Different foods vary in their energy value and thus in their caloric value. This caloric value is determined by the amount of fat, carbohydrate, protein and alcohol contained in the food.

Fat produces the most calories - 9 calories per gram (butter).
Carbohydrates, such as sugar and starch (pasta), and protein (chicken breast) produce 4 calories per gram.

Alcohol produces about 7 calories per gram.
Water, fibre, vitamins and minerals do not affect the caloric value of a food.

Our body uses calories to keep us moving, breathing, sleeping etc. If you eat more energy (calories) than you burn, your body will store the excess as fat.

For every 3,500 calories that you accumulate, you will gain roughly one pound of fat. Similarly, if you use more energy than you eat, your body will use the stored fat and convert it into energy, and you will lose weight.
The number of calories that you need per day to keep your weight steady depends on your age, sex, size, lifestyle (especially the amount of exercise you get), body composition, and heredity.

Calorie Requirements :

Adult women between the age of 25 and 45 years require 2,300 calories per day.

Women over 45 and below 65 require approximately 100 calories less.
Women over 65 years of age require 1800 calories per day.
An average man between the age of 25 and 45 requires 3200 calories a day.
Men over 45 require 3000 calories a day.
Men over 65 require 2550 calories approximately.
Note: This amount of calorie intake helps maintain normal health and weight.

Therefore, in order to lose weight, a person will have to either reduce the number of calories consumed everyday or increase the amount of exercise and energy they use, or if at all possible both!

Convenience food or food on the move

Working people today eat at least a third of their meals away from home. At home, you may use prepared and packaged foods to save time and energy. Eating convenience food is all right as long as you make sure that it does not violate the basic principles of variety, balance, and moderation. Choose items that help fill gaps in food groups.

Nearly all fast food places have salad bars, and if you take only a little dressing, salads are low in fat, salt, and calories whilst providing essential dietary fibre. When choosing convenience food for home use, read nutritional labels carefully to make sure it fits in with your eating plan.