Metabolically normal obesity more common than previously thought

14 August 2008

MedWire News: Metabolically normal obesity is more common than previously thought and may be partly due to low ectopic fat in the liver, show results from two studies published in the Archives of Internal Medicine.

It has been observed previously that a subset of individuals who have an obese body mass index (BMI) (30 kg/m² and above) are "relatively resistant to the development of the adiposity-associated cardiometabolic abnormalities that increase cardiovascular disease risk," say researchers led by Rachel Wildman (Albert Einstein College of Medicine, Bronx, New York, USA).

Wildman and team investigated the correlation between BMI and incidence of cardiometabolic abnormalities such as dyslipidemia, hypertension, high glucose, insulin resistance, and systemic inflammation, in a group of 5440 US individuals. The participants were a cross-sectional sample from the National Health and Nutrition Examination Surveys 1999-2004.

The results from this study demonstrated that 51.3% of overweight (BMI 25-29.9 kg/m²) and 31.7% of obese (BMI of 30 kg/m² or above) US adults over the age of 20 years were metabolically healthy (with 1 or no metabolic abnormalities). Of note, the researchers also found that 23.5% of normal weight individuals were metabolically abnormal (with 2 or more metabolic abnormalities).

In the second study, which attempted to determine possible causes for metabolically benign obesity, Norbert Stefan (University of Tübingen, Germany) and fellow researchers assessed the fat distribution of 314 German men and women with normal, overweight, and obese (insulin sensitive and insulin resistant) BMIs.

Participants' total body, visceral, subcutaneous, liver, and skeletal muscle fat distribution were measured using magnetic resonance tomography and spectroscopy.

Stefan et al found that ectopic fat in skeletal muscle and the liver and carotid intima-media thickness were both lower in insulin sensitive (4.3% and 0.54 mm) versus insulin resistant (9.5% and 0.59 mm) obese individuals, whereas distribution of total body, visceral, and subcutaneous fat were unchanged between obese groups.

The authors suggest their results indicate "ectopic fat in the liver may be more important than visceral fat in the determination of such a beneficial phenotype in obesity."

Wildman and colleagues conclude: "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: 1607-1608, 1609-1616, 1617-1624