Metabolism (RMR)
What is it and how does it work?
by Scott Murdoch, Ph.D.

Metabolism refers to the energy requirements needed to sustain life. In general, people require energy while at rest, during physical activity and even for food consumption (thermal effect of food).

What affects these three categories of energy requirements?

Resting Metabolic Rate (RMR)
RMR refers to the average rate of metabolism and is primarily affected by the amount of lean body tissue (i.e. muscle) one has. RMR usually contributes the largest component of total daily energy expenditure. Generally speaking, the higher the RMR, the more food one has to eat to support his or her energy requirements, which in turn makes it easier to get all the essential nutrients. On the other hand, those who have low RMR (usually resulting from lack of physical activity) find weight management difficult and will tend to eat less, resulting in inadequate nutrient intakes.

Physical Activity Equals Energy
Additionally, most physically active people have a higher RMR than people who are inactive. The reason for this is physical activity increases lean body tissue, which in turn raises the RMR.

Thermic Effect of Food (TEF)
The TEF refers to the increase in metabolic rate that results from ingesting a meal. The amount of energy expended from ingesting a meal approximates 5-10 percent of the energy in the meal itself. So a meal containing 1000 calories would use 50-100 of those calories for processing that food. Thus, the TEF makes a relatively small contribution to total daily energy expenditure.

Dietary restrictions are not the long-term answer to weight loss because the body has a memory and wants to protect itself. During periods when it isn't getting enough calories, the body starts to conserve energy, or store fat, by lowering its metabolic rate. Diets under 1000 calories stave the body, are detrimental to health, and do the opposite of what the person desires.

Other Factors Affect RMR
A man's RMR is generally higher than a women's because he typically has more lean body tissue. Age has an effect since from infancy through young adulthood there is a tremendous demand for energy placed on the body for growth and development. But as we age, our activity levels tend to decline, and with it our lean muscle tissue. If we can stay active throughout life, we typically don't see dramatic declines in energy expenditure and lean body tissue, and consequently the prevalence of chronic diseases associated with sedentary lifestyles so often found among the older population is lowered.

The goal for everyone should be to influence their RMR by incorporating some physical activity into their lifestyle. It doesn't have to be dramatic, but is needs to be regular.

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