Health Questions and Answers

Question: What is the difference between high-quality and low-quality protein?

Answer:

Protein is found in many foods from both animals and plants. But proteins vary greatly in their make-up of amino acids. Some proteins are greatly deficient in some amino acids which are important to protein synthesis. The “ideal” protein has an amino acid pattern which exactly matches the body’s needs. To be sure, these needs vary. The needs of a newborn baby are exactly met by the protein mixture in mother’s milk. The needs of an adult are somewhat different, being determined by replacement rather than by growth. Egg albumin is generally thought to be a good reference protein. One can grade a protein according to how closely it conforms to the composition of the reference protein.

The amino acid score is the measure of this. This is calculated by comparing the amount of an amino acid in the protein with the amount in the reference protein. The score of a protein is the score for its lowest amino acid, the limiting amino acid. Usually the limiting amino acid is either tryptophan, threonine, lysine, or methionine + cystine. Notice that this defines higher-quality protein as being composed of a higher proportion of the essential amino acids than the reference protein.

A fundamental way of looking at protein is its ability to promote synthesis. We can measure synthesis in a variety of ways but the oldest and most generally useful is to determine the dietary protein intake and measure the urinary nitrogen output. Assuming the patient is in steady state, the difference should be measure of a net protein synthesis. Some proteins support synthesis better than others, and these are high-quality proteins.

Lastly, one can define protein as its ability to produce growth. The protein efficiency ratio is the ability to promote  growth in an animal; it is ratio of the weight gain to the protein intake over a study period of several days or weeks.

Reference: Deitel M (ed): Nutrition in Clinical Surgery, 2nd ed. Baltimore, Williams and Wilkins, 1985.

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