Questions and Answers
Metabolism, quite simply, is the conversion of food to energy.
Metabolic rate is a measure of how much food, or fat, is converted to energy in a day. Resting metabolic rate (RMR) is the measurement of how much food, or energy, is required to maintain basic body functions such as heartbeat, breathing, and maintenance of body heat while you are in a state of rest. That energy is expressed in calories per day. So an RMR test shows how many calories you burn at rest, doing nothing more than sitting in a chair.
Each individual will react differently to dietary changes which is why a true measurement of RMR is so valuable. But if a patient reduces calories, don’t be surprised if their RMR also goes down. This may or may not be in conjunction with weight loss. It is critical to measure those changes periodically to make the necessary adjustments in diet and exercise to keep patients on track and avoid the dreaded “plateau.” (Martin)
Predictive formulas have been used for years because an actual measurement through Indirect Calorimetry has not been practical. But the truth is, while predictions might hold their ground statistically, when applied to individuals, they are woefully inadequate. In an examination of published articles examining the validity of various predictive equations, Frankenfield, Roth-Yousey and Compher found that even the best equation (Miffin-St Jeor) was only within 10% of measured results. While 10% may seem statistically acceptable, for the individual desiring to lose weight, an RMR estimate that is 10% higher (2200 kCals) than an actual measurement of 2000 kCals a day would be significant. That additional caloric intake would result in 21 pounds gained in one year! Additionally, these same researchers noted that errors and limitations with equations exist when applied to individuals, and that, “RMR estimation errors would be eliminated by valid measurement of RMR with indirect calorimetry.” (Frankenfield D, et al, J Am Diet Assoc. 2005 May;105(5):775-89.)
Additional researchers at the University of Pennsylvania noted that the obese have Resting Energy Expenditures that are particularly difficult to predict. Their findings read, “Caloric prescription for weight reduction must be tailored to individuals rather than recommending the same caloric intake to persons with varying metabolic rates.” (Foster GD, et al, Metabolism. 1988 May;37(5):467-72.)
Often after a significant weight loss, RMR is depressed even lower than expected relative to the change in body composition. Most researchers point to this as a key factor in the high rate of weight regain among the formerly obese. Identifying this post-diet RMR is a vital step that is key to long term weight loss success. It gives the information necessary to set an appropriate caloric goal for maintenance and teach a patient to eat within the constraints of their new metabolic requirements.