Protein: Requirements and Function
The second main component conceptualized by nutritionists as a key ingredient of a healthy diet is protein, that is used in many different ways to build up and repair human tissues. Proteins are basically chains of component parts called amino acids, and it is these amino acids which are the basic “currency” of protein nutrition. Twenty amino acids are common in foods, and of these nine are essential because humans cannot synthesize them from other nutrient molecules. Meat, fish, and eggs are animal-based and protein-dense foods that contain the complete profile of amino acids, basically because we are eating products that are very similar in composition to our own body tissues. In addition, some grains such as quinoa and buckwheat contain complete protein, while most legumes (peas, beans, soybeans, bean sprouts, products made from these) are high in proteins in a way that complements grains in the diet.
Protein Sources: Diet and Food System Aspects
For people who do not eat meat (a vegetarian diet) or who avoid all animal-based foods (vegan diets), the full complement of amino acids are accessed by eating milk and egg products or by eating a diversity of plant-based foods with proteins such as whole grains, nuts, and legumes. Legumes are particularly protein-dense and important in addressing the lack of amino acids in other plant-based foods. The combination of rice and beans is an oft-cited example of the complementarity of amino acids for a complete amino acid profile. Eating a wide range of plant-based foods is an excellent strategy to access the full complement of essential amino acids, as well as the diversity of mineral, vitamin, and fiber needs discussed on the next pages. Many of the most problematic diets are those that are highly monotonous due to poverty and/or inadequate knowledge about diet, with an excess or a sole dependence on a single starch source without legumes or animal products, or overconsumption of processed foods in comparison to fresh plant and whole grain foods. Where only a single grain is eaten, deficiencies of certain amino acids can result.
 These are phenylalanine, tryptophan, methionine, lysine, leucine, isoleucine, valine and threonine, which you can find in many introductory nutrition texts or resources online, if further interested. A ninth amino acid, histidine, is important in child growth and may also be vital to tissue repair, while another, arginine is essential for some growth stages and can usually be synthesized by healthy adults.