Metabolism, in its simplest sense, is the rate at which your body burns calories. In a broader sense it is complex network of hormones and enzymes responsible for converting food into fuel, while also determining how efficiently you burn that fuel. Many people think of metabolism as how easily they lose or gain weight.

Surprising to many is that the largest component of your metabolism, approximately 70%, is your basal metabolic rate (BMR). Your BMR determines how many calories are needed just to keep you alive and functioning. It is the energy used by your body to perform basic functions, such as breathing, brain health, keeping the heart beating and maintaining body temperature. For example, your brain requires approximately 109 calories per pound and weighs approximately three pounds. This means that 327 calories per day are required to maintain your brain. Other examples include your heart and kidneys, which require 200 calories per pound (approximate weight of an adult human heart is 5/8 of a pound; approximate weight of kidneys is ¼ of a pound). Your BMR decreases as you age and differs from person to person. Resting metabolic rate (RMR) is often used interchangeably with basal metabolic rate (BMR), though they are slightly different.

What influences my BMR?
Age – metabolism decreases five percent per decade after age 40
Amount of lean muscle – Muscle burns more calories than fat. The more muscle you have, the more calories you burn, even at rest. (For every pound of muscle you burn approximately 6 calories versus a pound of fat, which burns approximately 2 calories).
Gender – Males generally have a 10 to 15% faster BMR than females, as the male body has a larger percentage of lean muscle tissue.
Heredity – metabolic rate can be inherited from previous generations
Thyroid disorder – hypothyroidism (underactive thyroid gland) and hyperthyroidism (overactive thyroid gland) can slow down or speed up metabolism (these conditions occur in only 3 and .3 percent of the population)

How do I calculate my RMR?
To calculate your RMR, use the Mifflin-St Jeor equation (may be more reliable than the Harris-Benedict equation)

RMR = 9.99w + 6.25s – 4.92a + 166g-161
w = weight in kilograms; if you know your weight in pounds, divide by 2.2 to get your weight in kilograms
s = height in centimeters (1 foot = 30.48 centimeters, 1 inch = 2.54 centimeters);
a = age in years
g = gender = 1 for males, 0 for females
For example, the equation for a 30 year old, 120 lb, 5’4 woman would be as follows:
9.99(54.54) + 6.25(162.5) – 4.92(30)+ 166(0)-161 = 1251.80

This means that this woman requires approximately 1251.8 calories to maintain her body’s vital functions and her weight at rest.

The Eating Disorder Connection: Bringing it all Together
Often people with eating disorders, such as Anorexia Nervosa, restrict their caloric intake to below 1000 calories or even 500 calories per day. As illusrated in the example above, this is not even enough calories to maintain their basic body functions. Over time, if this calorie deficit continues, one’s body will begin shutting down.

Also, our bodies are very intelligent and if a continued calorie deficit is percieved, our metabolism will slow down to compensate for this deficit, meaning we are burning less calories than prior to the restriction. This starts a vicious cycle for someone with disordered eating behavior, as the result is that one usually has to restrict even more. There is hope though, after resuming a balanced diet sufficient to meet one’s caloric needs, one’s metabolism will also adjust once the threat of starvation is no longer present and your body begins to trust that it is getting the necessary nutrients.