Hope's Edge_ The Next Diet for a Small Planet - Frances Moore Lappe [88]
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