BMI Fails as a Cardiovascular Risk Factor - CME Teaching Brief® - MedPage Today:

"BMI Fails as a Cardiovascular Risk Factor

By Judith Groch, Senior Writer, MedPage Today
Reviewed by Zalman S. Agus, MD; Emeritus Professor at the University of Pennsylvania School of Medicine.
August 18, 2006

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ROCHESTER, Minn., Aug. 18 -- Because body mass index (BMI) cannot discriminate between fat and lean mass, it cannot reliably predict the outcome for patients with coronary artery disease, according to researchers here.
In fact, in a meta-analysis of 40 studies, including 250,152 patients with established coronary artery disease, outcomes for cardiovascular and total mortality were better for overweight and mildly obese groups compared with normal-BMI patients, found a study reported in the Aug. 19 issue of The Lancet. Adjustment for confounding factors did not change the findings.
"Rather than proving that obesity is harmless, our data suggest that alternative methods might be needed to better characterize individuals who truly have excess fat, compared with those in whom BMI is raised because of preserved muscle mass," said Francisco Lopez-Jimenez, M.D., of the Mayo Clinic here, and colleagues.
The meta-analysis, with a mean follow-up of 3.8 years, included studies with sufficient information to allow the calculation of unadjusted and adjusted relative risks or studies that gave the actual risk estimate for each BMI group, said Dr. Lopez-Jimenez. Most data were collected in the late 1980s and 1990s.
Coronary artery disease was defined as history of percutaneous coronary intervention, coronary artery bypass graft, or myocardial infarction. BMI was used as a measure of obesity.

The findings were:

Patients with a low BMI) (<20) had a one-third increased relative risk (RR)
for total mortality (RR=1.37, 95% CI 1.32-1.43), and cardiovascular mortality
(RR 1.45, CI 1.16-1.81) compared with risks for normal-BMI patients who served
as the reference group.
Overweight patients (BMI 25-29.9) had the lowest
risk for total mortality (RR=0.87, CI 0.81-0·94) and cardiovascular mortality
(RR=0.88, CI 0.75-1.02).
Obese patients (BMI 30-35) had no increased risk
for total mortality (RR=0.93, CI 0.85-1.03) or cardiovascular mortality (RR
0.97, CI 0.82-1.15). However, obesity was associated with higher total mortality
specifically among patients with a history of coronary artery bypass surgery.
Severely obese patients (≥35) did not have an increase in total mortality
(RR 1.10, CI 0·87-1.41), but they had an almost 90% higher risk for
cardiovascular mortality (RR 1.88 CI, 1.05-3.34).

These findings, the researchers wrote, could be explained by the failure of BMI to discriminate between body fat and lean mass. The better outcomes for overweight people may be because they have more muscle than normal-weight people.
In addition, lower BMI values have been related to low lean body mass, and BMI might not adequately reflect adiposity, the authors wrote.
Also, they suggested, because low and normal BMI groups were almost consistently associated with a lower prevalence of established cardiovascular risk factors, these groups were less likely to receive effective secondary prevention therapies, such as exercise, a healthy diet, and treatment for other risk factors.
Finally, they said, extensive data have shown that central obesity poses a greater risk for cardiovascular disease than BMI.
Among the study's limitations, the researchers mentioned the lack of individual data and possible publication bias (papers suggesting no connection between BMI were not submitted or accepted). However, this bias was probably not a factor in their study, they said. Finally, and most important, they wrote, is the substantial heterogeneity of the results, making it difficult to detect small differences across studies.
From a clinical standpoint, the researchers said, a fundamental question is whether weight loss or maintaining a normal weight can decrease cardiovascular event in patients with coronary artery disease.
Because exercise and diet are the main components of a weight-loss program, it can be assumed that interventions for overweight patients with coronary disease will reduce cardiovascular outcomes, including mortality. However, Dr. Lopez-Jimenez said, no randomized controlled trials have yet been undertaken to address this relationship.
Additional studies with different methods are needed, he said, particularly those with different weight-loss strategies and those that use other methods of identifying obesity. These might include body composition techniques to measure body fat and distribution that also account for lean mass.
Pending further information, patients with coronary artery disease who are truly obese should be encouraged to pursue interventions that reduce body fat, the researchers said.
In an accompanying comment in the same Lancet issue, Maria G. Franzosi, M.D., of the Istituto Mario Negri in Milan, Italy, asked the question: "Is the debate on the relation between BMI and mortality over?" Her answer: "The meta-analysis does not provide new information, but some useful implications can be drawn from it. BMI can definitely be left aside as a clinical and epidemiological measure of cardiovascular risk for both primary and secondary prevention."
She explained that the "BMI is not a good measure of visceral fat, the key determinant of metabolic abnormalities that contribute to cardiovascular risk. Estimates of the effect of obesity based on BMI are therefore too low."
She also pointed out that "the better outcomes in overweight and mildly obese people might be because these individuals have a greater lean mass than normal weight and severely obese people. An increased lean mass is related to physical activity and independently contributes to reduced coronary artery disease risk."
The contribution of body fat to cardiovascular risk requires integrated basic research to which retrospective analyses of existing databases cannot add relevant insights, she wrote. However, she emphasized, "Uncertainty about the best index of obesity should not translate into uncertainty about the need for a prevention policy against excess bodyweight, which must be strongly supported."