Just how many calories do our cells need to function well? The number is different for every person. You may notice on the nutritional labels of the foods you buy that the "percent daily values" are based on a 2,000 calorie diet - 2,000 calories is a rough average of what a person needs to eat in a day, but your body might need more or less than 2,000 calories. Height, weight, gender, age and activity level all affect your caloric needs. There are three main factors involved in calculating how many calories your body needs per day:

• Basal metabolic rate
• Physical activity
• Thermic effect of food

Basal Metabolic Rate (BMR) is the amount of energy your body needs to function at rest. To be precise, Basal metabolic rate (BMR), and the closely related resting metabolic rate (RMR), is the amount of energy 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 release of energy in this state is sufficient only for the functioning of the vital organs, such as the heart, lungs, brain and the rest of the nervous system, liver, kidneys, sex organs, muscles and skin. BMR decreases with age and with the loss of lean body mass. Increasing muscle mass increases BMR. Aerobic fitness level, a product of cardiovascular exercise, while previously thought to have effect on BMR, has been shown in the 1990s not to correlate with BMR, when fat-free body mass was adjusted for. New research has however come to light which suggests aerobic exercise does increase resting energy consumption. Illness, previously consumed food and beverages, environmental temperature, and stress levels can affect one's overall energy expenditure as well as one's BMR.

BMR is measured under very restrictive circumstances when a person is awake. An accurate BMR measurement requires that the person's sympathetic nervous system not be stimulated, such a condition whose prerequisite is that of complete rest. A more common and closely related measurement, used under less strict conditions, is resting metabolic rate (RMR).

BMR and RMR are measured by gas analysis through either direct or indirect calorimetry, though a rough estimation can be acquired through an equation using age, sex, height, and weight. Studies conducted by Spennewyn in 1990 found strong correlations between lean mass and metabolism based on indirect calorimetry measurements. Spennewyn discovered that lean tissue in men and women required approximately 16 calories per pound per day. Thus, once a lean mass was known it could be multiplied by 16 to reveal daily caloric needs based on the activity level of the individual. This method has been used in many health club environments to determine daily caloric needs.

Medical considerations

Each person's metabolism is unique due to his or her unique physical makeup and physical behavior. For some, this makes weight management a very difficult undertaking requiring sophisticated expertise. There are a number of medical adjustments to natural human processes that can affect one's metabolism.

Menopause affects metabolism but in different ways for different people, thus hormones are sometimes used to minimize the effects of menopause. Weight training can have a longer impact on metabolism than aerobic training, but there are no formulas currently written which can predict the length and duration of a raised metabolism from trophic changes with anabolic neuromuscular training. Gastric bypass surgery is used to reduce the content capacity of the stomach, bringing caloric intake down and lowering thermogenesis. Because the surgery significantly reduces caloric consumption, it will decrease BMR and RMR over time in the same fashion as aging, because the volume of the stomach is reduced. The stomach along with the rest of the digestive tract is a major contributor to BMR and RMR. Celiac disease, which reduces the ability of the stomach to digest food, may also reduce BMR and RMR. Celiac disease is fairly common, occurring in 1% of the U.S. population, with 2 million undiagnosed.

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