Metabolic Rate: Myths and Facts

Currently, there seems to be a great deal of confusion and misinformation regarding metabolic rate, specifically, the effects or lack thereof associated with various dietary supplements. Basal metabolic rate (BMR) is the amount of energy normally expended while at rest in a neutrally temperate environment, in the post-absorptive state (meaning that the digestive system is inactive, which requires about twelve hours of fasting in humans). The synthesis and use of ATP in this state is sufficient only for the functioning of the internal organs, such as the heart, lungs, brain, nervous system, liver, kidneys, sex organs, muscles and skin. Basal metabolic rate decreases with increasing age and with the loss of lean body mass. There are two factors related to exercise that influence BMR including both cardiovascular exercise and strength training. Acute bouts of cardiovascular exercise have been shown to increase BMR for a short time following the training session, but this is typically less than one hour and depends on level of physical conditioning. Additionally, increases in muscle mass associated with chronic strength training have also been shown to increase resting BMR continually until a decrease in lean body mass occurs. Other factors that influence BMR include illness, caloric intake, temperature, and stress can affect one's overall energy expenditure as these events require additional energy above BMR. An accurate BMR measurement requires that the person's sympathetic nervous system is not stimulated. A more common, related measurement is resting metabolic rate (RMR), which does not require as strict conditions as the measurement of BMR. BMR and RMR are measured by gas analysis through either direct or indirect calorimetry (measuring the amount of oxygen consumed and carbon dioxide released), though a rough estimation can be acquired through an equation using age, sex, height, and weight. Both basal metabolic rate and resting metabolic rate are usually expressed in terms of daily rates of energy expenditure. BMR is assessed under very restrictive circumstances when a person is awake, but at complete rest.

Some dietary supplements claim to increase RMR or BMR, which primarily include ingredients such as caffeine or ephedra/ephedrine. The basis of the increased BMR/RMR claim is based upon the active ingredients stimulating the central nervous system, as increases in central nervous system activity has been shown to increase BMR/RMR. The problem with these supplements is that the manufactures directly or indirectly claim that their products will result in an increase in fat metabolism, which cannot be true as the central nervous system cannot use fat as an energy substrate, but instead uses glucose as its sole source of ATP production. So, the claims of the manufacturers are both true and untrue...yes, you might have an increase in RMR/BMR, but no, you will not have an increase in fat metabolism.