A complete protein is defined by the Food and Nutrition Board of the National Academy of Medicine as having all nine amino acids essential for human health in sufficient concentration. What were the factors that went into calculating this profile that separates proteins into groups of "complete" vs. "incomplete"?
Foods are labelled as "complete proteins" if that food, consumed in isolation and only just enough to meet the estimated average requirement for protein, is able to meet human requirements for all essential amino acids.
Amino Acid Requirements
Wide differences in requirements for amino acids have been observed among similar individuals, so two different values are provided in the literature. Estimated Average Requirement (EAR) is an intake expected to be sufficient for an average (median) individual. By definition 50% of the population are above the median, and thus the intake would be insufficient. The Recommended Daily Allowance (RDA) is an intake expected to be sufficient for about 97% of the population.
Here are the EAR and RDA amino acid intake levels for adults aged 19 and older. Requirements for children are slightly higher.
Amino Acid EAR (mg/kg/d) RDA (mg/kg/d) ------------------------ ------------- ------------- Histidine 11 14 Isoleucine 15 19 leucine 34 42 lysine 31 38 Methionine + Cysteine 15 19 Phenylalanine + Tyrosine 27 33 Threonine 16 20 Tryptophan 4 5 Valine 19 24
The methods used to determine adequate intake for individual amino acids are too complex to summarize here. For more details, see the chapter on protein and amino acids in Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005).
The Reference Protein
In order to generate scores for different foods, we need a reference profile to compare against. The reference profile is generated by dividing the EAR for each amino acid by the overall EAR for protein (0.66 g/kg/d). If the reference profile was computed by dividing amino acid RDA by total protein RDA the results would be very similar. The reference profile is shown below.
Note: I observed a modest discrepancy between the presented values and what was expected based on the formula, so I have provided a second column showing the expected values based on the formula.
Amino Acid mg/g protein EAR ÷ 0.66 ------------------------ ------------ ---------- Histidine 18 16.7 Isoleucine 25 22.7 leucine 55 51.5 lysine 51 47.0 Methionine + Cysteine 25 22.7 Phenylalanine + Tyrosine 47 40.9 Threonine 27 24.2 Tryptophan 7 6.0 Valine 32 28.8
This is the standard profile used to define a complete protein. Note however, that because the EAR for amino acids is variable on several factors (eg. age, body composition) the definition of a complete protein is also variable.
Complete Nutrition with Incomplete Proteins
Even the so-called incomplete proteins can provide all essential amino acids in sufficient quantities, provided a larger amount of food is consumed.
For example, lentils (USDA SR28 Food #16069) is considered an incomplete protein because it has a lower methionine+cysteine (sulfur-containing amino acids, or SAA) content (21.6 mg/g) than the reference protein (25 mg/g). However, lentils alone could satisfy the SAA requirement if the total protein obtained from lentils was 16% higher than the total protein requirement.
Consider the example of a 70 kg person. The RDA for total protein would be 88 grams (based on 0.80 g/kg) and the RDA for SAA would be 1.33 grams. The SAA requirement could be met by consuming 900 kcal worth of lentils. Therefore even lentils in isolation, which are commonly considered an incomplete protein, can satisfy all amino acid requirements.
For further reading, the Food and Nutrition Board of the National Academy of Medicine publishes a freely-available book about dietary reference intakes for various nutrients, and this book contains 179 pages about protein and amino acids.