Hope's Edge: The Next Diet for a Small Planet (25 page)

In addition, the recommended allowances of protein are calculated for healthy people. Ill health and age, as well as genetic differences, could result in greatly differing needs. Genetic differences may play a role not only in our needs but also in our taste for foods (which may or may not be related to needs). In a recent study, adult identical twins were found more similar in their choices of foods, including the protein density of the diet, than were fraternal twins.
³

Figure 11. Hypothetical Mixed Plant and Dairy Diet (Just to Prove a Point)

Effects of Stress

Even more surprising, any individual’s need for protein can vary a lot. Physical stress—pain, for example—or psychological stress—even from exam pressure—can push your protein need up by as much as one-third. But remember, most of us eat almost twice the protein our bodies can use, so we can easily get the “extra” protein needed under stress from the protein already in most of our diets.

A World Health Organization report
⁴
discussed these stress conditions: (1)
heat:
unacclimatized individuals lose nitrogen (a primary component of protein) in heavy sweating; (2)
heavy work:
athletes and others may need additional protein when they are increasing their muscle mass, although the amount needed is not likely to be large (some studies, though not widely substantiated, suggest an additional 25 percent intake over the totals recommended here if you are building muscle mass); (3)
inadequate energy intake:
when overall calorie intake is not adequate, some dietary protein is used for energy and thus is not available to meet protein needs; (4)
infection:
infections, especially acute ones, cause some depletion of body nitrogen due to increased urinary excretion and poor intestinal absorption (as with diarrhea); these losses need to be replaced with additional protein during recovery.

The obvious conclusion is this: we should suspect any diet “expert” who claims that we
all
would do better on a high-protein or a low-protein diet. Instead of following a recommended allowance blindly, we should become better observers of our own body’s well-being, developing what protein researcher Williams calls “body wisdom.” Part of body wisdom is being aware of how you feel—your energy level, general health, and temperament. (Certain nutritional deficiencies negatively affect appetite and choice of foods, so just feeling “satisfied” is not enough.) Body wisdom also involves being a wise observer of your body’s condition: many types of nutritional deficiencies show up as deterioration in the hair, skin, and nails and in the slow healing of wounds.

Why Do We Need Protein Anyway?

Given protein’s importance to the body, perhaps it is not so surprising that a certain mystique grew up around it. We simply cannot live on fats and carbohydrates alone. Protein makes up about one-half of the nonwater components of our bodies. Just as cellulose provides the structural framework of a tree, protein provides the framework for animals. Skin, hair, nails, cartilage, tendons, muscles, and even the organic framework of bones are made up largely of fibrous proteins. Obviously, protein is needed for growth in children. Adults also need it to replace tissues that are continually breaking down and to build tissues, such as hair and nails, which are continually growing.

But talking about the body’s need for “protein” is unscientific. What the body needs from food are the building blocks of protein—amino acids, specifically the eight that the body cannot manufacture itself, which are called “essential amino acids.” Even more precisely, what the body actually requires are the carbon skeletons of these essential amino acids that the body cannot synthesize, although it can complete them by adding nitrogen, if the nitrogen is available. The body needs many more amino acids than just these eight essential ones. The body can, however, build the others
if
it has sufficient “loose” or extra nitrogen to build with. Thus, what is popularly referred to as the “protein” the body needs to eat are the eight essential amino acids and some extra nitrogen.

The body depends on protein for the myriad of reactions that we call “metabolism.” Proteins such as insulin, which regulate metabolic processors, we call “hormones;” other proteins, catalysts of important metabolic reactions, we call “enzymes.” In addition, hemoglobin, the critical oxygen-carrying molecule of the blood, is built from protein.

Not only is protein necessary to the basic chemical reactions of life, it is also necessary to maintain the body environment so that these reactions can take place. Protein in the blood helps to prevent excess alkalinity or acidity, maintaining the “body neutrality” essential to normal cellular metabolism. Protein in blood serum participates in regulating the body’s water balance, the distribution of fluid on either side of the cell membrane.

