Two new studies indicate that it's the location of the fat that matters because people of normal weight may be having fat at unwanted locations in their bodies.
In the first study, German researchers analyzed 314 people, ages 18 to 69, and divided them into four groups: normal weight, overweight, obese but still sensitive to insulin, and obese with insulin resistance.
People in the overweight and obese groups had more total body and visceral fat (abdominal fat around the organs) than those with normal weight.
But obese people with insulin resistance had more fat within their skeletal muscles and their livers than obese people without insulin resistance.
Obese people with insulin resistance also had thicker walls in the carotid arteries, an early sign of narrowing of the arteries -- a condition called atherosclerosis.
Insulin sensitivity and artery wall thickness were the same in obese people without insulin resistance and in normal-weight people.
"In conclusion, we provide evidence that a metabolically benign obesity can be identified and that it may protect from insulin resistance and atherosclerosis," the authors said.
"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," they added.
In a second study, researchers at the Albert Einstein College of Medicine, 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 said.
The studies are published in the August 11/25 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.