I'm confused! In Cosgrove's article on the hierarchy of fat loss, he claims we need to accept that RMR is largely a function of how much muscle you have on your body. Therefore, adding activities that promote muscle mass will elevate one’s metabolic rate. On the other hand, Jason Karp says that adding muscle mass from lifting weights doesn't have much effect on RMR and claims a 200 pound person who gains two pounds of muscle is only burning approximately 20 calories more per day. What does this mean for training for fat loss? Is cardio more efficient or weight training or an interval mixture of both or separating them out?
While it has become popular among fitness professionals to say that you must weight train to build muscle to increase your resting metabolic rate (RMR) and subsequently lose weight, this point is highly exaggerated and not supported by scientific research. Using one MET (3.5 ml/kg/min) as RMR, each pound of fat-free mass is calculated to burn 11.5 calories per day, a negligible amount since it takes a 3,500-calorie deficit between caloric consumption and expenditure to lose just one pound. Research has also documented this calorie-burning potential of fat-free mass. Even as far back as 1971, Holliday calculated that RMR in children and adolescents was equal to eight calories per pound of wet muscle per day.
While there is a positive relationship between fat-free weight and RMR among animals and humans with large differences in body weight, whether an individual can significantly increase his or her RMR is questionable. While only a couple of studies have shown that RMR (or total daily caloric expenditure) increases in response to weight training or endurance training, many have shown that it does not, with one study finding that RMR increased only in men. The study that found an increased RMR following endurance training examined older individuals, whose RMR is more likely to be influenced by exercise training given the age-associated loss in muscle mass (sarcopenia) and the associated loss in muscles’ metabolic activity. Broeder et al. found that RMR is not significantly different between people of different aerobic fitness levels and is independent of training status.
Even more to the point is that people lose weight only when they are in negative energy balance (i.e., caloric expenditure is greater than caloric intake). Research has shown that when people are in negative energy balance and losing weight, RMR actually decreases. Geliebter et al. found that RMR decreases during weight loss even when muscle mass is maintained by weight training. Since no research has shown that RMR is maintained much less increased when people are in negative energy balance, fitness professionals cannot say that weight training increases RMR that results in weight loss.
For fat loss, the effects of training are not about how much muscle you add to your body but rather about how you enhance the metabolic profile of the muscles, since it is the change in composition of the metabolically active portions of muscles that accounts for any change in RMR. For example, endurance training enhances fat oxidation by increasing skeletal muscle mitochondrial content and cellular respiratory capacity. Weight training (or long, intense endurance training), provided it depletes muscle glycogen, helps repartition post-exercise food intake, so ingested carbohydrates are used to replenish muscle glycogen stores rather than be stored as fat. Depleting muscles of glycogen (and then not consuming carbohydrates after your workout) threatens the muscles’ survival since carbohydrates are the preferred fuel of muscles. In response to this threat, muscles “learn” how to use fat more effectively. With the right training stimulus, over time muscles become better fat-burning machines. If you want your clients to lose weight, the old tenet is still true: expend more calories than you consume.
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