Body mass index (BMI) - the commonly used measure of obesity - cannot reliably predict the outcome for patients with heart disease, concludes an Article in this week's issue of The Lancet.
This is because BMI is an unreliable indicator of obesity, say the researchers.
Doctors already know that obesity is a risk factor for developing heart disease. However, how obesity affects people with established heart disease has been unclear because previous studies have had contradictory results.
To investigate, Francisco Lopez-Jimenez (Mayo Clinic College of Medicine, Rochester, MN, USA) and colleagues combined data from 40 studies involving about 250,000 people with heart disease; the average follow-up was four years. Most of the studies used BMI as a measure of obesity.
The investigators found that patients with a low BMI had a higher risk of death than those with a normal BMI. Overweight patients had better survival and fewer heart problems than those with a normal BMI. Obese people who had had bypass surgery had a higher death rate when compared with people with a normal BMI, while severely obese people had a higher risk of a heart-related death but not death from other causes.
The better outcomes for overweight people may be because they have more muscle than normal weight people, state the authors. The results therefore demonstrate the inability of BMI to discriminate between body fat and lean muscle, they conclude.
"Rather than proving that obesity is harmless, our data suggest that alternative methods might be needed to better characterise individuals who truly have excess body fat, compared with those in whom BMI is raised because of preserved muscle mass," explains Dr Lopez- Jimenez.
In an accompanying Comment Maria Grazia Franzosi (Istituto Mario Negri, Milan, Italy) states: "BMI can definitely be left aside as a clinical and epidemiological measure of cardiovascular risk . . . 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."