A recent study in humans demonstrated that frequent episodes of intentional weight loss can actually make individuals more prone to weight gain. Similar studies have not been conducted in animals. Because metabolic adaptations to calorie restriction appear to be universal from species to species, it is probably safe to assume that pets would also be prone to weight gain with frequent bouts of weight loss.

A sustained commitment to a lifestyle change rather than the short term "yo-yo" effect of on-again, off-again dieting is probably a healthier strategy for us and our pets.

The Study

Researchers in Finland compared weight gain patterns for 2,000 sets of twins. Twins were chosen to decrease the genetic variability of metabolism and behavior between individuals. Subjects with a history of two or more episodes of dieting progressively gained more weight than their non-dieting co-twin over 25 years than those subjects that only dieted once compared to their non-dieting co-twin over the same time period. The researchers concluded that the diet itself promoted weight gain independent of genetics. Other research supports their claim.

Supporting Research

Studies have shown that calorie restriction or dieting promotes metabolic changes in the body to resist further weight loss. Immediate hormonal changes that occur with the feeling of hunger signals the portion of the brain that controls the thyroid gland to decrease the production of the hormone, thyroxine. Blood levels of thyroxine determine the rate of cellular activity in the body. As thyroxine levels fall in the bloodstream, cellular activity slows and fewer calories are needed to sustain resting metabolism.

Hormonal changes also affect the active or non-resting metabolic rate of muscle and fat cells. Dieting muscle requires less energy to perform the same task it did prior to dieting. Fat cells become resistant to breakdown for energy. In fact, the body shifts to promote fat production. Dieting cells shift to using carbohydrates for energy rather than fat, further decreasing fat loss.

Carbohydrates, fats and protein all require a portion of the calories they contain for their own digestion and absorption from the intestines. Proteins require 15-25 percent of their calories, carbs require 5-15 percent of their calories, and fats require 2-3 percent of their calories for this purpose. This is called the thermic effect of food. During dieting, the thermic effect of food for carbs, fats, and proteins decrease so the body uses fewer calories for the digestion and absorption of food, contributing to the slowdown of weight loss.

Although there are fewer of these metabolic studies in cats and dogs, studies in both species have confirmed that weight gain after induced obesity and dieting is faster and requires fewer calories than needed to induce obesity. This would imply similar metabolic changes during dieting in cats and dogs.

What’s New with the Study

The results of the Finnish study suggest that metabolic efficiency changes have lasting effects. Most studies follow metabolic changes over 1- 2 years, with few extending five years. The 25 year period studied by the Finnish researchers suggest that metabolic changes may last much longer and possibly indefinitely. Obviously, more research is needed to confirm this, but the implications are important. Rather than seeking diet solutions for ourselves and our pets, we should look at weight management as a complete lifestyle change in eating, feeding, and exercise habits. Recent statistics indicate the average daily calorie intake in the U.S. across all age groups is 3,770! This is 770-1,000 more calories than needed for active men and women, and far more than necessary for children and less active individuals.

Our pets are also enjoying this calorie largess. This degree of overeating and overfeeding by us and our pets certainly warrants an analysis of lifestyle rather than the newest diet craze.

Dr. Ken Tudor

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