All foods are made of one or more of seven types of essential nutrients - so called because they are necessary to sustain life in the body. These types of nutrients are:

• Carbohydrates
• Fats
• Proteins
• Phytochemicals (the hundreds of vital substances in plants)
• Vitamins
• Minerals
• Water

Nutrition is the study of how to provide these nutrients in the right amount for good health. Changing these nutrients into smaller, simpler substances for ready use within the body is the process of digestion. Its main activity is breaking down and extracting. Digestion entails:

• Breaking down carbohydrates into simple sugars (glucose)
• Breaking down fats into fatty acids and glycerol
• Breaking down proteins into amino acids
• Extracting phytochemicals from plant-based foods

Carbohydrates, fats, and proteins are the only three nutrients that contain heat-producing energy. They are the fuels that the body burns in order to provide it with the heat energy it needs. The amount of heat generated when food is oxidized is measured in units of heat called calories. One calorie represents the quantity necessary to raise the temperature of a kilogram (roughly one quart) of water by 1C (1.8F).

These three nutrients burn at a different rates - some providing instant but short-lived flames," others burning slowly and efficiently for the warmth needed on a cold winter's night. Fats contain about 225 calories per ounce and are the slowest-burning fuel - the coal. Proteins contain about 115 calories per ounce and burn almost twice as fast as fats - they are the hardwood. Carbohydrates (including sugars) contain 115 calories or less per ounce and burn quickly - they are the softwood and kindling. However, since carbohydrates should be eaten in greater proportion than fats or proteins, they can provide consistent, efficient service.

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