Ho-ho to the no-no on yo-yo.

If you're overweight, think twice before going on a severe weight loss diet. Most persons who diet eventually gain back the weight they lost, so they try over with the same result. And we all know the dangers of this yo-yo dieting--more difficulty in losing weight the next time around, replacement of muscle tissue with fat, decreased metabolism, increased risk of diabetes and heart disease, etc.

Right?

Wrong, says the National Task Force on the Prevention and Treatment of Obesity. After reviewing nearly 30 years of research on weight cycling (Journal of the American Medical Association, October 19, 1994), the panel concluded, "There is no convincing evidence that weight cycling in humans has adverse effects on body composition, energy expenditure, risk factors for cardiovascular disease, or the effectiveness of future efforts at weight loss."

Yet standard textbooks on nutrition and dietetics now present the detrimental effects of weight cycling as established fact--and some researchers have even suggested that remaining obese may be preferable to undergoing repeated failed attempts to permanently reduce body weight.

What the panel found were many studies so poorly carried out that it was impossible to draw honest conclusions from them. Some studies, for example, have shown that a person who diets away pounds lost five years earlier and then regained may end up with a higher proportion of fat to muscle.

What they failed to take into account, however, was that time alone may have been responsible for fat replacing muscle that had deteriorated with age. Other studies in which yo-yo dieters were found to have developed more fat around the middle--converting from "pear" to the higher risk "apple" shape--lacked evidence that the appropriate techniques had been employed to measure abdominal fat.

Some claims made for the negative results of yo-yo dieting are probably true--but repeated dieting doesn't necessarily cause these results. Finding that it's harder to lose weight the next time, for example, may be due simply to one's discouragement with the process rather than an inherent inability to lose the weight again--you just don't try as hard the next time. Also, the emotional distress that often accompanies repeated dieting efforts may be the cause of the yo-yo dieting pattern rather than its effect.

These findings of the task force do not mean, of course, that one should take on-and-off dieting lightly. "Obese individuals who undertake weight loss efforts should be ready to commit to lifelong changes in their behavioral patterns, diet, and physical activity," said the report. The risk of being overweight is something to be concerned about--and even reducing one's weight by 10 or 15 pounds permanently can be beneficial.

The bottom line: Don't let concerns about the possible risks of repeated dieting deter you from making the effort to lose some weight and keep it off--even if you don't do very well the first time.