Tuesday, August 12, 2008

More on Paul Campos

One thing that still bothers me about Paul Campos's book The Obesity Myth is that he focuses almost entirely on BMI studies to make his case that bodyfat isn't relevant to health or mortality. He constantly points out that there's a U-shaped curve in which underweight people (by BMI) have higher mortality, people technically classified as moderately overweight (by BMI) have lower mortality overall, and obese people then have the highest mortality.

But BMI studies don't tell us whether bodyfat is healthy or unhealthy. That's because BMI doesn't correlate very well with bodyfat in the first place. The most recent study I could find was this: Romero-Corral et al, "Accuracy of body mass index in diagnosing obesity in the adult general population," International Journal of Obesity 32 no. 6 (June 2008): 959-66. From the abstract:
Body mass index (BMI) is the most widely used measure to diagnose obesity. However, the accuracy of BMI in detecting excess body adiposity in the adult general population is largely unknown.

METHODS: A cross-sectional design of 13,601 subjects (age 20-79.9 years; 49% men) from the Third National Health and Nutrition Examination Survey. Bioelectrical impedance analysis was used to estimate body fat percent (BF%). We assessed the diagnostic performance of BMI using the World Health Organization reference standard for obesity of BF%>25% in men and>35% in women. We tested the correlation between BMI and both BF% and lean mass by sex and age groups adjusted for race.

RESULTS: BMI-defined obesity (> or =30 kg m(-2)) was present in 19.1% of men and 24.7% of women, while BF%-defined obesity was present in 43.9% of men and 52.3% of women. . . . However, in the intermediate range of BMI (25-29.9 kg m(-2)), BMI failed to discriminate between BF% and lean mass in both sexes. CONCLUSIONS: The accuracy of BMI in diagnosing obesity is limited, particularly for individuals in the intermediate BMI ranges, in men and in the elderly. A BMI cutoff of> or =30 kg m(-2) has good specificity but misses more than half of people with excess fat. These results may help to explain the unexpected better survival in overweight/mild obese patients.
Note the bolded text, because it rather starkly undermines Campos's whole argument that because BMI isn't linearly correlated to mortality, people shouldn't worry about being fat. BMI /= bodyfat.

What's more, there's a whole area of medical literature that, as far as I can tell, Campos's book completely ignores (again, it was a very poor endnoting system). Researchers have found for years that bodyfat cells produce inflammatory agents (such as C-reactive protein or its precursors).

One recent study isolated this process experimentally, by studying specific fat cells extracted from plastic surgery patients and observing the excretion of cytokines and resistin; on exposing the fat cells to statins or aspirin, the production of inflammatory agents declined. Another similar study isolated higher levels of interleukin-6 -- another inflammatory marker -- in blood that came from visceral fat in the abdomen.

In turn, chronic inflammation seems to cause cancer, heart disease and strokes, etc.

So when we are learning more and more about the specific biological mechanisms by which fat cells make a person less healthy, it's a waste of time to point out how BMI (not an accurate measure of bodyfat) and mortality are correlated.

To be sure, Campos is basically right that except for the morbidly obese, it doesn't make sense to worry about lowering your BMI or even your bodyweight per se. But it's still true that people should think about lowering bodyfat (or, rather, eating a healthy diet and exercising with high intensity several times a week, which, in my experience, inevitably does the trick).


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