Protein is the quintessential nutrient that every cell in the human body requires for growth or repair. The antibodies that protect us from disease, the enzymes needed for digestion and metabolism, and hormones like insulin are all proteins. Cholesterol travels through the bloodstream attached to lipoproteins (fat-carrying proteins). Connective tissue made from protein forms the matrix of bones. Keratin, still another type of protein, is used by the body to make hair and nails.
With so many essential functions linked to protein, you might assume that it should make up the bulk of your diet, but this is not the case. In an ideal balanced diet, only 10 to 12% of your daily calories should come from protein. Healthy adults only need 0.36 g per lb (0.8 g per kg) of body weight of protein every day, though if you exercise regularly, you may need more. Thus, a person weighing 154 lbs (70 kg) requires 56 g of protein per day—the amount in a 3-oz serving of chicken.
Proteins are made of amino acids. The human body requires 20 different amino acids to build all the proteins it needs. Of these, 11 can be made in the body, but the other nine, referred to as essential amino acids, must come from the diet. Just as the letters in the alphabet are joined to make words, so too are amino acids arranged in an almost infinite number of different ways to form the more than 50,000 different proteins in the body.
DNA (deoxyribonucleic acid), the genetic material that is found in the nucleus of each body cell, provides the blueprint for how amino acids are arranged to form individual proteins. With the exception of oils and pure sugar, all foods contain at least some protein, but its quality varies according to the amino acids it provides.
Animal protein (with the exception of gelatin) provides all nine essential amino acids in the proportions required by the body and is therefore referred to as complete, or high-quality, protein. Unfortunately, though, many animal proteins also come with relatively high amounts of saturated fat.