How muscle mass and heart rate affect the body's metabolism.
When it comes to weight loss, it's all about calories. Burning more calories than the body takes in results in weight loss and taking in more calories than are burned means a weight gain.
Metabolism is the number of calories the body burns and is made up of three components: resting metabolic rate, physical activity and a small amount needed for food digestion. Increasing the body's metabolism has the potential to enhance weight management because the number of calories needed each day would be higher (and so the amount of food needed to fuel that need would be greater).
While many popular publications would lead you to believe otherwise, metabolism can only be boosted in two ways: increasing the body's muscle mass and increasing the body's heart rate.
Losing some muscle mass is considered to be an expected part of weight loss because a smaller body needs less muscle to move the body's weight around. Resistance (i.e. weight) training as part of a comprehensive exercise plan can increase lean muscle mass in weight-stable individuals and minimise loss of lean tissue during weight loss. While a few studies have demonstrated otherwise, most studies show that it is not feasible to decrease body weight and simultaneously increase lean muscle tissue.1
A popular theory concerning weight loss, increased metabolism and exercise states that it is feasible to create a sustained increase in metabolism through regular aerobic workouts. The studies that have evaluated this theory for scientific merit have shown very mixed results. The general finding for studies done in humans is that, if this phenomenon exists, the impact on overall metabolism is very small.2 Further, there are no studies that have directly linked the 'after effects' of exercise with significant weight loss.
Heart Rate's Impact
A person's heart rate has an impact on metabolism the higher the heart rate, the more calories burned. Stimulants, whether from a prescribed medication, dietary supplement, or a caffeine-containing beverage, all work to increase the heart rate and fire up the nervous system.
In a controlled study that looked at giving stimulants, including amphetamines or a placebo, weight loss was greater with the stimulant because heart rate and blood pressure were raised and food intake was decreased as a side effect of being 'hyper'.3
Most dietary supplements and herbal compounds that are sold over the counter to promote weight loss contain stimulants, though at a lesser strength than that found in amphetamines. These products are often marketed as having 'fat burning' properties, which are achieved because they increase heart rate and blood pressure and decrease appetite. They can work, but they do so at a cost to the body. Negative side effects include insomnia, nervousness, anxiety and, in extreme cases, death.
Increasingly, some of these products (including the ones that were most effective, like fen-phen and ephedra) have been banned because of their negative side effects.
Demling RH, DeSanti L. Effect of a hypocaloric diet, increased protein intake and resistance training on lean mass gains and fat mass loss in overweight police officers
. Ann Nutr Metab. 44(1):21-9, 2000.
Speakman JR, Selman C. Physical activity and resting metabolic rate
. Proc Nutr Soc. 62(3):621-34, 2003.
3Makris AP, Rush CR, Frederich RC, Kelly TH. Wake-promoting agents with different mechanisms of action: comparison of effects of modafinil and amphetamine on food intake and cardiovascular activity. Appetite. 42(2):185-95, 2004.