Weighty issues

By Dr. Ananya Mandal, MDDec 2 2010

A new study shows that the ideal body mass index or BMI is between 20 and 24.9. BMI is calculated by dividing weight in Kilograms by the square of height in meters. The study authors examined nearly 1.5 million adults and found that even being slightly overweight but not obese, are enough to raise the risk of an early death.

Michael Thun, a co author, said, “Having a little extra meat on your bones—if that meat happens to be fat—is harmful, not beneficial.” The team looked at 19 studies involving mostly white adults, and noted each person's body mass index and whether he or she died during the study period. Those who were overweight but not obese were 13 percent more likely to die prematurely than their normal-weight counterparts. They noted that the “healthiest” BMI was between 20 and 24.9 which may vary depending on race or ethnicity. “Overweight” BMIs range from 25 to 30; “obese” from 30 to 40; and “morbidly obese” from 40 and above.

The team was led by Amy Berrington de Gonzalez, an investigator at the National Cancer Institute in Bethesda, who said, “We conducted the study to try and clarify the relationship between BMI [body mass index] and all-cause mortality, in part to answer two questions: what the optimal BMI range is and the risk associated with being overweight, or having a BMI of 25 to 30… There’s particular uncertainty with regard to this second question.” The team she said selected only subjects who were healthy non-smokers. “Both poor health and smoking can bias the relationship between BMI and mortality… It’s one of the possible explanations of the previous study findings that found the protective effect of being overweight,” she explained.

The paradoxical protective effects of being overweight were also explained by Dr. Robert Kushner, clinical director of Northwestern Memorial Hospital's Comprehensive Center on Obesity in Chicago. He said, “In certain subgroups, such as people with certain types of cancers, there may be a protective effect of being overweight.” Dr. Phillip Schauer, director of The Cleveland Clinic Bariatric and Metabolic institute said, “Some of these studies that found that being slightly overweight was protective didn’t necessarily assess for the type of body fat… For example, fat on the belly is worse than fat on the buttocks or thighs.”

The findings were published on Thursday in the New England Journal of Medicine.

However a biostatistician said that the association between overweight participants and mortality is small, and is likely due to confounding variables. Bruce Levin, professor and chair of the Department of Biostatistics at Columbia University's Mailman School of Public Health in New York said, “What [the authors] don’t point out is that the study model they used might also incompletely adjust for unmeasured confounding factors.” De Gonzalez agreed to these errors and said, “The increased risk for overweight people was small… You can never rule out confounding, but we ruled out everything we had information on.”

Assessment of only white adults may be another limitation. Schauer said, “Different ethnic groups, especially Native Americans, African Americans and people from South Asia seem to tolerate obesity less and are more prone to obesity-related comorbidities… The association might be stronger in those groups.”

The study also found an association between being underweight and a higher risk for mortality. Schauer said, “With a BMI below normal, we see a rise in mortality. People who have cancer could lose body mass… It could be an indicator they have another chronic illness.”

In conclusion mast experts agree that the message is clear. “The association between BMI and premature mortality has been known for quite a while, but it strengthens the whole argument,” said Schauer.