Function

• Regardless of your activity level, your body uses a significant amount of calories each day. Calories, which are a measurement of energy, are burned by the body to do everything from high-intensity workouts to breathing, chewing and sleeping. Before you can determine the number of calories your body needs, you must understand the factors which determine your energy requirements. Your age, weight, gender and activity level will determine your intake. In addition, three factors determine how calories are burned: your activity level, your basal metabolic rate (BMR) and the thermic effect of food.
  
  

BMR

• Your BMR accounts for about 60 to 70 percent of your total calories burned. It includes the calories that you burn while doing the most basic functions, such as sleeping, showering and breathing. A quick way to estimate the amount of calories you would need to consume to account for your BMR is to multiply your weight by 10. Or you can choose to use the more accurate Harris Benedict Equation. For men: BMR = 66 + (13.7 X weight in kg) + (5 X height in cm) - (6.8 X age in years). For women: BMR = 655 + (9.6 X weight in kg) + (1.8 X height in cm) - (4.7 X age in years). The general rule of thumb is that you should never consume less than your BMR.
  
  

Thermic Effect of Food

• The process of eating your food also burns calories. Known as the thermic effect of food, the process burns about 10 percent of the calories you consume. Proteins, which are the hardest to digest, have the greatest thermic effect. Fats, on the other hand, which are easy to process, have the lowest thermic effect.
  
  

Activity Level

• The higher your activity level is, the more calories you will need to consume. Your activity level isn't just based on the number of workouts you manage to squeeze into a week. Activities such as doing the dishes, walking to work, talking on the phone and driving also burn calories. Activity levels are broken into five categories and may account for 20 to 50 percent of your daily calorie use. To ensure that you are consuming enough calories for your activity level, refer to the following Harris Benedict Formula.
  
  Sedentary: BMR x 1.2
  
  Lightly Active: BMR x 1.375
  
  Moderately Active: BMR x 1.55
  
  Very Active: BMR x 1.725
  
  Extra Active: BMR x 1.9
  
  

Food Calories

• Food calories are found in proteins, carbohydrates and fats. Your body needs all three in order to function. According to the USDA, about 45 to 65 percent of the calories in your diet should come from carbohydrates, 10 to 35 percent should come from protein and 20 to 35 percent should come from fat. Aim for a diet that consists of low fat dairy products, lean proteins and a high amount of grains, vegetables and fruits.