Does Dieting Increase Or Decrease Metabolic Rate?

Similar to the gas burned by your car while idling, your body requires a baseline amount of energy to survive. This energy expenditure is your basal metabolic rate. Dieting tends to decrease your metabolic rate, especially if your weight-loss regimen involves severe calorie restriction. Regular physical activity while dieting may reduce the magnitude of reduction in your metabolic rate.

Size Matters

A variety of factors influence metabolic rate, but your body size is the primary determinant. The larger your body mass, the more energy you need to keep it functioning. Your body mass decreases as you lose weight, which causes a reduction in your metabolic rate. Think of your extra pounds as weights strapped to your body. As the weights are removed, it gets progressively easier to move -- that is, it is less work. Less work equates to fewer calories burned. Therefore, any successful diet leads to a reduction in your metabolic rate that is at least proportional to the amount of weight lost.

Starvation Reaction

When you diet, you probably want to lose weight as quickly as possible. Restricting your caloric intake too drastically, however, may sabotage your weight loss by triggering a starvation reaction. As an innate survival mechanism, your body responds to extremely low caloric intake by reducing your metabolic rate to conserve energy. If you were indeed starving, this response would be a potentially life-saving metabolic adjustment. Going into starvation mode while dieting, however, makes it more difficult to lose weight. The reduction in metabolic rate associated with a starvation reaction is significantly greater than what would be expected from the loss of body mass caused by your weight loss.

Muscle Mass

Your muscles account for approximately 20 to 30 percent of your resting metabolic rate, according to Dr. Fahad Javed and colleagues in an April 2010 article published in the "American Journal of Clinical Nutrition." With severe calorie restriction, muscle proteins may break down to generate needed energy. Loss of muscle mass may lead to an unwanted reduction in your metabolic rate. Steps to help prevent this development include not restricting your caloric intake excessively, consuming a sufficient amount of dietary protein and participating in regular physical activity. Weight lifting and other types of resistance training are particularly good options for preventing loss of muscle mass and a dramatic reduction in your metabolic rate.

How Much Calorie Restriction?

One of the trickiest aspects of dieting is finding the metabolic "sweet spot," the level of caloric intake that leads to consistent weight loss without causing a steep decline in your metabolic rate. The American Dietetic Association recommends a daily deficit of 500 to 1,000 calories below the amount needed to maintain your current weight. For example, if your maintenance intake is 2,100 calories, limiting your daily caloric consumption to 1,100 to 1,600 calories will lead to an average weight loss of 1 to 2 lbs. weekly.