Or perhaps you should ask, what isn’t protein? Remove the water from your body, and protein accounts for around 75 percent of what’s left. Your hair, skin, muscle, bone and almost every other body part or tissue is made up of protein; you are protein!

But what does this have to do with protein in your diet? Dietary protein is often described in terms of amino acids, which are the building blocks your body uses to make the proteins that keep you alive. Your body needs 22 different amino acids (called essential amino acids) in order to make all the necessary proteins. Thirteen of these amino acids are made by the body itself, but nine of them must come from food, which is why you need protein in your diet.

The foods we eat for protein can be described in two ways:

Complete protein – These are foods which contain all nine amino acids that you need in your diet. All meat and other animal products are sources of complete proteins. These include meat, poultry, fish, shellfish, eggs, milk, and milk products.

Incomplete protein – These are foods that are low in protein or contain only some of the nine amino acids you need in your diet. Vegetable protein is usually considered incomplete because it is missing one or more essential amino acids. However, as long as you eat a variety of foods throughout the day, it’s easy to still get all nine amino acids without eating animal products.

Protein is body food. It is used to build new tissue, which is why there is an increased need for protein during periods of growth such as in infancy, childhood, adolescence and pregnancy. Here are some of the things protein does: 

• Provides the structure for muscles, hair and blood
• Repairs worn-out body tissue proteins resulting from general “wear and tear” to the body
• Supplies emergency energy when there is not enough carbohydrate or fat in your diet
• Helps transport important nutrients such as iron and cholesterol throughout your body
• Enhances your immune system
• Builds cardiac (heart) muscle
• Contributes to numerous essential body secretions such as hormones and enzymes. The only protein-free body fluids are urine and bile.

Protein and muscle. Excess protein won't help to build bigger muscles. Any extra protein you get is simply converted to, and stored as, fat. Carbohydrates are actually the best form of “fuel” for muscles exercised for long periods. In fact, a diet high in protein and fat, but low in carbohydrates, can impair athletic performance.