Protein is different from the other macronutrients because it contains nitrogen. Because of this, protein has unique structure, function, and digestion in the human body. Proteins can be found in all kinds of foods but are most commonly associated with animal tissue (meat). Molecularly, proteins are built out of animo acids. When these amino acids bind together they form strands upon strands of animo acids called peptides. As the peptides bind and fold on each other, polypeptides are formed and a protein is made.

Meat contains all 20 amino acids and therefore is considered a “complete” protein. The protein in grains, nuts, and vegetables lack certain amino acids making them “incomplete” proteins. Although they are incomplete, when you match incomplete proteins in a meal, a complete protein can be formed. This matching is known as complementary proteins. Grains lack the amino acids lysine, and legumes lack the amino acids methionine and tryptophan. When you combine a grain such as rice and legume such as beans, you consume a complete profile of amino acids.

Protein’s function in the body is to maintain structure, repair damages, maintain hormone balance, provide antibodies in the immune system, synthesize enzymes, transport nutrients, and an energy reserve. Muscle is composed primarily of protein, and without protein to maintain our muscles, we wouldn’t be able to move around. When the muscle takes damage by cuts, bruises, disease, or sport-related training they must be repaired by protein. Our immune system contains a vast array of defense mechanism to protect our body from pathogenic foreign invaders. All antibodies made by our immune system are built from proteins. Enzymes are paramount in metabolic function – they are also made from proteins. When protein is taken in properly through the diet the body can function normally and appropriately. Proteins like carbohydrates yield 4 calories per gram of protein. The body will not use protein as a source of energy until all carbohydrates are consumed and fat stores are utterly depleted. The recommendation for protein is different for each person. Based on a 2,000 calorie diet, an average male should consume 0.8 grams of protein per kilogram of body weight or around 10-35% of their total calories.