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Not just the weight, but where it is

Researchers are learning why belly fat carries greater disease risk than fat elsewhere

March 08, 2004|Jane E. Allen | Times Staff Writer

Pear-shaped people may have more trouble losing weight -- from their hips, thighs and posterior, specifically -- but it's the apple-shaped folks who need to redouble their efforts. Their fat is more dangerous.

Researchers have known that carrying extra pounds around the belly and upper body increases the risk of hypertension, heart disease, stroke, diabetes and cancers of the breast, ovary and prostate. Now they're learning why.

The two deposits of belly fat -- the subcutaneous layer and a deeper layer -- both function like mini-organs, with blood vessels, connective tissue, immune cells recruited from bone marrow and the ability to store and secrete hormones. The inflammatory hormones, enzymes and fatty acids released by this fat increase the risk of diabetes, hypertension and high triglycerides.

"We've probably discovered 10 hormones in the past decade that fat can make," said Susan Fried, a professor of medicine at the University of Maryland School of Medicine in Baltimore. Two were discovered in just the last year. "A new one is being discovered every two to three months," she added.

First, scientists found that fat made leptin, a hormone that regulates appetite, inflammation and reproduction. They subsequently found that it makes the inflammatory hormone interleukin-6, which tells the liver to release more triglycerides and takes the brakes off glucose production.

Fat makes at least one hormone that's beneficial, the anti-inflammatory hormone adiponectin, which protects against atherosclerosis and helps muscles burn fat efficiently. But too much fat slows production of this hormone. Low levels have been linked to heart disease in adults, and new research presented Friday at an American Heart Assn. conference found that adiponectin levels are significantly lower in overweight children and young adults, which probably sets them up early for heart disease.

Such complex biochemical activity belies the fact that fat has its place, biologically speaking -- it's been key to human survival for about 50,000 years. Being able to store fat in the belly was a hedge against starvation, while immune cells in the fat protected against infection. But most people in modern cultures suffer from the consequences of dietary excess.

Over the long term, such excess becomes much more than a superficial problem.

The subcutaneous fat layer, which lies just beneath the skin and can be easily pinched, is damaging enough, but it's only the beginning. Excess inches at the waist often suggest the presence of visceral fat. That deeper layer includes the omentum, a sheet of fat that hangs from the bottom edge of the stomach over the intestines.

Visceral fat contains more metabolically active fat cells than those that accumulate between the skin and muscles of the abdomen or the hips, thighs and buttocks. It's particularly unhealthy because it releases more inflammatory hormones and fatty acids than other fat. Worse still, it sends them right through the portal vein into the liver, where they adversely affect glucose metabolism, blood pressure regulation and production of unhealthy blood fats called triglycerides.

The enhanced understanding of fat has led some experts to say that a person's shape can be a more meaningful indicator of obesity and disease risk than the often-touted body mass index. BMI, an expression of the relationship of weight to height, doesn't distinguish between fat and muscle, so it can be particularly misleading for people who tend to accumulate fat in the upper body, including those of Asian ancestry, Latinos and adults over 50, said Dr. David Heber, director of the UCLA Center for Human Nutrition.

"Weight-to-height ratio doesn't tell the whole story," he said. BMI has helped track the epidemic of obesity -- where one in two Americans today is overweight -- but for individuals, "shape is more important than weight."

Whether people who gain weight tend to be pear-shaped or apple-shaped depends on many factors, including genetics, how much they exercise, and hormones. Women have a higher percentage of body fat than men, mostly because their bodies make the reproductive hormones estrogen and progesterone. Those hormones cause them to lay down fat in the lower body, where it resists weight loss.

The good news, nutrition and obesity experts say, is that once you lose the excess fat, you can improve your health and lower your disease risks. Diet and exercise actually shrink the deep visceral abdominal fat faster than the shallower, unsightly blubber that's the bane of overweight people.

Canadian researchers reported in February's Medicine & Science in Sports & Exercise that even modest increases in physical activity can improve men's cardiovascular fitness and reduce abdominal fat, regardless of BMI.

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