time of survey, the normal category combines low-risk stable-weight individuals with high-risk individuals that have experienced weight loss. In contrast, only the low-risk stable-weight group is categorized as normal weight using maximum BMI.**

Use of maximum BMI reveals that estimates based on BMI at the time of survey may substantially underestimate the mortality burden associated with excess weight in the US.

The mortality risk of obesity has very likely been greatly underestimated. The key point is that some illnesses cause weight loss, thus skewing the statistics, i.e. more normal weight people ill, fewer overweight. Eliminate that problem, and the true risk of obesity rises.

The author found, using the maximum BMI method, that obesity accounted for a full 33% of all adult deaths in his sample, while using BMI at time of death leads to an estimation of only 5%.

Don’t let anyone tell you the sophistry that obesity isn’t all that bad.