what is Calorie?

Calorie:

A calorie (cal) is the amount of heat energy required to raise the 

temperature of 1g of water 1°C from 14° to 15°C. A kilocalorie (kcal) is the amount of heat required to raise the temperature of 1000g of water 1°C.

When broken down one gram of each carbohydrates ,protein and fat releases 4, 4, 9 kcal of energy respectively.

To calculate the energy available from a food, multiply the number of grams of carbohydrate, protein, and fat by 4,4, and 9, respectively. Then add the results together.

For example, 1 slice of bread with a tablespoon of peanut butter on it contains 16 grams carbohydrate, 7 grams protein, and 9 grams fat :

16g carbohydrate x 4 kcal/g = 64 kcal
7g protein x 4 kcal/g                 = 28 kcal
9g fat x 9kcal/g                           = 81 kcal
Total                                                = 173 kcal

From this information, we can calculate the percentage of kcalories each of the energy nutrients contributes to the total. To determine the percentage of kcalories from fat, for example, divide the 81 fat calories by the total of 173 kcalories:

81 ÷ 173 = 0.468 (rounded to 0.47)
Then multiply by 100 to get the percentage:
0.47 x 100 = 47%

(Reference: Understanding Nutrition 7th Edition by Eleanor Noss Whitney and Sharon Rady Rolfes )

The general guide line is that approximately 57%, 30%, 13% of energy should come from Carbohydrates, Fats and Protein respectively.