NPU: Other Factors Affecting Protein Utilization

Let’s return again to Household Number Four, the lacto-ovo-vegetarian teacher. As a scientist who weighs 180 pounds, he knows that he cannot just eat any foods that contain 74 grams of protein, the amount recommended for his body weight, and be sure he is getting all he needs. He judiciously matches foods with complementary amino acid profiles: rice with soybeans, wheat with beans, peanuts with milk, sunflower seeds with peanuts, brewer’s yeast, wheat germ, etc.


He also takes into consideration other factors that affect what his fellow scientists call net protein utilization (NPU). The NPU of any given food describes its usefulness as a protein source, it’s quality as a protein. NPU involves two factors: the chemical makeup (“biological value”) of the protein that is, the ratio of amino acids to each other, and digestibility. If you don’t eat enough fiber with your meals like many Americans you will probably digest considerably less of the protein you consume. And if you don’t consume enough of your calories as carbohydrates, some of the protein you eat will just be burned as fuel.

Even more important is how you cook protein. If you cook most protein foods under too high heat, that can interfere with both digestion and absorption. This poses a special problem for people who eat pork. It must be cooked very thoroughly to destroy the trichinosis causing trichinae organisms. In the process, some of the protein content is lost. This dilemma can be avoided by leaving pork out of our diets altogether.

Toasting bread makes some of its lysine content unavailable through binding; this can be compensated for by complementing the toast with another food group such as legumes, which are rich in lysine (try toasted, whole grain black bread with baked beans), or with a complete protein such as cheese (spread your morning toast with cottage cheese), or milk but make sure it is fresh milk; drying milk also makes the lysine less available.

On the other hand, cooking beans and other legumes make the protein more available. Heat deactivates chemicals called trypsin inhibitors which are found in uncooked soybeans and some other legumes.

Using these guidelines of chemical makeup and digestibility, nutritionists have determined NPU figures for the major protein sources. On a scale of one hundred, the NPUs for selected foods are:

Eggs: 94 Cheese: 70
Milk: 82 Meat and poultry: 67
Fish: 80 Tofu (soybean curd): 65

Nuts, seeds, grains, and legumes all foods that play an important part in vegetarian diets range in NPU from 40 to 60. If you want to get complete protein from the delicious Mexican classic of tortillas with frijoles refritos (refried beans), you will probably have to eat more of it than you would eat of a meat dish to get an equivalent quantity of usable protein. But this fact in no way suggests that the tortillas and beans are inferior as a protein source.

Everyone who pays attention to his or her health will have some objection to one or more of the major protein sources. Vegetarians don’t eat meat; some of us are allergic to cheese or other foods; others have to avoid milk or eggs because of their fat content. No single food is the answer to everyone’s protein needs.

The only solution is to obtain a full spectrum of dietary benefits by eating as many combinations of a protein food as possible. Happily, this is the most pleasurable alternative: with care and the right information, you can be sure of getting a balanced protein intake that also provides variety and delicious taste.

How to Increase Your NPU

The most important key to increasing the NPU of the protein you eat is food combining. Eggs are the only food that contains the eight essential amino acids in the ideal mixture. But you couldn’t get all your protein by eating eggs, even if you wanted to: they only contain about 6 grams of protein each.

So you must turn to other protein sources, the foods that don’t contain the ideal mixture. They, too, provide protein, even though they do it less efficiently.

The NPU of incomplete proteins is determined by what is called the “limiting” amino acid. This is the one present in the lowest proportion relative to the ideal egglike ratio of amino acids. For instance, a slice of bread may contain half the amount of lysine which would be needed for the protein in it to be complete, but relatively correct proportions of the other seven essential amino acids. As a result, your body might be able to use only one-quarter of the total protein contained in the bread.

On the other hand, if you eat the bread with a glass of fresh milk, peanut, sesame, or sunflower butter, the low lysine content will be compensated for by that in the milk. As a result, you’ll get a complete protein inefficiently usable form.

You could also just eat greater quantities of grains and cereals to compensate for those foods* insufficient levels of lysine. But a far safer way of avoiding lysine deficiency which may cause nausea and dizziness would be to complement the grain with a lysine-rich food group such as legumes. Eating whole wheat noodles with a sauce containing eggs and grated cheese would do away with the problem altogether. Adding a mixed salad of lettuce, dandelion greens, carrots, tomatoes, and alfalfa sprouts would give you an almost completely balanced meal.

If you do plan vegetarian meals in a way that compensates for missing amino acids, you can easily eat a nutritionally sound diet. This has been confirmed by a number of studies sponsored by eminent authorities, including the Beth Israel Hospital Committee and the National Academy of Science. Even pregnant mothers and growing children can get sufficient protein from a vegetarian diet according to Dorothy Lane and M. G. Hardinge Vegans or pure vegetarians (those who eschew eggs and dairy products) may need dietary supplements to supply iron, B vitamins, and other nutrients; but they need not lack protein.


NPU net protein utilization describes the usefulness of any food as a source of protein, in terms of both digestibility and completeness of amino acid makeup. Charts that measure the NPU of various protein sources show average NPU. How you cook protein (as slowly and as little as possible is best, except for beans), and how much fiber you consume (fiber in the diet increases protein absorption), affect the actual NPU of the foods you are eating.

The NPU of eggs is 94 percent of the protein in eggs. Milk and fish NPUs are around 80; cheese 70; meat, poultry, and soybean curd about 65. The protein in nuts, seeds, grains, and legumes ranges from 40 to 60 percent usable; but these amounts can be increased by combining these incomplete sources, or adding complete protein foods to them so that the “limiting amino acid” the one present in the lowest proportion relative to the ideal is increased. For example, milk (containing extra lysine) can supplement the lysine in short supply even in whole grain bread.

A vegetarian even a vegan diet can supply all the protein you need if you are careful about food combining. Some effective combinations include:

grains and beans
grains and seeds
grains and nuts (or nut butter)
beans and seeds
nuts and seeds
any of these with eggs, milk, cheese,
brewer’s yeast, wheat germ, or sprouts

Soybeans are particularly effective since they contain more complete protein than other beans. Sprouting grains, other beans, and seeds also add to their NPU.


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