Although it is well established that obesity ups the risk of numerous health problems, including type 2 diabetes, osteoarthritis and some types of cancer, it has also been observed that the lowest mortality is not associated with the lowest BMI. Rather, epidemiological studies have demonstrated that people with BMIs in the overweight or obese categories may have a decreased risk of death. New research investigates this association with respect to cardiovascular disease mortality, and also finds what may be a partial explanation.
In a meta-analysis of 36 studies of people who had cardiac revascularization procedures (procedures to improve blood supply to the heart) after a heart attack (MI), Dr. Abhishek Sharma from the Department of Medicine, Maimonides Medical Center, Brooklyn, NY and colleagues examined the relationship between BMI and the risk of death, and death from cardiovascular disease. Contrary to what one might expect, and in agreement with other epidemiological studies, the risk of these endpoints was highest in the underweight group (BMI < 20). After an average follow-up period of nearly 2 years, patients with a BMI of 25-29.9 (overweight) had the lowest risk of death from cardiovascular disease, and increasing BMI didn t increase that risk.
In a second study, Dr. Alban De Schuter from John Ochsner Heart & Vascular Institute, Ochsner Clinical School, The University of Queensland School of Medicine, New Orleans, LA and colleagues examined the relationship between body composition and mortality. In particular they were interested in explaining the association between increasing BMI and decreasing risk of mortality the obesity paradox.
These investigators assessed (from records) patients with normal left ventricular function (that is, no evidence of heart failure). They divided the patients into groups according to their BMI classifications, and for each one calculated body fat (BF) and a lean mass index (LMI = [1-BF] x BMI).
After a follow up of about 3 years, they analyzed the risk of death associated with these indices of body composition. First, they found that a higher BMI was associated with significantly lower mortality. Further, the higher LMI group had a significant 29 percent lower risk of mortality. For obese patients, they found that both BMI and BF were associated with higher mortality, and LMI was still found to be protective. These authors concluded that Body composition could explain the inverse J shape of the mortality curve noted with increasing BMI [obesity paradox]. Body fat seems to be protective in this cohort only if no adjustment was made for LMI. Lean mass index seems to remain protective in obese patients even when BMI is not.
ACSH s Dr. Ruth Kava comments While the first study above supports the findings of others that increasing BMI could be protective, the second provides at least a partial explanation. Aswe have explained, BMI does not distinguish between lean tissue and fat. She continued Thus to the extent that an increased BMI indicates increased lean tissue, one might reasonably expect that risk of disease and death could be attenuated. Since this was a retrospective study that relied on calculated, not measured values, it is to be hoped that future research will refine and extend these conclusions.