Body shape: in the eye of the receptor?

Body shape: In the eye of the receptor?

The shape of our bodies -- whether bottom-heavy or thick-in-the-middle -- may be related to hormone-binding structures on the surface of fat cells. As a result, while we may be able to shrink in size, changing our proportions through dieting may be nearly impossible, says Rudolph L. Leibel of Rockefeller University in New York City.

At the American Heart Association's 15th Science Writers Forum, held this week in New Orleans, Leibel discussed recent data from his group showing that responses by certain hormone receptors bound on fat cells (adipocytes) are influenced by a person's gender, pregnancy and age. An understanding of how these receptors, called alpha.sub.2 and beta.sub.1, are stimulated could help explain the relationship between obesity and conditions like heart disease, and thus could offer ways to reduce fat-related health risks, says Leibel.

The notion of who's fat and who's not can be "a nasty business," according to Leibel, who says the fat cell should be regarded "with the same sort of respect as the brain cell or the endothelial cell [lining blood vessels]." Fat, he says, "is not simply something put there to torture us, or to make money for the diet industry." Instead, it serves as an efficient way to store energy. Although our 30 billion fat cells account for only 0.1 percent of the body's total cell population, they contain nearly all the readily available energy, Leibel notes.

But too much fat can be dangerous, causing increased risk of heart disease, diabetes and stroke. Studies have shown that a male-pattern weight gain--concentrated primarily in the abdomen area--is associated with more heart disease than is a female-type weight gain concentrated in the thighs and buttocks (SN: 7/16/83, p.44). The exact reasons for this are unclear, but the observations have focused attention on patterns of fat deposition.

At Rockefeller, scientists are asking how the shape of the body is controlled, with the hope of reducing specific fat deposits someday. Along with genetics and the number of fat cells, adipocyte size is considered important in determining the amount of body fat. The New York group's research suggests that cell size is regulated by receptors, found on all fat cells, for the hormones norepinephrine and epinephrine. But while these socalled alpha.sub.2 and beta.sub.1 receptors bind both hormones, the relative numbers of each type of receptor activated during binding apparently affect fat distribution, says Leibel.

For example, when the beta.sub.1 receptor is the one predominantly stimulated, there is an increase in intracellular production of lipase, an enzyme that breaks down fat. When the alpha.sub.2 receptor is stimulated, however, this fat destruction is inhibited and fat remains inside, making the fat cell even fatter.

Determining how these cell receptors relate to body shape and subsequent health risks is a matter of comparing apples and pears, since most overweight males have apple-like shapes and the majority of overweight females are pear-shaped. In a recently completed study of fat cells taken from various body sites, Leibel and his co-workers found that men and women essentially have equivalent beta.sub.1 activity in the buttocks. Also, both sexes have higher alpha.sub.2 activity in the buttocks than in the abdominal area -- meaning, as Leibel explains, that both sexes are likely "to hang on to fat in their [buttocks]." But he says gender makes a difference when it comes to potbellies: Men tend to have more alpha.sub.2 activity, and therefore fat retention, in the abdomen than do women, according to the study.

The researchers also observed that fat tissue serves as an energy depot during breast-feeding. A woman's receptor response changes during lactation, with the alpha.sub.2 activity in the hips and thighs temporarily dropping in a form of spot reducing, making fat available for use elsewhere in the body. If scientists were able to determine how these localized changes take place, says Leibel, "we would clearly have a very powerful tool for the clinical management of obesity and related problems."

If there were a way to lose weight just from the abdomen, the amount of weight loss needed to gain health benefits would not have to be so great, says Leibel. But how such selective slimming will come about is a mystery. Designing a drug specific enough to properly alter receptor activity will be a difficult task, he says. He also notes that liposuction, a procedure used to remove fat from under the skin surface, cannot reduce the deeper, more important fat deposits.

And results from his current research sound depressing for those fighting a bulge here and there. In a preliminary study of 18 women undergoing weight loss through dieting, he discovered that body proportions stay constant, while the alpha.sub.2 and beta.sub.1 receptors "go hay-wire." In measuring the waist- to hip-size ratio, Leibel and his colleagues found no change in body proportion when comparing the pre-diet figure to that after a 10 percent weight loss. What they did see were large changes in the performance of both receptors in the abdominal and buttock region. Leibel emphasizes that this research is preliminary and the scientists are not yet able to explain why these apparent receptor changes do not seem to affect body shape.