Amino acids (AA) contain both amine and carboxyl functional groups. Most are the building blocks for protein and are absorbed into the bloodstream following digestion of ingested animal and/or vegetable protein sources. However, not all proteins in the diet have the same nutritional value because each contains different proportions of the essential (or indispensable) AA. The essential and non-essential (or dispensable) AA terminology (see Figure 12) refers to whether or not a specific AA can be synthesised by the body at a rate sufficient to meet the normal requirements for protein synthesis. When sufficient essential AA are present, the protein is considered ‘first-class’ or ‘complete’, e.g. dairy products, eggs, fish and meat. In contrast, plant proteins are described as ‘second-class’ or ‘incomplete’ proteins and must be combined to equal ‘complete’ proteins. Specifically, if combinations of grains plus legumes (peas, beans and peanuts), grains plus nuts or seeds, and/or legumes plus nuts or seeds are consumed throughout the day, adequate amounts of the essential AA are available and protein synthesis is normal (these are often called complementary proteins). Otherwise, growth and/or tissue repair is impaired. As a result, strict vegetarians need to plan their diet carefully to ensure their daily ingestion of plant foods provides them with adequate quantities of each essential AA. Further, several AA (known as conditionally indispensable) including arginine, cysteine, glutamine, proline, tyrosine and perhaps others, can become essential under conditions of stress, e.g. trauma, exercise, etc.