The finding comes from a study of risk factors for diabetes and heart disease in 5,440 obese, overweight, and normal-weight U.S. adults by Albert Einstein College researchers Rachel P. Wildman, PhD, Judith Wylie-Rosett, EdD, and colleagues.
"We used to think all fat did was store energy," Wylie-Rosett tells WebMD. "Now we know that fat tissue is hormone-producing tissue. It may act differently in different people."
Clues to what's going on come from a second study looking at 314 German adults with traditional risk factors for type 2 diabetes and heart disease: a family history of type 2 diabetes, obesity, or a personal history of high blood sugar or gestational diabetes.
Close examination revealed a wide range of true diabetes/heart disease risk factors. For normal-weight and overweight people, risk was linked to belly fat. But for obese people, risk wasn't so much linked to belly fat as it was to having a fatty liver.
Belly fat signals fat accumulation around the organs of the body. Bodies that don't get much exercise tend to grow this kind of fat.
Similarly, obese people who get at least moderate physical exercise tend to have less fatty livers than those who don't exercise. Fortunately, there's a lot a person can do about this, says study researcher Norbert Stefan, MD, of the University of Tubingen, Germany.
"The higher an obese person's activity level, the larger the decrease in liver fat," Stefan tells WebMD.
"It may be the fat-and-fit phenomenon," Wylie-Rosett agrees. "In our study, the obese people with better risk profiles tended to have more physical activity. And the normal-weight people with worse risk factors tended to have characteristics associated with lower physical activity levels."
Warning: Whether or not you're obese, being fit doesn't mean being without risk. It's all a matter of probability, says Lewis Landsberg, MD, director of the Northwestern University obesity center.
"For any particular disease, there are many people with risk factors that do not get the disease, and many people without risk factors who do," Landsberg tells WebMD. "We've known for a long time that although obesity is a risk factor for heart disease, many obese people don't have that risk. But across the population, those with more body fat will have an increased incidence of heart disease. And those with the apple-shaped, upper-body obesity are at greater risk than those with the pear-shaped, lower-body obesity."
Exercise Cuts Risk Even Without Weight Loss
Landsberg notes that it is quite possible for obese people to be fit. That's because fitness is more than weight loss. It means losing the visceral fat around internal organs, improving insulin sensitivity, cutting blood pressure, and much more.
On the other hand, being obese isn't healthy.
But Stefan says some obese people are in particular need of an intensive lifestyle-change program. His team is now exploring the effects of such a program.
"At the moment, we see that the lifestyle intervention in general has effects on reducing liver fat more than total body fat," he says. "Many people stop the program because they are not happy with a body-weight decrease of just a few kilograms. But the liver fat goes down very much, and their metabolism increases very much. They must realize this has benefits. So it is important to tell people to stick with it, even if they don't lose much weight."
Interestingly, not everyone gets the same benefit from the same amount of exercise.
"The most important factor is not how much you exercise, but what the effect your exercise has in increasing your fitness," Stefan says. "It looks like at the same level of exercise, some people increase their fitness and others don't. It looks like there are fitness non-responders. And those non-responders don't have that good an effect of exercise on liver fat."
These "exercise non-responders" may need more exercise. Or they may require diabetes drugs to get their risk factors under control. Stefan and colleagues are testing these interventions in ongoing studies.
"We are seeing fitness as the most important factor, and then on top of that reduction of intake of carbs has a strong effect," he says. "So far, our recommendations are increased physical activity, four hours per week of moderate activity, and reduced carbs. We also recommend a reduced intake of saturated fat."
Reports on the Stefan and Wildman/Wylie-Rosett studies -- as well as an editorial by Landsberg -- appear in the Aug. 11/25 issue of Archives of Internal Medicine.