Last, and of great importance, new protein synthesis is needed to form antibodies to fight bacterial and viral infections.

How Much Is Enough?

The protein allowances I use in this book are those recommended by the Committee on Dietary Allowances of the National Academy of Sciences, Food and Nutrition Board. It’s interesting to learn how the committee arrives at these recommended allowances. Keep in mind that the procedure is full of
assumptions
(some of which are disputed within the scientific community),
estimates, and averages
. Realizing this, R. J. Williams’s advice takes on even greater importance: observe your own body carefully to find out what is best for you.

To come up with the recommended allowance for an entire population, the committee followed four steps:

Step 1. Estimating average need
. Since nitrogen is a characteristic and relatively constant component of protein, scientists can measure protein by measuring nitrogen. To determine how much protein humans need, experimenters put subjects on a protein-free diet. They then measure how much nitrogen is lost in urine and feces. They add to this an amount to cover the small losses through the skin, sweat, and internal body structure. For children, additional nitrogen for growth is added.
The total of these nitrogen losses is the amount you have to replace by eating protein
, and is therefore the basis of the average protein requirement for body maintenance—24 grams of protein for a 154-pound man (also expressed as .34 gram of protein per kilogram of body weight).

Step 2
.
Adjusting for individual differences
. To allow for individual differences and to cover 97.5 percent of the population, the committee sets this protein requirement 30 percent above the average, arriving at 30 grams per day of protein for a 154-pound man, or .45 gram per kilogram of body weight per day. This assumption that 30 percent above the average requirement will cover 97.5 percent of the population is one of the issues in dispute by nutritionists.

Step 3. Adjusting for normal eating compared to experimental conditions
. Scientists have discovered that protein is used less efficiently when people are eating a normal diet containing some extra protein than when they are eating at or near their protein requirement, as they do under experimental conditions. Apparently, when people are deprived of protein their bodies compensate by more fully using what’s there and excreting less. So to account for the less efficient use of protein in ordinary eating patterns, the committee adds another 30 percent. This brings the allowance up to 42 grams for a 154-pound “average” American man, .57 gram per kilogram of body weight per day.

Step 4. Adjusting for protein usability
. The protein in our food is not fully used by the body. The above estimates are all based on an ideal “reference protein” (I’ll explain this fully in the following section). Scientists estimate the average usability of protein in the U. S. diet at 75 percent Therefore, the allowance of 42 grams of protein for a 154-pound male is pushed up to 56 grams because it is assumed that only 75 percent of what is eaten is actually used. For a 128-pound woman, the average American female, the corresponding allowance is 44 grams.

So now you know how the National Academy of Sciences arrived at the recommended protein allowances that are used throughout this book. Since it is set 30 percent above the average, it is
more than most people need
. Some protein authorities, however, believe that the allowance still may not be high enough to include 97.5 percent of the population.

Special Needs

While I’ve said that protein complementarity is not necessary for most of us, it does come in handy for those who must increase their protein intake without increasing calories. This is true for those whose bodies are under special stress, especially pregnant and breast-feeding women. A pregnant woman is advised to up her protein intake by an additional 30 grams a day (a 68 percent increase) but her calorie intake by only 300 calories (a 15 percent increase). A breast-feeding woman is advised to add 20 grams of protein to her diet but only 500 more calories; that would be 45 percent more protein but only 25 percent more calories. With these high protein needs, it becomes important to make the most of all the protein you eat, and combining complementary protein can help do that.

Many are concerned about the protein needs of children. Actually, they do not need more protein in relation to their calorie intake than adults. But because infant and young children cannot digest certain plant foods as easily as adults, some special care is needed in meeting their needs on a largely plant food diet. Michael and Nina Shandler have provided guidance for parents in
The Complete Guide and Cookbook for Raising Your Child as a Vegetarian
(Schocken Books, 1981). It also contains sound nutritional advice for pregnant women.

