Among people with chronic obstructive pulmonary disease (COPD), usually emphysema, a low fat-free body mass index appears to be associated with greater mortality, Danish and UK researchers report.

There is a clear association between decreasing body mass index (BMI) and mortality both in the general population and in those with COPD, Dr. Jorgen Vestbo of Hvidovre Hospital and colleagues note. They say that, in general, the harmful effect of a low BMI is due to the low fat-free component of body mass.

The researchers point out that fat mass is a “metabolic inactive energy source,” but fat-free mass “contains the metabolic active organs, skeletal muscle being the largest of these organs.”

As they put it, “It seems biologically reasonable to assume that loss of or a low fat-free mass is unfavorable.”

To determine the association between fat-free body mass index distribution and COPD prognosis, the researchers used data from almost 1900 COPD patients who were followed for an average of 7 years.

In those with a normal BMI, 26 percent had a fat-free body mass index lower than that in the lowest 10 percent of the general population, the researchers report in the American Journal of Respiratory and Critical Care Medicine.

BMI and fat-free body mass index were associated with the likelihood of dying during the study period. The mortality rate was 57 percent among those with a fat-free body mass index below the 10th percentile, compared with 32 percent for those with a higher fat-free body mass index.

In light of these findings, the researchers conclude that “assessment of fat-free body mass provides important information in COPD and should be considered in the routine evaluation of patients with this condition.”

SOURCE: American Journal of Respiratory and Critical Care Medicine, January 1, 2006.