Metabolic Adaptation Part II: Cardio, Weight Training and Fat Loss

In part one of our blog on Metabolic Adaptation, we established that the speed at which one loses weight is largely determined by the size of one’s daily caloric deficit.  How that deficit is created is a function of how much we eat (calories in), coupled with how much we move (calories out).  Conventional approaches to weight loss typically marry a reduced calorie diet with an exercise program that is most often grounded in some type of aerobic exercise.  Indeed, the American College of Sports Medicine recommends >150 minutes of moderate intensity exercise per week, coupled with a reduced calorie intake in order to enjoy clinically significant weight loss (1).  The reason aerobic exercise is generally favored over weight training is based on the simple fact that aerobic exercise burns more calories per unit of time than weight training, and weight loss requires a caloric deficit.  It stands to reason then that exercises that burn the most calories, will have the greatest effect on daily caloric expenditure, and will therefore lead to the greatest losses in fat.  However, successful weight management should view the weight loss process through two separate but equally important lenses- one which views how we take weight off, and one that views how we keep it off.  Unfortunately, conventional approaches to weight loss place way too much emphasis on losing weight, and more often than not, zero emphasis on how to keep weight off.  

Because aerobic exercise burns more calories per unit of time than weight training, it is the most often prescribed “go to” activity for weight loss.  However, the research examining exercise and weight loss shows only a modest effect of exercise on weight loss, unless that exercise is also coupled with a dietary intervention (1).  Furthermore, while aerobic exercise may burn a lot of calories, not all of the adaptations that are brought about by aerobic exercise are desirable when the goal is to improve body composition.  For example, chronic aerobic exercise often brings about decrease in muscle mass, and this is especially true when that exercise is coupled with a reduced calorie intake.  Unfortunately, many weight loss professionals assume, expect, and accept that muscle mass will fall when one is trying to lose weight.  As a general rule, most weight loss studies will show that 20%-30% of weight loss comes from muscle when obese people undertake a weight loss program (2).  This is concerning, as there are several advantages to having higher levels of muscle mass.  Some of these are:

  • leads to a higher resting metabolism
  • helps to regulate core body temperature
  • helps with glucose and metabolic control
  • directly relates to quality of life as we age
  • helps to maintain skeletal integrity

Most importantly, it has been shown that there is a direct relationship between how much muscle you lose during weight loss, and how likely you are to regain that lost weight.  

Because we know that coupling calorie restriction with aerobic exercise can potentially lead to losses in muscle that can sabotage your weight loss efforts, we question the efficacy of using aerobic exercise as the primary form of activity during weight loss.  Because of this, we suggest using resistance training as the main form of exercise, despite the fact that it has been shown to be ineffective at inducing weight loss.  We recommend weight training not because it’s effective at helping to induce a calorie deficit, but rather because it is effective at maintaining or increasing muscle mass while one is simultaneously losing fat.  Viewing weight loss this way requires one to look deeper into the “calories in, calories out” approach, focusing more on what kind of weight one’s losing, rather than how much.  Conventional approaches to weight loss view exercise simply as a way to burn calories and contribute to a greater daily caloric deficit.  Within the context of a weight loss program, if exercise is viewed as a means to prevent muscle loss, rather than a means to induce fat loss, then the exercise and nutritional approaches we take change dramatically from what is traditionally recommended for weight loss.   


  1.  J.E. Donnelly, S.N. Blair, J.M. Jakicic, M.M. Manore, J.W. Rankin, B.K. Smith  Appropriate physical activity intervention strategies for weight loss and prevention of weight regain for adults.  Med Sci Sports Exerc, 41 (2009), pp. 459-471
  2. Heymsfield, S. B., Gonzalez, M. C., Shen, W., Redman, L., & Thomas, D. (2014). Weight loss composition is one-fourth fat-free mass: a critical review and critique of this widely cited rule. Obesity reviews : an official journal of the International Association for the Study of Obesity, 15(4), 310–321.

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