Your personal calorie needs are unique, depending on such factors as age, gender, body size and activity level. A number of calorie-estimation equations developed from these values can be used to determine how many calories you need each day. The USDA MyPyramid website has a guide for how much food you need to eat based on your daily caloric requirements.
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As you get older, your caloric needs tend to decrease for a number of reasons, including lost lean body mass, reduced physical activity and negative metabolic adaptations from chronic low-calorie intake, according to the "American Journal of Physiology." A 25-year-old man weighing 170 pounds probably needs significantly more calories than a 75-year-old man at the same weight. Fight this decrease in caloric needs by maintaining your normal levels of activity and food intake as you age.
At similar weights men typically have higher caloric needs than women because of differences in body composition. Women maintain larger stores of body fat for child-rearing purposes, and body fat burns significantly fewer calories than does muscle. Essential body fat stores for women are 10 to 13 percent compared to only two to five percent for men, according to ACE Fitness.
Body Size and Composition
As you grow taller, your weight increases to support your larger body frame. However, your body composition plays a large role in your caloric needs. You need many more calories if most of your weight is muscle, compared with fat. One pound of muscle burns about three times the number of calories as a pound of fat, according to ACE Fitness.
Physical Activity Level
While you cannot change your age or gender, you can control your activity level, which is considered the largest modifiable factor of caloric expenditure. In fact, physical activity represents about 15 to 30 percent of our daily calorie needs, based on activity factors from Kansas State University. For example, walking or jogging one mile burns approximately 100 calories. Resistance training such as weightlifting not only burns calories but also promotes maintenance and development of lean body mass. More lean body mass means more calories burned every day!
Estimating Your Daily Calorie Needs
Two equations are commonly used to estimate calorie needs: Harris-Benedict and Mifflin-St. Jeor. The values obtained by these equations are then multiplied by an activity factor representing your typical weekly physical activity. The resulting number is your average estimated daily calorie needs. For example, a 180 pound man who is 5 feet 10 inches, 35 years old and works out a few times a week needs about 2,500 calories per day, while a 140 pound woman who is 5 feet 5 inches, 50 years old and is sedentary requires about 1,600 calories per day. To calculate your personal calorie needs, CaloriesPerDay.com has a helpful tutorial. Both equations are considered estimates, and your true calorie needs may be somewhat greater or less than the number calculated. Please consult a physician before starting any weight loss or weight gain program.