Calories And Energy Requirements by Jane Thurnell-Read

People often talk about calories - too many calories or the number of calories in a chocolate bar, but what exactly are calories.

A calorie is a unit of heat energy. It is the amount of energy required to raise the temperature of 1 gm of water 1o C from 14.5 o C to 15.5 o C. The energy derived from food is measured in kilocalories (kcal). Kilocalories are also popularly known as calories. Kilojoules and mega joules are being used more and more as measures of energy.
1kcal = 4.2 kJ

Energy is needed for:

• The basic processes of life (e.g. breathing, metabolism, etc.)
• Muscular movement
• The creation of new tissues (growth and cell replacement)

Energy is derived from food. Protein, fat, carbohydrate and alcohol all contain chemical energy that can be transformed and used by the body. The body needs (has a physiological requirement for) carbohydrate, protein and fat, but no physiological requirement for alcohol.

• 1 gm of carbohydrate yields 4 kcal (17 kJ)
• 1 gm of protein yields 4 kcal (17 kJ)
• 1 gm of fat yields 9 kcal (37kJ)
• 1 gm of alcohol yields 7 kcal (29 kJ)

It takes about 3% of energy in the fat found in food to convert it to body fat.

It takes about 23% of energy in carbohydrate to convert it to body fat.

On average an adult man requires 2550 kcal per day and an adult woman 1940 kcal per day. This is a guideline only as there are enormous variations depending on activity levels, metabolic efficiency, etc. Energy needs increase for the repair necessary after surgery or severe injury.