11.1: Evaluating Dietary Protein

To make protein, cells must have 20 kinds of amino acids. We can’t make 9 of these, so we must get them in our diet. We can make the other 11 (or ingest them ready-made). For example, the amino acid alanine isn’t needed in the diet. We can make it by taking the amino (-NH₂) off of an “extra” amino acid and adding it to the pyruvate made from glucose (Chap. 9).

Foods are evaluated for protein in two ways: (1) Quality: Is the food’s content of amino acids similar to what we need? Proteins have different amounts of each amino acid. Corn protein, for example, has a lot of the amino acid leucine and very little tryptophan, compared to what we need. (2) Amount: How much protein is in the food?

Protein Quality

High-quality protein has an amino acid content that closely matches what we need. Animal foods generally have higher quality protein, e.g., the amino acid make-up of chicken meat is more in line with what we need. Other factors affect quality but are relatively minor. For example, a poorly digested food means a lower-quality protein, but we usually eat food in forms that are almost completely digested—we eat rice cooked, not raw.

Amino Acid Content

Virtually all proteins have all 20 amino acids* —the standard 20 needed to make protein (Table 11-1). Any other amino acid in a protein is created after the protein is made on a ribosome (see Chap. 10). For example, the amino acid hydroxyproline is made by adding “a hydroxy” (-OH) to proline in collagen.†

Table 11-1: The 20 Kinds of Amino Acids Needed to Make Protein

Hydroxyproline can’t be used to make protein; there isn’t a genetic code for it, as there is for proline. Cells can use only the standard 20, no more or no less, to make muscle protein or any other protein, although ads for supplements may tout: has 24 amino acids!

Since we can make 11 of the 20, evaluation of amino acid content focuses on the other 9 (Table 11-1). The closer the proportions of the 9 amino acids in a protein match our protein, the higher its quality. In other words, cannibals eat the highest quality protein. A supplement labeled, has 9 amino acids, probably wouldn’t sell as well as the one touting 24, though it might if it were labeled, has only the 9 amino acids required in the diet!)

Among plant proteins, soybean is near the top in quality, and wheat and corn near the bottom. As shown in Figure 11.1, the content of essential amino acids in corn isn’t a very good match to what we need.

Egg protein is the gold standard by which protein quality is measured. Protein in a hen’s egg is evenly divided between white and yolk. (Eggs have only a small amount of carbohydrate; all of the fat, cholesterol, iron, and carotene/vitamin A is in the yolk.) The high quality of egg protein isn’t surprising. After all, the egg represents the compact package of nutrients needed to make a bird!

Looking at an egg this way, it’s no wonder that it’s rich in cholesterol. It takes a lot of cells to form a chick, and cholesterol is an essential part of cell membranes. Cholesterol is a fancy molecule (Figure 5.2)—it takes a lot of energy to make it. There isn’t space to store much energy in an egg, so it makes sense to have a lot of ready-made cholesterol—and the correct amino acids in the right amounts.

^{*The protein powder gelatin lacks the amino acid tryptophan—it’s destroyed in the process of making gelatin from collagen in animal skin, bone, etc. Gelatin is used to make gummy bears, marshmallows, Jell-O, capsules for drugs, etc.
†Collagen is an abundant protein in animal tissue. Vitamin C is needed to convert proline to hydroxyproline, making the collagen more stable. Many of the symptoms of the vitamin-C-deficiency disease scurvy (bleeding gums, etc.; see Chap. 1) are due to the instability of collagen in tissues.}

Figure 11.1: Relative Proportions of Essential Amino Acids in Egg and Corn Protein

Limiting Amino Acid

Since all 20 amino acids are needed to make protein, protein synthesis can’t continue if even one is lacking (Figure 11.2). One amino acid can’t substitute for another. For example, lysine is the limiting amino acid in corn (Figure 11.1). It’s present in the lowest amount relative to our need, so it limits how much protein our cells can make from the amino acids in corn.

Figure 11.2: Combining Proteins with Different Limiting Amino Acids

Complementing Proteins

Keep in mind that the comparisons of protein quality in Figure 11.1 are for single kinds of protein. In reality, we eat meals—a mix of proteins. The quality of plant proteins is enhanced when those with different limiting amino acids are eaten together (Figure 11.2). An amino acid deficit of one protein is offset by the amino acid content of a different, complementary protein. This is what’s meant by complementing proteins.

Many people throughout history have been vegetarians by circumstance, so it isn’t surprising that plant foods have been combined “properly” long before the existence of amino acids was known. Lysine is the limiting amino acid in grain (e.g., corn, wheat, rice, oats, barley), whereas methionine is usually the limiting amino acid in legumes (e.g., soy, lima, garbanzo, pinto, kidney beans—and peanuts, which are really legumes, not nuts).

Grain and legumes have different limiting amino acids, so eating them together provides higher quality protein. Grains are high methionine, low lysine.* Legumes are low methionine, high lysine. Grain-legume combos include tortillas and beans, falafel (chickpeas/garbanzo beans in pita bread), baked beans and bread, tofu (soybean curd) and rice, and a peanut butter sandwich.

Nuts and seeds (e.g., sesame, pumpkin, sunflower) are often eaten with grains (e.g., stir-fried vegetables and cashew nuts over rice). Legumes and seeds also are a good mix, as in the Middle Eastern dish hummus, a chick pea-sesame seed combo. In a modern twist, Japanese scientists envisioned genetically complementing soy and rice proteins—inserting the gene for soybean protein into rice’s DNA.

^{*Lysine is sold as a feed additive. It’s the limiting amino acid in grain, so adding it to the grain fed to animals improves growth (more meat for the same amount of feed). It’s big business. In 1998, two former Archer-Daniels Midland Co. executives were indicted for criminal price-fixing of lysine.}