Resting metabolic rate (RMR; usually measured in calories) is defined as the amount of energy expended in a state of complete rest (i.e. asleep) per day.  Metabolic rates are highly individual and depend on age, weight, lean body mass and historical caloric intake.  Extraneous factors such as prolonged periods of stress and low body fat may also affect RMR.¹  It is important to note that like most physiological equilibriums, metabolic rate is not static -- it is dynamic in nature.



Lean body mass, or fat free mass (FFM), is total body mass excluding fat tissue.



A calorie (kcal, Cal) is simply a unit measure of energy.  It is the most commonly used measurement of energy in biochemistry, metabolism and nutritional science.  The analogous SI unit is the Joule (J).  Its significance in nutrition is the measurement of the energy value of food, metabolic rate and expenditure via exercise.  Because a calorie is a magnitude of a physical quantity, its value does not change.



The caloric baseline is the basal metabolic rate plus the average expended energy through daily activity and regular exercise. In other words, it is when calories consumed are equal to calories expended, or calories are net zero.  Caloric baseline is the number of calories that maintain body weight at a specific point in time. Another expression for caloric baseline is "maintenance calories".



A caloric deficit results from net negative calories, or when calories consumed are less than those expended.  A calorie deficit is achieved either by reducing calories below caloric baseline, increasing energy expended through exercise (often via cardio), or a combination of both.  It follows that a caloric deficit results in weight loss.  It is known that smaller calorie deficits are more optimal maintaining metabolic rate and thus in sustaining weight loss than are larger ones.²

Conversely, a caloric surplus results from net positive calories, or when more calories are consumed than are expended.  A caloric surplus is achieved either by increasing calories above caloric baseline, decreasing energy expended through exercise (often via decreased cardio), or a combination of both.  It follows that a caloric surplus results in weight gain.



Because of the dynamic nature of the metabolism, metabolic rate can be manipulated and optimized. The goal with reverse dieting is to maximize the BMR by slowly shifting the metabolic equilibrium via small, regular caloric increases.  For a given bodyweight, there is a point where any increase in calories are surplus and result in weight gain over time.  At this point, the individual's metabolic rate is maximized.


a, b.  The proportion of lean body mass either lost or gained through caloric deficit or surplus is dependent on nutrition and training.  In caloric deficit, high protein diets and resistance training are shown to conserve lean body mass independently and in combination.  Resistance training is shown to conserve lean body mass in a caloric deficit and promote muscle protein synthesis (MPS) in maintenance or surplus, and in some cases deficits.³


  1. MacDonald, L. (2010). Why Big Caloric Deficits and Lots of Activity Can Hurt Fat Loss. Body Recomposition. Retrieved from
  2. Bryner, R., Ullrich, I., Sauers, J., Donley, D., Hornsby, G., Kolar, M., & Yeater, R. (1999). Effects of resistance vs. aerobic training combined with an 800 calorie liquid diet on lean body mass and resting metabolic rate. J Am Coll Nutr., 18(2), 115-21. Retrieved from
  3. Geiliebter, A., Maher, M., Gerace, L., Gutin, B., Heymsfield, S., & Hashim, S. (1997). Effects of strength or aerobic training on body composition, resting metabolic rate, and peak oxygen consumption in obese dieting subjects. Am J Clin Nutr., 66(3), 557-63. Retrieved from