4.
Protein Complementarity: The Debate

I
N THE PREVIOUS
chapter I tried to dispel the myth that you need lots of meat to get the protein you need, while confessing that
Diet for a Small Planet
had helped create a new myth—that to get the protein you need without meat you have to conscientiously combine nonmeat sources to create a protein that is as usable by the body as meat protein. Protein complementarity is not the myth; it works. The myth is that complementing proteins is
necessary
for most people on a low- or nonmeat diet. With a healthy varied diet, concern about protein complementarity is not necessary for most of us.

Nonetheless, for several reasons, I would like to explain briefly protein complementarity. The first reason is that it is useful for people with a considerably higher than average protein need, including pregnant and breast-feeding women. Second, understanding protein complementarity does disprove any notion that animal protein is uniquely qualified to meet nutritional needs. But perhaps my real reason is simply that it fascinates me, particularly since I realized that complementary protein combinations evolved spontaneously as the basis of virtually all of the world’s great cuisines.

If all proteins were the same, there would never have been a controversy about preferable sources for humans; only quantity would matter. Proteins, however, are not identical. The proteins which our bodies use are made up of 22 amino acids in varying combinations. As already noted, 8 of these amino acids cannot be synthesized by our bodies; they must be obtained from outside sources. These 8 essential amino acids (which I will refer to as “EAAs”) are tryptophan, leucine, isoleucine, lysine, valine, threonine, the sulfur-containing amino acids (methionine and cystine), and the aromatic amino acids. (Histidine is also necessary for children.)

Our bodies need all of the EAAs simultaneously in order to carry out protein synthesis. If one EAA is missing, even temporarily, protein synthesis will fall to a very low level or stop altogether. We also need the EAAs in differing amounts. In most food proteins all of the EAAs are present, but one or more of the EAAs is usually present in a disproportionately small amount, thus deviating from the most utilizable pattern. These EAAs are called the “limiting amino acids” in a food protein.

Let us put together these three critical factors about protein:

Of the 22 necessary amino acids, there are 8 that our bodies cannot make but must get from outside sources.

All of these 8 must be present simultaneously.

All of these 8 must be present in the right proportions.

What does this mean to the body? A great deal. If you eat protein containing enough tryptophan to satisfy 100 percent of the utilizable pattern’s requirement, 100 percent of the leucine level, and so forth, but only 50 percent of the necessary lysine, then as far as your body is concerned, you might as well have eaten only 50 percent of
all
the EAAs. Only 50 percent of the protein you ate was used
as protein
. The protein “assembling center” in the body cells uses the EAAs at the level of the “limiting amino acid;” that is, at the level of whichever EAA happens to be least present. The surplus amino acids are released to be used by the body as fuel as if they were carbohydrates.
Figure 12
gives you a graphic illustration of what this means.

One reflection of how closely the amino acid pattern of a given food matches that which the body can use is what nutritionists term the “biological value” of a food protein. Roughly, the biological value is the proportion of the protein absorbed by the digestive tract that is retained by the body. In other words, the biological value is the percentage of absorbed protein that your body actually uses. There is, however, another question: how much gets absorbed
to begin with
by the digestive tract? That is what we call “digestibility.” So the protein available to our bodies depends on its biological value
and
its digestibility. The term covering both of these factors is “net protein utilization,” or NPU. Quite simply, NPU estimates how much of the protein we eat is actually available to our bodies. (See
Figure 13
.)

The NPU of a food is largely determined by how closely the essential amino acids in its protein match the body’s one utilizable pattern. Because the protein of egg most nearly matches this ideal pattern, egg protein is used as a model for measuring amino acid patterns in other food. The amino acid pattern of cheese nearly matches egg’s pattern, while that of peanuts fails utterly. You can guess then that the NPU of cheese is significantly higher than that of peanuts. The difference is great—70 as compared to about 40.

In
Appendix D
I provide basic protein information on over a hundred commonly eaten foods. I give the amount of usable protein (total protein adjusted by NPU scores), their amino acid strengths and weaknesses, and the contribution of one serving to meeting an average person’s daily protein allowance.

