Obesity can trigger inflammation in the fat cells found just below the skin, creating an environment that has been linked with the development of both diabetes and heart disease, two new studies indicate.
The findings suggest that people need to worry about all types of body fat, not just the deeply embedded fat that earlier work had focused on. But the new work also hints that some face a higher risk than others.
In the body, most fat clusters under the skin in what’s known as subcutaneous adipose tissue. Much of the rest, called visceral fat, accumulates within muscle and between organs deeper inside the body. For more than a decade, studies have shown that obesity triggers visceral fat to begin spewing hormonelike chemicals called cytokines. These proinflammatory chemicals have been linked with metabolic syndrome, a constellation of abnormalities that can include impaired insulin sensitivity (known as insulin resistance), fat buildup around the waist, high blood pressure and low concentrations of HDL, the good cholesterol.
“There’s been this sort of ill-proven idea that subcutaneous adipose tissue is not harmful and that visceral adipose tissue is the vicious demon that makes us sick,” says Gökhan S. Hotamisligil of the Harvard School of Public Health in Boston, who was not involved in the studies. The new data, he says, reinforce the fact that subcutaneous fat is far from benign.
In one study, Ishwarlal Jialal of the University of California, Davis Medical Center in Sacramento and his colleagues sampled blood and subcutaneous fat from 65 men and women. Most were clinically overweight or obese, and 39 had metabolic syndrome.
Subcutaneous fat from people with metabolic syndrome was rife with macrophages, a type of immune cell that normally kills unwanted cells and invaders. But here, Jialal says, macrophages “appeared to be conspiring with fat cells to produce a host of proinflammatory cytokines,” whose levels correlated with an individual’s insulin resistance.
This proinflammatory environment was absent in people without metabolic syndrome — even among people who were very obese, the researchers report in the November Journal of Clinical Endocrinology & Metabolism, in an article posted online August 24. Although obesity predisposes people to develop metabolic syndrome, concludes Jialal, it takes more than excess fat to trigger it.
A related trial, published in the July 12 Journal of the American College of Cardiology, probed inflammation and metabolic impairments associated with subcutaneous fat in 109 obese men and women and 17 lean individuals. Some had diabetes but none had known heart disease.
“We found no inflammation in the fat of lean individuals,” who also had healthy blood vessel function, notes cardiologist Noyan Gokce of Boston University School of Medicine. In contrast, roughly 70 percent of the obese people had metabolic syndrome or at least some abnormal metabolic parameters in blood tests. These people also showed evidence of inflammation-promoting fat and impaired blood vessel function.
But the big surprise, Gokce says, was that the other roughly 30 percent of the obese people had very little inflammation inside their fat, little evidence of insulin resistance and vascular function that was the same as in lean people. “So it seems that the quantity of fat may not be nearly as important as its quality.”
The new data “open up the idea that maybe we should revisit subcutaneous fat as a major player in the metabolic consequences of obesity,” says Alyssa Hasty of Vanderbilt University School of Medicine in Nashville.