Complete protein

A complete protein supplies all nine essential amino acids in roughly the amounts needed to maintain lean tissue.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 224.} A protein low in one or more of them is incomplete.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 224.} Except for gelatin, animal proteins are complete, while vegetable proteins tend to lack enough of various essential amino acids.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 224.},^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 225.} ISSA uses eggs and peas as examples of complete and incomplete proteins, respectively.^{3International Sports Sciences Association, Fast Track Study Guide: Nutrition, p. 3.}

Protein quality also depends on digestibility, and plant proteins are generally less digestible than animal proteins.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 197.}

In practice

• Complete sources include eggs, dairy, meat and seafood; the IPTA textbook lists soy as the main complete plant food.^{5International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 501.}
• Grains are limited in lysine and threonine, while legumes are low in methionine, so combining them offsets the gaps.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 225.} They don’t have to be eaten in the same meal, just within the same diet.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 225.}
• Vegetarian and vegan clients can meet their needs with legumes, vegetables, seeds, nuts, rice and whole grains across the day.^{6Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 198.}
• Schoenfeld reports little evidence that the specific protein source matters for muscle growth when a client eats enough animal-based food.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 225.}