How important a question this is for many potential vegetarians! Our culture seems to be obsessed with obtaining enough protein. And yet protein is one of the easiest nutrients to get. By an inappropriate choice of foods, a person might be deficient in vitamins A or C; but it is almost impossible to be protein-deficient on a calorically adequate diet. To see why this is so, we need to look at protein requirements as a percentage of calories.

Protein as a Percentage of Calories

Protein, fat, and carbohydrate--the three major components of common foods--all contain calories, in about this ratio:

1 gram of protein = 4 calories

1 gram of carbohydrate = 4 calories

1 gram of fat = 9 calories

Thus, if a potato weighing 100 grams contains 76 calories and 2.1 grams of protein, we say that it contains 2.1 x 4 = 8.4 calories as protein, or about 11% calories as protein.

According to the National Research Council, an adult male requires 2700 calories and 56 grams of protein. The 56 grams of protein represent 224 calories, or about 8.3% of calories as protein. For the adult female, the figure is about the same: 2000 calories and 44 grams of protein, or about 8.8% of calories as protein.

If wheat has 17% of calories as protein, potatoes 11%, broccoli 45%, corn 15%, and so on, then all of these foods provide enough protein on a calorically adequate diet, even if you eat nothing but potatoes, wheat, and broccoli. In fact, of the common plant foods, almost all provide more than 10% of calories as protein. Only the fruits, as a rule, contain less; but this is not going to be a problem unless one is trying to live on an all-fruit diet.

Table 1. Protein content of some common plant foods (100 gram dry portion) -- as a percentage of total calories

In 1971, a revolutionary new book came out espousing the virtues of a meatless diet. It became a million-copy bestseller and convinced many people to try vegetarianism or become vegetarians. That book was Diet for a Small Planet by Frances Moore Lappé.

In her book, Lappé put forward as her central concept the idea of "protein complementarity"--the idea that vegetarians should eat different kinds of proteins at a single meal in order to get the same quality of protein which was found in meat. Countless thousands of vegetarians thereafter referred to Lappé's charts and tables and struggled to understand the intricacies of balancing tryptophan, lysine, methionine, and all the other amino acids.

The basic idea was this: while meat contains all of the amino acids, plant foods were deficient in one or more of the eight "essential" amino acids. Therefore, balance plant foods weak in one amino acid but strong in a second amino acid, with other plant foods strong in the first but weak in the second. Simple, right? Well, simple to some people, but not so simple to others, who eventually gave up the effort and went back to a meat-based diet out of fear of missing one or another of the amino acids.

And yet the central thesis of this best-selling book, one which even today many vegetarians believe in, is false. There's no question that you need all of the amino acids. But virtually all plant foods have all of the essential amino acids; and not only are the amino acids there, they are present in more than enough quantity to meet the needs of normal adults, if you are on a calorically adequate diet.

It's true that plant foods have more of the requirements of some amino acids than of others. Rice is strong in tryptophan, methionine, and valine, and weak in isoleucine and lysine. But rice protein sufficient to provide 100% of our quantitative protein needs, also provides 265% of the adult male requirement for lysine and 266% of that for isoleucine. (It provides 400% or more of all of the others.)

The same is true for virtually all other plant foods. In fact, some plant foods which do not quite provide the requirement for total protein, such as sweet potatoes, do provide the minimum requirement for all of the essential amino acids.

The whole idea of "protein complementarity" got started in 1914 when Osborne and Mendel published a paper on rat nutrition. They noticed that baby rats fed a plant food diet did not grow as fast as other rats who ate the same diet plus a lysine supplement. Conclusion: these plant foods needed a lysine supplement.

Unfortunately, the nutritional requirements of rats and humans are quite different, and this was quickly demonstrated by experiments on humans. Studies in which humans have been fed wheat bread alone, or potatoes alone, or corn alone, or rice alone, have all shown that these plant foods contain not only enough protein, but enough of all of the essential amino acids, to support growth and maintenance of healthy adults. Particularly striking were the experiments involving rice: not only was the rice protein more than adequate, it was adequate when only about 2/3 of the calories were provided through the rice. This means that the actual requirement for protein for most individuals is actually less than 8% of calories as stated by the National Research Council; the NRC has padded its figures with a "safety factor" which many individuals do not need.

A few sample plant foods are shown with their "limiting amino acid" content in the accompanying table. (Limiting amino acids are the amino acids the food contains the least of in relation to human nutritional requirements.)

Table 2. Limiting amino acid content of selected "Low-Protein" plant foods

There are some ways you can become protein deficient, but it's pretty hard. One way is not to get enough food. We sometimes see people in famine areas with bloated bellies who are suffering from protein deficiency. They are also suffering from deficiencies of calories, iron, calcium, and vitamins A through Z. In short, they are "starving to death," and their problem is not so much lack of protein as it is lack of everything.

Another way to become protein deficient is to get almost all of your calories from alcohol and/or sugar. Sugar contains no protein! Hard liquor contains virtually no protein (beer contains very small amounts). So if you are an alcoholic sugar junkie, you may be in danger of protein deficiency. Another possible source of deficiency is that infants may be fed foods which they cannot digest.

Because of the sufficiency, or overabundance, of plant protein, animal products (milk, cheese, and eggs as well as meat, fish, and poultry) are completely unnecessary for adequate protein nutrition. Breast milk, incidentally, which has provided human infants with adequate protein for hundreds of thousands of years, provides 6% of calories as protein--far less than that of whole cow's milk, which contains 22% of calories as protein.

Not only is plant protein sufficient, it is often superior to animal protein. Excessive protein consumption is now strongly linked to bone weakness and osteoporosis. Studies done on calcium loss have shown that as protein consumption increases, so does calcium loss. Not only that, the protein in meat (which is higher in the sulfur-containing amino acids) causes a greater calcium loss than the same quantity of protein in soybeans! So as far as preventing calcium loss and the possibility of osteoporosis, plant protein is superior in this case to animal protein.

The word on protein complementarity is: forget it. The whole idea that this is necessary is a myth. Frances Moore Lappé has now essentially reversed herself on this issue, saying that getting enough protein "is much easier than I thought." It's good to get a variety of foods, because you need all the various vitamins and minerals--not because of protein. If you get plenty of a variety of plant foods, regardless of your combining techniques (or lack thereof), and you get enough calories (not too difficult for most of us), it's almost impossible to be protein deficient. Protein is one of the easiest nutrients to get.

--Keith Akers

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