In the last few years, nutritionists have learned that the NPU ratings (see
Appendix C
) tend to overestimate the usability of protein by the body; that is, the NPU values we use are too high. Most of these ratings were determined in experiments in which people’s protein intake was grossly inadequate. At this low level, it turns out, the body uses protein more efficiently than when the diet contains adequate protein. This is especially true for the less usable plant proteins.
¹
What this means is that in a protein-adequate diet, we may have to eat slightly more of any given food to get the amount of usable protein indicated. Scientists are now proceeding with experiments based on this new understanding, but in the meantime, all we have are the current NPU scores.

Figure 12. The Problem of a “Limiting Amino Acid”

Figure 13. What Is “NPU”?

Source of Data:
Department of Agriculture Handbook No. 8, 1968; and
The Amino Acid Composition and Biological Value of Some Proteins
FAO, Rome. Courtesy of Dr. Isabel Contento, Department of Nutrition Education, Teachers College, Columbia University; adapted from chart in 1971 edition of
Diet for a Small Planet

Figure 14. The Food/Protein Continuum

This discovery does not call into question the perspective put forth here. It has been taken into account in setting protein allowances.

Is Meat Necessary?

Those who insist on the superiority, or indispensability, of meat as a protein source focus on both the large quantity and the high quality of protein in meat. Plant protein is seen as inferior on both counts. The result is that animal and vegetable protein are thought of as comprising two separate categories. In fact, this is a common mistake in our thinking about protein. It is much more useful and accurate to visualize animal and vegetable protein along a continuum.

Figure 14
, “The Food/Protein Continuum,” will help you see the range of protein variability on two scales: protein quantity, based on the percent of protein in the food by weight; and usability, based on the NPU of the protein. (Weights for grains and legumes are calculated for
cooked
food.)

Quantity
. When judging foods with the percentage of protein as the criterion, generalization is difficult. It is clear, however, that plants rank highest, particularly in their processed forms. Soybean flour is over 40 percent protein. Next come certain cheeses, such as Parmesan, which is 36 percent protein. Meat follows, ranging between 20 and 35 percent. Cooked beans, peas, and lentils have between 5 and 10 percent protein; though it might surprise you, eggs, milk, and yogurt are in the same range. There are, of course, other plants—some fruits, for example—that contain too little protein to even appear on the scale. (We are concerned here only with plants that are widely used as sources of protein.)

Warning: this quantity scale is misleading. It gives the percent by weight, yet the real issue in evaluating a protein source is not weight but calories. Can we get enough protein from a food without getting too many calories? Looked at from this angle, most plant foods qualify and some excel. Eating vegetables such as broccoli, cauliflower, mushrooms, and spinach,
you get the same amount of protein for each calorie that you get with meat
. But it would be difficult to get a full day’s protein allowance from cauliflower (unless you were prepared to eat 20 cups!). Nonetheless, such vegetables can contribute substantially to meeting our protein needs.

Usability
. The protein usability scale generally ranges from NPU values of about 40 to 94. Clearly, animal protein occupies the highest rungs of this scale. Meat, however, is not at the top. It places slightly above the middle, with an average NPU of 67. At the top are eggs (NPU of 94) and milk (NPU of 82). The NPUs of plant proteins generally range lower on the continuum, between 40 and 70. But protein in some plants, such as soybeans and whole rice, approaches or overlaps the NPU values for meat.

Complementing Your Proteins

Because different food groups have different amino acid strengths and weaknesses, eating a mixture of protein sources can increase the protein value of a meal; here’s a case where the whole is greater than the sum of its parts. The EAA deficiency in one food can be countered by the EAA contained in other food. For example, the expected biological value of three parts bread and one part cheddar cheese would be 64 percent if eaten separately. Yet, if eaten together, their biological value is 76 percent because of the complementary relationship. The “whole” is greater largely because cheese makes up for bread’s lysine and isoleucine deficiencies. Such protein mixes
do not result in a perfect protein
that is fully utilizable by the body (only egg is near perfect). But combinations can increase the protein quality as much as 50 percent above the average of the items eaten separately.