Thursday, August 31, 2006

Serious flaws in the latest big obesity study

The latest study of "obesity" has received what appears to be totally uncritical mention in the press so I think it is time I pointed out just some of the glaring problems with it. For starters, however, I reproduce the original journal abstract below:

Overweight, Obesity, and Mortality in a Large Prospective Cohort of Persons 50 to 71 Years Old

By Kenneth F. Adams et al.

Background: Obesity, defined by a body-mass index (BMI) (the weight in kilograms divided by the square of the height in meters) of 30.0 or more, is associated with an increased risk of death, but the relation between overweight (a BMI of 25.0 to 29.9) and the risk of death has been questioned.

Methods: We prospectively examined BMI in relation to the risk of death from any cause in 527,265 U.S. men and women in the National Institutes of Health-AARP cohort who were 50 to 71 years old at enrollment in 1995-1996. BMI was calculated from self-reported weight and height. Relative risks and 95 percent confidence intervals were adjusted for age, race or ethnic group, level of education, smoking status, physical activity, and alcohol intake. We also conducted alternative analyses to address potential biases related to preexisting chronic disease and smoking status.

Results: During a maximum follow-up of 10 years through 2005, 61,317 participants (42,173 men and 19,144 women) died. Initial analyses showed an increased risk of death for the highest and lowest categories of BMI among both men and women, in all racial or ethnic groups, and at all ages. When the analysis was restricted to healthy people who had never smoked, the risk of death was associated with both overweight and obesity among men and women. In analyses of BMI during midlife (age of 50 years) among those who had never smoked, the associations became stronger, with the risk of death increasing by 20 to 40 percent among overweight persons and by two to at least three times among obese persons; the risk of death among underweight persons was attenuated.

Conclusions: Excess body weight during midlife, including overweight, is associated with an increased risk of death.

1). What most glaringly identifies the sample concerned as not a random one is of course the percentage of women. A random sample would comprise about 50% women but there were actually twice as many men as women in this sample. So the population to which the findings may be generalized is essentially unknown, though a guess that it is a population who were worried about their health would probably not be too far astray.

2). The results were "adjusted" for physical inactivity. That is entirely inappropriate. Overweight people undoubtedly exercise less so the adjustment in effect creates an artificial population with no relevance to the real world. It is also possible that the adjustment for alcohol intake was inappropriate.

3). The overall results were, as usual, that people of middling weight lived longest. It was only in selected subsets of the sample that people of middling weight died somewhat younger. It is those subsets, however, that have attracted most media attention. If it were my practice in my own research to generalize from arbitrary subgroups of non-samples, I could prove anything too.

4). BMI is now in any case a rather contentious index of "obesity", for the amusing reason that in some populations it shows that overweight people live longer, as indeed it did in the present study.

5). The article looks at obesity at only one point in the lifespan. Weight tends to increase unevenly with age so that some people become overweight in later life who were not previously so. So what is true of those who are overweight in later life may not at all be true of (say) childhood obesity, and vice versa. This is a lacuna rather than a flaw in the study but it is yet another reason why the results of the study should not be generalized.

Since the conclusion given in the journal abstract is wildly inappropriate to the data, however, the media can hardly be blamed for their dramatizations, for once. It would seem that the prestigious academic journal -- NEJM -- in which the study appeared has gone the way of the BMJ in becoming a largely politically correct organ.

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