Some Obese Individuals Appear Metabolically Healthy

August 2008 - Some obese individuals do not appear to have an increased risk for heart disease, while some normal-weight individuals experience a cluster of heart risks, according to two reports in the August 11/25 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.

"The prevalence of obesity is increasing worldwide, and this epidemic is accompanied by a high incidence of type 2 diabetes mellitus and cardiovascular disease," the authors write as background information in one of the articles. Research indicates that in addition to overall obesity, the way body fat is distributed may influence risk for heart disease and diabetes. For instance, individuals with fat within the abdominal cavity-estimated by measuring waist size-appear to be at higher risk for insulin resistance (a pre-diabetic condition that occurs when the body fails to respond to the hormone insulin) and for having an unhealthy cardiovascular risk profile.

In one study, Norbert Stefan, M.D., and colleagues at the University of Tbingen, Germany, studied 314 individuals age 18 to 69 (average age 45). The researchers measured participants' total body fat, visceral fat (abdominal fat around the internal organs) and subcutaneous fat (fat under the skin) using magnetic resonance tomography. Insulin resistance was measured using an oral glucose tolerance test. The individuals were then divided into four groups: normal weight, overweight, obese but still sensitive to insulin and obese with insulin resistance.

Those in the overweight and obese groups had more total body and visceral fat than those at a normal weight, and there was no difference between obese groups.

However, obese individuals with insulin resistance had more fat within their skeletal muscles and their livers than obese individuals without insulin resistance. In addition, those who were insulin-resistant had thicker walls in their carotid arteries, an early sign of atherosclerosis (narrowing of the arteries, a heart disease risk factor).

Individuals in the obese-insulin sensitive group did not differ from the normal-weight group in insulin sensitivity or artery wall thickness, the authors note. "In conclusion, we provide evidence that a metabolically benign obesity can be identified and that it may protect from insulin resistance and atherosclerosis," they write. "Furthermore, our data suggest that ectopic [misplaced] fat accumulation in the liver may be more important than visceral fat in the determination of such a beneficial phenotype in obesity."

In a second study, Rachel P. Wildman, Ph.D., of the Albert Einstein College of Medicine, Bronx, N.Y., and colleagues assessed body weight and cardiometabolic abnormalities (including high blood pressure, elevated triglycerides and low high-density lipoprotein or "good" cholesterol) in 5,440 individuals participating in the National Health and Nutritional Examination Surveys between 1999 and 2004. Participants were considered metabolically healthy if they had none or one abnormality and metabolically abnormal if they had two or more abnormalities.

"Among U.S. adults 20 years and older, 23.5 percent (approximately 16.3 million adults) of normal-weight adults were metabolically abnormal, whereas 51.3 percent (approximately 35.9 million adults) of overweight adults and 31.7 percent (approximately 19.5 million adults) of obese adults were metabolically healthy," the authors write. Normal-weight individuals with metabolic abnormalities tended to be older, less physically active and have larger waists than healthy normal-weight individuals. Obese individuals with no metabolic abnormalities were more likely to be younger, black, more physically active and have smaller waists than those with metabolic risk factors.

"These data show that a considerable proportion of overweight and obese U.S. adults are metabolically healthy, whereas a considerable proportion of normal-weight adults express a clustering of cardiometabolic abnormalities," the authors write.

"Further studies into the behavioral, hormonal or biochemical and genetic mechanisms underlying these differential metabolic responses to body size are needed and will likely further the identification of possible obesity intervention targets and improve cardiovascular disease screening tools."

Arch Intern Med. 2008;168[15]:1609-1616 and 1617-1624

Source: American Medical Association (AMA)