Maintaining your body temperature is a function of your metabolism -- specifically your basal metabolic rate, or BMR. Your BMR is the amount of calories you burn at rest and can account for between 50 and 80 percent of all the energy you use. Individual caloric requirements will vary based on age, activity level, gender and body composition -- muscle requires more energy than fat.
Understanding Calories
A calorie is a unit of energy, it measures heat -- specifically it's a measurement of the amount of energy required to raise the temperature of 1 gram of water by 1 degree C. Nutritionists use the word calorie to describe the potential food has to give your body energy. The more calories in food, the greater the amount of energy you will have to use. Excess energy that you can't use immediately is stored for later use -- as a form of fat known as triglycerides.
Metabolism
There are three parts to your metabolism. Your BMR, which maintains body temperature and keeps you alive is the energy used to circulate blood, keep your heart beating and the many other involuntary chemical processes happening constantly in your body. Then there is the energy used for movement and the energy used for digestion -- called thermogenisis. Up to 80 percent of the calories you eat are used by your BMR, about 20 percent go to physical activity and only 5 to 10 percent are used for digestion. Digesting protein requires more energy than digesting fats or carbohydrates.
Calories and Metabolism
The number of calories you need to maintain your basal metabolic rate depends on your age and gender. The National Institutes of Health notes that a woman needs a minimum of 1,200 calories and a man needs at least 1,500 calories each day to maintain basic metabolic function. Then you need to add calories for your activity level -- the more exercise you do, the more calories you'll need to add to maintain your weight. If you want to lose weight, you may not need to add more calories -- but you must eat the minimum calories needed to maintain your BMR. If you don't consume enough energy, your body thinks you are starving and slows your metabolism to conserve energy -- it's a biological survival mechanism.
Calculating Your BMR
To calculate your BMR, you'll need to know your weight in pounds, your height in inches and your age in years. Women should use this formula: BMR = 655 + 4.35 x weight in pounds + 4.7 x height in inches - 4.7 x age in years. Men should use this formula: BMR = 66 + 6.23 x weight in pounds + 12.7 x height in inches - 6.8 x age in years. To determine how many calories you need to maintain your metabolism, including movement and digestion, simply multiply your BMR by 1.2 if you are sedentary; by 1.375 if you are lightly active; by 1.55 for moderate activity and by 1.75 if you are very active. This formula is known as the Harris Benedict Equation.
References
- Better Health Channel; Metabolism Explained; September 2009
- MayoClinic.com; Metabolism and Weight Loss -- How You Burn Calories; October 2009
- Medline Plus: Tips for Losing Weight
- American College of Sports Medicine; Metabolism is Modifiable With the Right Lifestyle Changes; March 2009
- BMI Calculator: Harris Benedict Equation
