BMI: DOA?

Researchers at the Mayo Clinic in Minnesota recently reviewed 40 studies comprising over 250,000 heart patients; average follow-up time was just under four years. Their conclusion? The Body-Mass Index (BMI) is not useful in predicting mortality in patients with coronary artery disease (CAD). More to the point, fatter people with CAD don’t die at higher rates than leaner ones, with the exception of morbidly obese people, whose relative risk of death from cardiac events was 1.88 times the norm of patients studied.
The findings were published in this week’s issue of The Lancet, which, if you’ve never heard of it, is the British answer to the New England Journal of Medicine in terms of prestige and overall reputation. You can read the summary here, but you have to register (it’s free) at the Lancet site first.


The researchers, who emphasized that obesity and overweight are still grave risk factors for various medical problems, concluded that the reason the BMI fails to correlate well with outcomes in this population is that it fails to discriminate between lean body mass (i.e., muscle) and blubber.
The researchers know a lot more than I do, and I haven’t read the whole study, but I admit to a bit of skepticism here. Most people with high BMIs — in fact, most people, period — are not heavily muscled. Patients with CAD and high BMIs are even less likely than younger, healthy people to be built like linebackers. I’d be willing to bet that close to 100% of the high-BMI patients studied by the Mayo team were, or are, simply fat. And if I had to wager a guess — and if Orac or one of the other medical types reads this, maybe he or she can jump in — I would further speculate that the higher mortality in low-BMI CAD patients was tied to factors such as the wasting that occurs in late-stage congestive heart failure and other co-morbidities. But the study summary notes that the team, not surprisingly (the Mayo isn’t generally associated with amateur hour), controlled for confounding factors.
Naturally, Big Fat Blog is pleased to report these findings, but as usual muddles some important details. Paul McAleer correctly notes that the Reuters article to which he links gives an annual-deaths-from-obesity-figure — 300,000 – that the Centers for Disease Control (CDC) lowered to 112,000 over a year ago. But his claim that “the BMI is useless as used today to gauge one’s health” is overreaching and is certainly not supported by the findings of the Mayo group, which looked only at heart patients. Apparently some folks have forgotten that people become ill and die from numerous health problems experienced at far higher rates in obese and overweight persons than in leanfolk.
That said, were I fat, inclined toward righteousness, and not all that inclined to dig below the surface, I too would be wondering why the researchers were so adamant about insisting that excess weight is pestilential when their own study suggested just the opposite, at least with respect to one organ system; that the researchers could only speculate about the reason(s) for their findings makes their claims about blotation seem all the more shaky.

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  1. #1 by natural cynic on August 19, 2006 - 6:12 pm

    There are some problems with the supposed take-home message of this study. The Lancet study was a meta-analysis of people with pre-existing cardiovascular disease, therefore its applicability to the general population is limited. And it is also limited to those with BMI’s less than 35, which cuts out the morbidly obese.
    Incidentally, there is also a fairly large study out in American Journal of Clinical Nutrition of individuals over the age of 75 that concluded that there was little difference in overall death rates for those who have BMI’s less than 35 except for the expected higher death rates among those who have the lowest BMI.
    There are two important things that must be considered. One that is probably trivial in these reports is the effect of muscularity on BMI. Someone who is lean yet has highly developed muscles will be in the overweight or obese category [BMI’s of 25-30 and 30-35, respectively]. It is probably intersting to note that some who originally had a high muscularity when young – american football players – do have a significantly higher cardiovascular death rate.
    The most important thing to consider is where the fat is. BMI does not discriminate betweeen those with peripheral adiposity, which has been shown to be relatively benign, and central adiposity, which definately increases cardiovascular risk. If the fat is mostly located on the hips and thighs, it does not have any direct effect on the blood markers for CVD [cholesterol fractions, triglycerides, hyperinsulinemia]. There is one possible indirect effect – arthritis that results in hypomobility. In contrast, central obesity, which is due to intraperitoneal fat appears to be related to a number of CVD risk factors – hyperlipidemia, hypercholesterolemia, insulin resistance, hypertension and possibly hyperinflammatory markers. Central obesity is the typical pot belly and can easily be measured with a waist circumference or waist/hip ratio.
    so…
    [snark]lean – the good
    pot belly – the bad
    thunder thighs – … [/snark]

  2. #2 by Kevin Beck on August 19, 2006 - 6:20 pm

    Thanks, natural cynic. I missed the fact that people with BMIs > 35 were excluded, so my use of the term “morbidly obese” (40 and above, I believe) was inappropriate; what I should have written is that those patients with the highest BMIs among those studied had a RR of cardiac death of 1.88.
    If the BMI were scrapped in favor of DEXA scanning to assess true subcutaneous and intraperitoneal fat, I don’t doubt that the correlations now observed between BMI and certain disease states would only be strengthened and that those areas in which the the BMI seems less applicable would be amenable to DEXA-result/disease analyses. But obviously this is not feasible.

  3. #3 by natural cynic on August 19, 2006 - 7:42 pm

    Isn’t DEXA used primarily for bone scanning for osteoporosis? A CAT scan will illuminate intraperitoneal and subcutaneous fat if real measurements are needed. But, for most diagnostic purposes, it is not necessary, since a waist circumference combined with blood lipids will usually make an open-and-shut case for excess i.p. fat depots.

  4. #4 by Mouth of the Yellow River on August 20, 2006 - 12:39 am

    Ni Hao! Kannichi Wa!BMI/central obesity is only one of an increasing number of whole organism parameters of metabolic syndrome (syndrome X) whose etiology is chronic positive imbalance of calories in versus calories burned.Whether the imbalance of where the excess calories come from–fat, carbohydrate or protein–make a difference is a second generation question.BMI is a powerful fascination because it is visual, superficial, looks versus unseen parameters of chronic high blood pressure, high LDL/HDL, high cholesterol, glucose intolerance, insulin resistance (type II diabetes) and fatty liver diseases.Each of these are symptoms of which we do not know whether they are independent, upstream or downstream of the others in terms of cause and effect.Were the dietary habits of these patients, e.g. calories in versus calories out, source of calories, and other parameters of metabolic syndrome measured?Western and developing societies are faced with a major dilemma of which only the tip of the iceberg was approached in Morgan Spurlock’s Super-Size Me. Since the early 20th century depression years mind over matter and more than ever today moves onward. But how much does the virtual mind depend on the calories in/calories out ratio of its whole body housing?Metabolic Syndrome is likely to be an adaptive response intrinsic to current stage of societal evolution in eating and exercise habits. The degree of reversibility versus irreversible imprinting is a key unknown as well as its inheritability.MOTYR

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