Metabolic Rate and Eating Disorders

By: Philip Walker, M.S.

Metabolism refers to the sum of all vital processes in which energy and nutrients from food are made available to and utilized by the body.  The minimum level of energy to sustain the body’s vital functions, not including the energy cost of digesting and absorbing, or engaging in physical activity, is referred to as the resting metabolic rate (RMR).  Metabolism differs from person to person, depending on a variety of factors.  Metabolism slows down with age, and gender differences play an apparent part in the function of the metabolism.  The average RMR for a woman is between 5-10% lower than that of men, due partly to the fact that women have a larger percentage of body fat and smaller muscle mass than most men.

There is a substantial decline in metabolism in underweight people who have restricted their food intake.

Research shows that the metabolism adapts to different conditions in order to defend a particular level of weight or energy balance.  If a person begins to restrict, or undertakes a fast of more than a week or two his or her metabolism slows down.  In a person who has a history of restricting, metabolism has been shown to have a greater “slow down” effect.  Studies have shown a decrease in RMR of up to 30-50% in underweight people.  Further studies found individuals on a constant reduced caloric intake lost an average of 40 grams of fat a day in the first month, 20 grams per day the second month, 10 grams per day the third month, until fat loss stopped altogether.  Therefore, a drop in metabolic rate is counterproductive in individuals trying to lose substantial amounts of body fat.  This is another example of the physiological adjustments inherent in the human body to sustain life.

The RMR declines within 24-48 hours after caloric restriction and is followed by a rapid decline after one week.  The RMR will then stay relatively unchanged throughout the remainder of the food restriction period.  The RMR of underweight exercisers have the same effects as underweight individuals who are sedentary.  Consequently, exercise appears to have no effect on RMR during restrictive periods.

Therefore, the resting metabolic rate should be a major concern for professionals treating anorexia and bulimia. This is apparent, not only during the restrictive phase itself, but also during refeeding and especially during the maintenance period.  The Walker Wellness Clinic objectively measures energy expenditure, nutritional intake, body composition, and resting metabolic rate to determine an accurate physiological status.  Therefore, the patient’s current physiological condition, nutrition requirements, and healthy weight range are established based on a number of individualized assessments to avoid guesswork, an excessive weight gain, and uncertainty.