Calories are a unit of energy and are more correctly expressed as kilocalories (kcal). Food energy is stored in the body as fat, muscle proteins or carbohydrates. For the same amount of weight, fat (9 cal/gram) has more than twice the calories of protein or carbohydrates (sugars and starches, 4 cal/gram).
A key factor in determining how many calories an individual can take in while maintaining weight is the resting metabolic rate (similar to the throttle-idle fuel consumption rate). This determines how many calories a person uses daily without any activity. These calories are expended just by breathing, muscular activity of the heart and other essential functions of living. Resting metabolic rates may vary as much as 400 calories per day in different individuals. Since approximately 3500 calories are equivalent to a pound of fat, 400 calories per day make a three and a half pound difference per week.
The average adult male will burn about 1800-2500 calories with normal activity each day. This includes a resting metabolic rate of about 1200-1500 calories per day and activity calories of 600-1000 per day. Additional exercise may increase this by several hundred calories. Different activities have different "caloric burn rates". Women have rates about 10% less than men. Lean muscle burns calories over 20% more efficiently than fat. Thus a muscular person will burn more calories without any activity than an overweight (overfat) person who has the same weight. Resting caloric rate decreases with age past approximately thirty years in both men and women. Lean muscle mass also decreases with age.
(published with permission in writing from:http://www.pilotfriend.com/aeromed/medical/aviation_medicine.htm)
