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Keeping It Off : Maintaining: Scientists have come up with a number of theories about why dieters' weight always seems to come creeping back.

June 27, 1991|GORDON BAKOULIS BLOCH | Bloch is a contributing editor of Health

Backsliding is almost universal among dieters. It leaves feelings of frustration and failure--along with a fresh layer of fat.

But weight-control experts are finally focusing on the mysteries of maintenance. Studies indicate that enzymatic changes in a dieter's body may be partly responsible when pounds creep back. Other findings suggest that the tendency to be heavy and regain pounds easily is etched in the genes. While you can't change the biological and genetic forces that conspire to put pounds back on, there's plenty you can do to fight them.

For some time, scientists have grappled with the question of why some bodies gravitate toward leanness while others tend to be heavy. Much research has focused on the "set-point" theory--the idea that after pounds are shed, the body inexplicably will return to a specific weight.

One explanation offered for set points stems from various studies that found that a dieter's metabolism tends to decelerate during dieting and remain slowed even when normal eating is resumed. This would mean that calories are burned more slowly, allowing weight to creep up easily.

New research disputes this widely accepted phenomenon, however. In one study, obese women who lost weight did experience significant drops in resting metabolic rate (RMR) while dieting, but their RMRs did not remain low.

"Our study, which monitored metabolism over the course of a year, indicates that once you've stopped dieting and achieved a stable body weight, your RMR increases to a level that's appropriate for your new weight," says the study leader, Thomas A. Wadden, associate professor in the department of psychiatry at the University of Pennsylvania School of Medicine in Philadelphia.

However, he cautions, this doesn't mean RMR will soar back up to pre-diet levels, "so to maintain a lower weight, you will have to watch your calorie intake carefully."

The latest research into the mechanisms that may stymie weight maintenance involves lipoprotein lipase (LPL), an enzyme in fat tissue thought to contribute to obesity in two ways. First, it extracts triglycerides (fats) from blood and converts them into a form in which they can be stored in the fat cells, which plumps them up. Second, animal experiments suggest that LPL may signal the central nervous system to stimulate increased food consumption.

It's no surprise that obese people tend to have high levels of LPL. But what is startling is that dieting doesn't cause their LPL levels to drop. In fact, weight loss may make their LPL levels surge.

Dr. Philip Kern and colleagues at Los Angeles' Cedars-Sinai Medical Center put nine obese men and women (average weight: 300 pounds) on a supervised very-low-calorie diet. The subjects shed an average of 93 pounds in six to nine months. LPL was measured at the start of the study and after each subject had maintained the weight loss for three months. All of their LPL levels had increased significantly. The heavier they were to begin with, the greater the rise in LPL. Kern thinks that their LPL levels rose to nudge their bodies into reestablishing an obese state.

Kern's findings may help explain why the formerly obese find it so tough to keep off pounds. But does LPL hinder the dieting efforts of those with comparatively little to lose? Probably not, Kern speculates. He believes there is a "threshold of weight" that needs to roll off before LPL activity increases.

It's unclear what that threshold, if it exists, might be. Kern speculates dieters would have to drop 10% to 15% of their body weight for LPL to kick in (his subjects lost 15% to 50%, though the exact threshold probably varies among individuals).

Still, normal-weight folk who lose a few pounds often put them back on as easily as obese people do, and two recent studies may provide some clues as to why. Though neither focuses on weight maintenance per se, they do reveal that the tendency to sustain a particular weight may be hereditary.

In one of the studies, scientists at the University of Pennsylvania in Philadelphia compared the lifelong health records of 93 pairs of identical twins who were raised apart. They found that genetic factors were twice as important as childhood environment in determining adult weight.

A study conducted by Claude Bouchard and colleagues at Laval University in Quebec yielded similar results. Twelve pairs of male identical twins, average age 21, were put on a three-month overfeeding program. Six days a week, each twin was fed 1,000 calories over the amount he normally consumed.

All 24 men gained--up to 29 pounds--and all experienced an increase in body-fat percentage and, specifically, in estimated subcutaneous fat (located just beneath the skin and thought to increase the risk of diabetes and heart disease).

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