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I have often heard people saying that any combination of a cereal (rice, wheat, maize, etc) and a legume (pulses, pea, gram, etc.) provides the complete range of amino acids.

Is it true?

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Yes, that is correct. Grains and legumes, for example, can be called complementary amino acid sources because when you combine the one with the other, you get all of the essential amino acids. Nuts and seeds are also complementary to legumes because they contain tryptophan, methionine, and cystine.

App for calculating amino acid complements

An online tool on MyFoodData will show how well two or more food sources fare viz. amino acid combination. The above link shows, as an example, how well lentils pair with rice. Check out the projected amino acid breakdown (see table below) and percent of daily recommended intake achieved by your chosen food combination (see image below).

Amino Acid Lentils (Cooked) Cooked White Rice Total
Weight (g) 198 158 356
Tryptophan (mg) (%RDI) 160 (57%) 49 (17%) 209 (75%)
Threonine (mg) (%RDI) 640 (61%) 152 (14%) 791 (75%)
Isoleucine (mg) (%RDI) 772 (55%) 183 (13%) 955 (68%)
Leucine (mg) (%RDI) 1295 (47%) 351 (13%) 1646 (60%)
Lysine (mg) (%RDI) 1247 (59%) 153 (7%) 1401 (67%)
Methionine (mg) (%RDI) 152 (21%) 100 (14%) 252 (35%)
Cystine (mg) (%RDI) 234 (81%) 87 (30%) 321 (112%)
Phenylalanine (mg) (%RDI) 881 (101%) 228 (26%) 1109 (127%)
Tyrosine (mg) (%RDI) 477 (55%) 142 (16%) 619 (71%)
Valine (mg) (%RDI) 887 (49%) 259 (14%) 1146 (63%)
Histidine (mg) (%RDI) 503 (72%) 100 (14%) 602 (86%)
Arginine (mg) 1380 354 1734
Alanine (mg) 746 246 993
Aspartic acid (mg) 1976 400 2376
Betaine (mg) ~ 0 0
Glutamic acid (mg) 2770 828 3598
Glycine (mg) 727 193 919
Proline (mg) 746 201 947
Serine (mg) 824 223 1046

Amino acids from combining lentils and rice as a % of recommended daily intake

Another app on calculator.net can tell you how much protein the American Dietetic Association (ADA), The Centers for Disease Control and Prevention (CDC), and World Health Organization (WHO) think you should be consuming based on your age, gender, height, weight, and personal activity level, and body fat %, if you choose the more accurate Katch-McArdle BMR formula (click ➕ Settings). Here is a sample output:

basic protein intake recommendations generated by the calculator

Scientific basis

The Essential amino acid Wikipedia article also observes that sufficiently caloric consumption of varied foods is adequate to achieve the recommended protein intake and that combining foods with complementary amino acids provides for complete proteins:

Foodstuffs that lack essential amino acids are poor sources of protein equivalents, as the body tends to deaminate the amino acids obtained, converting proteins into fats and carbohydrates. Therefore, a balance of essential amino acids is necessary for a high degree of net protein utilization, which is the mass ratio of amino acids converted to proteins to amino acids supplied.

Complete proteins contain a balanced set of essential amino acids for humans. Whole foods plant and natural animal sources provide all of the essential amino acids. Near-complete proteins are also found in some plant sources such as quinoa.

The net protein utilization is profoundly affected by the limiting amino acid content (the essential amino acid found in the smallest quantity in the foodstuff), and somewhat affected by salvage of essential amino acids in the body. It is therefore a good idea to mix foodstuffs that have different weaknesses in their essential amino acid distributions. This limits the loss of nitrogen through deamination and increases overall net protein utilization.

The following table of protein source vs limiting amino acid is then provided:

Protein source Limiting amino acid
Wheat lysine
Rice lysine
Maize lysine and tryptophan
Legumes methionine/cysteine pair and tryptophan
Egg, chicken, milk none; egg is the reference for complete protein

The article continues:

The amino acid distribution profile is less optimal in plant foods than in animal foods. but it is not necessary to consume plant foods containing complete proteins as long as a reasonably varied diet is maintained. Numerous pairs of different plant foods can provide a complete protein profile. Certain traditional combinations of foods, such as corn and beans, or beans and rice, contain the essential amino acids necessary for humans in adequate amounts. The official position of the Academy of Nutrition and Dietetics is that protein from an appropriate planned combination of a variety of plant foods eaten during the course of a day can be nutritionally adequate when caloric requirements are met.

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