In some older adults, a normal body mass index (BMI) may be hiding an elevated risk of death from cardiovascular disease, Geisel researchers have found.
Assistant professor of medicine John Batsis led a team of researchers in a study of a sample of 1,528 individuals, age 60 and over, from the National Health and Nutrition Survey (which had a 12.9-year follow-up, on average). All individuals had their percentage of body fat tested using a bioelectrical impedance analyzer, and, according to their BMI, all were at normal weight. BMI is calculated as a ratio of weight to height.
Batsis and his team looked at all-cause and cardiovascular mortality rates and cardiovascular risk factors for those with normal weight and differing degrees of percent of body fat. They compared people with normal weight obesity (NWO) (that is, people who were not obese or overweight according to their BMI, but according to bioelectrical impedance had a high percentage of body fat and would be considered obese) to those with lower levels of body fat.
Batsis found no difference in overall mortality between people with NWO and those with lower degrees of body fat. But there were some important differences between the two groups. Women with NWO had higher short-term cardiovascular mortality rates compared to women with lower body fat. And for men, the rate of long-term cardiovascular mortality was about four times higher for those with NWO than for those with lower body fat.
The study has serious implications. Women 60 and over who have a normal weight according to their BMI but have a high percentage of body fat may be at higher risk of early cardiovascular death. "By ignoring this sub-group, one potentially overlooks targeted therapeutic approaches that could attenuate morbidity and mortality, even in older people," the researchers wrote in the American Journal of Cardiology.
"You need to be able to identify these patients [with NWO] so you can aggressively manage their risk factors," Batsis says. "BMI, particularly in older adults, is a poor surrogate for obesity because there are changes in body composition it does not account for, including fat and adequate muscle mass."
Batsis and his colleagues believe there needs to be a better tool than BMI to measure obesity. "The question here is, is waist circumference any better? Is waist-hip ratio any better? We looked at [these measures] in separate studies and the jury is still out," he says. "It's likely a combination of the two."