Yo-Yo? No-No!

Many people succeed in losing weight, only to regain it. They then try a different diet and achieve similar results: weight loss followed by a seemingly inevitable weight and fat gain. It’s often called yo-yo dieting. Health experts differ in their opinions of the possible harmful effects of such dieting. The majority of people who go on diets do so to lose excess fat, but because of the media emphasis on thinness, as exemplified by certain stick-figure television stars, others begin diets even though they aren’t fat. That’s particularly true of young women.

A new study examined the effects of a typical yo-yo pattern of dieting in five healthy young women, aged 22 to 34.1 The subjects went through two dieting cycles over a 180-day period, during which they lost nearly nine pounds in the first month, only to regain all the lost weight and more by eating whatever they wanted for the next 14 days. None of the women engaged in any type of exercise.

Body-composition analysis of the women found that fat mass remained unchanged but that there was a significant loss of lean body mass, or muscle. That was likely due to the lack of exercise; exercise, especially weight training, is known to maintain lean body mass during dieting conditions.

Even more insidious, however, were the other changes. When you restrict calories without exercising, the body decreases secretion of norepinephrine as a means of avoiding excess energy expenditure, which would lead to even greater lean-mass loss. The sensitivity to norepinephrine, which can constrict blood vessels, remains even when calories are increased. So as a consequence of restricted dieting without exercise, the blood vessels constrict, leading to high blood pressure. That’s what happened to the women in this study.

In addition, the women’s thyroid levels remained low after the diet. That, too, is likely a defensive mechanism of the body. The net effect, however, was a lower resting metabolic rate, which set the stage for a quick weight rebound.

This study highlights two important aspects of dieting. The first is that dieting without exercise is an exercise in futility. Any fat lost is quite likely to rebound, since the loss of muscle leads to lower daily calorie use. Another point is the possible negative health consequences of dieting cycles, such as higher blood pressure.

1 Kajioka, T., et al. (2002). Effects of intentional weight cycling on non-obese young women. Metabolism, 51:149-154.