Belly fat seems so simple. Eat too many deep-fried mozzarella sticks while thinking about maybe possibly going to the gym someday, and your belly will eventually start growing. Nothing complicated about that.

But there's a lot going on beneath our over-stretched shirts. Scientists say that belly fat is strangely complex and widely misunderstood. If you want to do something about the bulge, you should know that slimming down can be complicated too.

"How you lose fat may be just as important as how much fat you lose," says Dr. Samuel Klein, professor of medicine and nutritional science at Washington University School of Medicine.

Belly fat is worth taking seriously. Doctors have known for a long time that a big midsection is a strong — but imperfect—warning of potential health problems such as heart disease and diabetes. "If you see someone with a big belly, that's a person who is probably at higher risk," Klein says. "Someone who just has a big butt probably has less risk."

It's important to realize that belly fat isn't created equal. Much of the fat in the stomach area lies directly under the skin. This so-called subcutaneous fat can really weigh you down. But as far as scientists can tell, it doesn't seem especially hazardous to your health. Klein led a 2004 study showing that patients could instantly lose 30 pounds or more of subcutaneous fat through liposuction without seeing any of the health benefits normally associated with weight loss, such as lower blood pressure, lower cholesterol and greater sensitivity to insulin.

When it comes to health, fat under the skin seems to be far less important than fat that wraps around the internal organs, also known as visceral fat. "Visceral fat is linked to diabetes and cardiovascular problems even more than BMI," or body mass index, says Dr. Steven Grinspoon, professor of medicine at Harvard Medical School and director of the program in nutritional metabolism at Massachusetts General Hospital. Nobody knows why visceral fat seems so risky. One possibility, according to Grinspoon, is that it may cause harmful inflammation.

But Klein believes that visceral fat may just be a side effect of an unhealthy lifestyle, not a direct threat. He adds that about one out of four obese people seems to have perfectly fine cholesterol, blood pressure and insulin sensitivity, as if all of that fat didn't really matter. You can't know for sure if a large-bellied person is on the road to health problems without a battery of tests, he says.

Most people who have a lot of obvious belly fat also have a lot of visceral fat, Klein says. And usually vice versa. He suggests a test that should be tried only on close acquaintances (or possibly yourself). "If someone has a big belly, just try to pinch it. If you can get a big chunk, they have a lot of subcutaneous fat. If you can't, it's probably mostly visceral fat."

Visceral fat lies beyond the reach of any suction tube. If you want to rid yourself of this possible risk factor for heart disease and diabetes, you'll have to take another approach. One proven formula, Klein says: cutting calories and getting more exercise. When people slim down through diet and exercise, visceral fat disappears twice as fast as subcutaneous fat, he says.

If you want to quickly trim your waistline without surgery, you might consider a noninvasive "body shaping" procedure. Plastic surgeons, for example, can use high-intensity ultrasound to destroy fat cells, tighten the collagen in the skin and remove a couple of inches from the waist — not enough for the outside world to notice, maybe, but enough to make a difference. "Patients often say that their pants fit better," says Dr. Mark Jewell, a plastic surgeon in Eugene, Ore., and a past president of the American Society for Aesthetic Plastic Surgery.

Unfortunately, such noninvasive approaches are financially out of reach for many Americans. And if a person has too much belly fat, Jewell says, the ultrasound waves won't be able to reach deep enough to do much good. Almost all patients with a BMI over 30 need to slim down before getting the procedure. "I sometimes send them back to a personal trainer," Jewell says.

health@latimes.com