Metabolism refers to the processes that the body needs to function. Basal Metabolic Rate is the amount of energy expressed in calories that a person needs to keep the body functioning at rest. Some of those processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles. Basal metabolic rate (BMR) affects the rate that a person burns calories and ultimately whether you maintain, gain, or lose weight. Your basal metabolic rate accounts for about 60 to 75% of the calories you burn every day. It is influenced by several factors.

Body Composition

The more muscle you have, the higher your metabolic rate tends to be even at rest. Muscle burns 3-5 times more calories than fat does.

Age

The metabolic rate is highest during the periods of rapid growth. As you get older, the amount of muscle decreases and metabolism naturally slows about 2-5% per decade after age 40 due to decrease in lean mass and a greater percentage of body fatness.

Weight

The heavier you are, the more calories you need. That's one reason it's easier to lose weight at the start of a diet, and harder later. The less you weigh, the fewer calories you need.

Gender

Women in general, have a metabolic rate about 5-10% lower than men even when of the same weight and height. Men generally burn more calories at rest than women because they naturally have more muscle.

Body Surface Area

The greater your body's surface area or skin area, the higher your BMR. Tall, thin people have higher BMRs.

Endocrine Glands

The thyroid hormones are the principal regulators of the metabolic rate. When the supply of thyroxin is inadequate, the BMR may fall 30 to 50%. If the thyroid is hyperactive the BMR may increase to twice the normal amount. The BMR in women fluctuates with the menstrual cycle. There is an average of 359 calories per day difference between its high point and low point. Pregnancy also increases metabolic rate.

Secondary factors can also affect metabolic rate. If the body perceives starvation either by real starvation or by extreme dieting, a person's metabolic rate can go as much as 50% below normal. Diets below 1,000 calories a day can decrease metabolic rate. The body is programmed for survival and interprets the reduction in calories as starvation, and all systems slow down to conserve energy.

During sleep, the rate falls about 10% below that of waking levels. Fever increases the metabolic rate about 7% for each degree rise in body temp. How much a person's muscles as relaxed affects the amount of energy used. The less relaxed the muscles are, the greater the metabolic rate. Emotional strain can cause increased tension and thus increase metabolic rate. That being said, do relax and get adequate sleep. People with sleep deprivation tend to have slower metabolisms and higher levels of cortisol, the hormone that can cause fat storage.

In addition to the factors that influence BMR, two other factors regulate how many calories your body burns each day:

1. The Thermic Effect of Feeding (TEF) is the amount of calories you use to digest, absorb, transport, and store the food you consume. This accounts for about 10% of the calories used each day.
2. The Thermic Effect of Activity (TEA) is the rate at which you burn calories while exercising and with normal movements. This accounts for about 30% of caloric needs. An inactive personal usually requires 30% more calories above basal, whereas a lightly active person might need 50% above basal, a moderately active person 75%, and a very active person 100%.

For more information about how to boost your metabolism with exercise: Understanding Metabolism: How to Boost Yours with Exercise