Chubby bellies deprive their owners of more than a svelte figure, according to new research. Abdominal flab in middle-aged rats causes insulin resistance, a common precursor to diabetes that afflicts old people disproportionately. Surgically removing the fat restores insulin function, suggesting that the condition develops because fat accumulates around the internal organs.
As people age, they acquire beer bellies and love handles. Their bodies also lose the ability to absorb glucose from the blood, a condition called insulin resistance. Circumstantial evidence links insulin resistance to visceral fat, the blubber found in bellies, but not to subcutaneous fat, which molds love handles. Researchers have debated whether visceral fat is a byproduct or the cause of insulin resistance.
To investigate, Nir Barzilai and colleagues at Albert Einstein College of Medicine in New York City cut out fat from adult rats with insulin resistance. When subcutaneous fat was removed, the rats fared no better. But when visceral fat was removed, the rats recovered and began to extract glucose from their bloodstream just as well as intact young ones did, the team reports in the October issue of Diabetes. The results suggest that visceral, but not subcutaneous, fat cripples insulin function.
To determine how fat might alter glucose uptake, the researchers measured the production of blood-borne molecules that fat cells secrete. They found that removal of visceral fat changes the signals sent out by subcutaneous fat cells. They produced about two-thirds less of the messenger RNA templates for the hormone leptin, which controls appetite and is overproduced in insulin-resistant people. There was also less TNF-a, a protein thought to stymie insulin by blocking its biochemical pathways. This observation implies that visceral fat causes insulin resistance by changing the output of molecules made by a wide range of fat tissues.
"This is the first demonstration that I know of that removal of visceral fat fixes the resistance problem," says endocrinologist Michael Schwartz of the University of Washington, Seattle. Endocrinologist Robert Schwartz of the University of Colorado Health Sciences Center in Denver suggests how diet could factor into the equation: calorie restriction--which is known to reduce insulin resistance--might alter metabolism by influencing the types of fat that calories build.