Newly discovered differences in the fat cells of men and women may help explain why men often have potbellies while women are more likely to have big hips and thighs, a researcher says.
Studies conducted at Rockefeller University in New York found differences between the sexes in both the makeup and number of these cells.
They show that, typically, women have more fat cells than men do in their thighs, hips and bottoms. But the fat cells in men's bellies are more likely than those in women's to accumulate fat.
Learning how to tinker with the biological controls that rule how fat cells take in and give up their cargo could open new ways for making people slim and healthy.
The latest research, conducted by Dr. Rudolph L. Leibel, was presented at a recent meeting of the American Heart Assn.
According to one estimate, 34 million American adults are overweight, and a third of that number are severely obese. Although too much fat is unhealthy, studies have shown that where the flab rests on the body is at least as important as how much excess poundage is carried.
Apples and Pears
In general, potbellies are associated with a higher risk of heart disease and diabetes. Extra weight down lower on the body is not linked with these diseases.
Experts believe this may be one reason why men, who are often shaped like apples, have a high risk of heart attacks, while women, who are often shaped like pears, have far less risk.
Two factors control the amount of body fat--the total number of fat cells and the size of these cells. Although the human body has about 30 billion fat cells, they are not evenly distributed.
The number and the location of fat cells are probably set early in life and cannot be lowered. People lose weight by making their fat cells skinnier.
"Control of the size of fat cells is extremely complex," said Leibel. "These are not just little bags of oil."
More Alpha for Men
He and colleagues have shown that the size of fat cells is controlled in part by molecules on the surface of the cells that are called alpha and beta receptors. Alpha receptors stimulate the cells to take in and store fat, while beta receptors prompt cells to get rid of fat.
Some cells have more alpha receptors, while others have more beta receptors, and the researchers believe that this has something to do with how readily they will give up their load of stored energy.
"Men and women tend to have roughly equivalent amounts of beta activity in their behinds--not to put too fine an anatomic point on it--and both tend to have relatively higher alpha activity in their behinds than in their bellies," Leibel said. "Men, however, differ from women in that they have more alpha in their bellies than women do.
"We believe that this difference in the distribution of alpha receptors may partly explain the tendency of the male to look like an apple rather than a pear. We think that the female shape has to do with differences in (fat) cell number" in the hips, thighs and bottom.
He said women need to carry about 80,000 calories of fat in order to get pregnant. This fuel is necessary to carry and feed a baby. The most efficient place to store this energy is the lower part of the body.
Carrying extra fat in the abdomen is dangerous, he said, because it interferes with the breakdown of insulin in the liver. This, in turn, raises the blood pressure and causes diabetes.
Dr. Janet Robishaw of the Geisinger Clinic in Danville, Pa., who also studies receptors, cautioned that fat storage "is a lot more complicated than just looking at receptors."
What happens inside the cell after a receptor is stimulated also probably plays a big role in how fat is dealt with, she said.
Leibel said a major research goal is to find ways to help people take off pounds in a specific part of the body without requiring major overall weight loss.
One way to accomplish this might be to selectively turn on or off the alpha and beta receptors. However, Leibel noted, genes and sex hormones obviously are also important in shaping the body, so it may be hard to have much impact simply by manipulating these receptors.