Rochester, MN - New ways of distinguishing excess body fat from lean mass to identify truly obese patients are needed to understand the link between body weight, mortality, and cardiovascular risk, new research suggests [1]. A new meta-analysis combining information on more than 250 152 patients with CAD and BMIs ranging from low to severely obese indicates that it may be time to scrap BMI as a risk factor.

"The main problem is the way obesity is defined," senior author on the paper, Dr Francisco Lopez-Jimenez (Mayo Clinic, Rochester, MN), told heartwire. "BMI is nothing but how much you weigh divided by a factor of your height. And that assumes that weight is body fat, and if you do develop some muscle, that doesn't count."

The study appears in the August 19, 2006 issue of the Lancet.

Lead author Dr Abel Romero-Corral (Mayo Clinic) and colleagues conducted their analysis by combining 40 studies that had a mean follow-up of 3.8 years and included data on BMI as well as mortality and cardiovascular events. They report that patients with a low BMI (<20) had an increased relative risk for total mortality and cardiovascular mortality compared with normal-weight individuals (BMI 20-24.9). Likewise, patients with the highest BMIs (>35) had the highest risk for cardiovascular mortality, but no increased risk of overall mortality. By contrast, overweight individuals (BMI 25-29.9) had a lower risk for both end points, while obese patients (BMI 30-34.9) appeared at no greater or lesser risk than normal-weight individuals.

Explaining the findings, Lopez-Jimenez points out, "In patients with heart disease, where the mean age is about 55 or 60 years, many of them are pretty sedentary and therefore have very limited muscle mass, a good amount of fat, but are still in the lean category according to BMI. But on the other hand, patients may have muscle mass and little amounts of fat but be classified in the clinically overweight category."

Far from proving that obesity is harmless, the findings suggest that alternative methods are needed to better characterize individuals who are truly fat as compared with carrying more weight in the form of muscle, the authors write. "Additional studies with different methods are needed, including randomized clinical trials with different weight-loss strategies or using other ways to identify obesity, such as body-composition techniques to measure body-fat percentage and distribution that also accounts for lean mass."

There are very expensive tests for people with heart disease . . . but for something that seems to be very important—that is, how much fat you have—we still rely on something that is, in my point of view, so simplistic.

The most practical way, says Lopez-Jimenez, is to measure waist circumference or waist-to-hip ratio. "A patient who has excess fat will rarely have a small waist," he told heartwire, adding that, in the paper, the few studies that actually measured waist circumference or waist-to-hip ratio showed that a larger waist was linked to worst outcomes, "which makes sense with everything else."

He continued: "Probably what patients need is to have a more comprehensive assessment of body fat. There are very expensive tests for people with heart disease, . . . but for something that seems to be very important—that is, how much fat you have—we still rely on something that is, in my point of view, so simplistic."

Still unknown is the unique contribution of higher muscle-to-fat ratio, which may be merely a surrogate of increased physical fitness. Future research is needed to assess the link between high muscle mass, high BMI, and clinical outcomes, Lopez-Jimenez said.

In an accompanying editorial, Dr Maria Grazia Franzosi (Instituto Mario Negri, Milano, Italy) points out that BMI is the most easily measured proxy for obesity but, agreeing with the study authors, believes it is not sophisticated enough to provide important information [2].

Uncertainty about the best index of obesity should not translate into uncertainty about the need for a prevention policy against excess body weight.

However, Franzosi also points out that the mean follow-up of 3.8 years may not be sufficient to fully appreciate the link between being underweight, overweight, or obese on progression of CAD and, ultimately, mortality.

"BMI can definitely be left aside as a clinical and epidemiological measure of cardiovascular risk for both primary and secondary prevention," Franzosi writes. "Uncertainty about the best index of obesity should not translate into uncertainty about the need for a prevention policy against excess body weight, which must be strongly supported."

Interestingly, the findings, while corroborated by other recent studies, fly in the face of the established lower-is-better dogma for BMI, only now being overturned. Lopez-Jimenez told heartwire that the Lancet meta-analysis actually mirrors an earlier study done by the same group ultimately published in 2004, which showed overweight patients to have lower mortality and similar risk of cardiovascular events when compared with normal-weight patients [3].

"At that time, we had a hard time publishing that paper because it was completely against the beliefs of most people, including the editors of many journals. They would send it back to us saying, this doesn't make sense."

If a patient has a BMI of 39, that's not muscle mass.

Asked whether the study justifies sidelining BMI when calculating cardiovascular risk, Lopez-Jimenez pointed out that it has little utility in different groups, but for different reasons. In people with a BMI over 35, he said, actually calculating BMI, or for that matter using more precise tools for measuring body fat, is a waste of time: "You don't need anything fancy to say that someone truly has a lot of fat," he explained. "If a patient has a BMI of 39, that's not muscle mass."

Where more sophisticated tools are needed, he says, is in patients with a BMI lower than 30 or 35, where the balance of muscle and fat may be more important.

Very low BMI also warrants close scrutiny, he told heartwire. "Patients who have a very low BMI may have the wrong idea that they are perfectly fine, when probably what they have is no muscle mass at all. We should not be so content, just because we have a very skinny patient. The typical example is the little old lady, 75 years old, who is very skinny. I don't think that is the ideal situation, because her BMI is low because she has no muscle mass at all."