A common occurrence in fat-loss diets is a decrease in the dieters' resting metabolic rates. In some cases - particularly when a decreased caloric intake is combined with no exercise or only aerobic exercise - this decreased metabolism results from the loss of lean mass, such
as muscle. Your resting metabolism is a reflection of your lean- mass content, which explains why most men have faster metabolic rates than women. Men have more muscle.
Other factors that can influence resting metabolism while dieting include the amount of calories you consume and the composition of your diet. For example; if you consume either too few calories or too few carbohydrates for an extended period, your metabolism will drop. This leads to the well-known "plateau effect" that sets in after a few weeks of dieting. What's happening is that the body is interpreting the lack of calories or carbs as a starvation situation. To prevent the loss of vital lean tissue, including muscle and organs, the body converts active thyroid hormone into an inactive version called "reverse T3." Since thyroid hormone controls the metabolic rote, this conversion is analogous to turning a thermostat to a low setting.
But another, often overlooked, factor in decreased resting metabolism while dieting is hydration, or fluid status. Many people who decrease their food intake also inadvertently lower their fluid intake. Since water plays a vital role in the transport of nutrients, as well as in other important chemical reactions in the body, lowering fluid intake itself may have a blunting effect on resting metabolism. Scientists from the University of Utah recently tested the hypothesis that decreased fluid reserves will lower one's resting metabolic rate. They tested the resting metabolism of six women and then gave them each a 40-milligram dose of Lasix, a potent diuretic drug. This led to a rapid 2% average loss in bodyweight in the women.
The women showed a marked decrease in their resting metabolic rates, a decreased ability to use fat and an increased use of carbohydrate. This study underscores the importance of drinking large quantities of fluids while dieting to produce optimal results.