Evaluating Protein Content

Most plant proteins are considered "incomplete" proteins because they are relatively low in one or more essential amino acids. The "incomplete" label is somewhat misleading: all plant proteins do contain all essential amino acids.

But in most cases, levels of one amino acid or another are insufficient for human needs. Grains are typically low in lysine; beans are typically low in the sulfur amino acids, methionine and cysteine. However, the level of sulfur amino acids in soybeans is higher than in other beans, and as a result, soy protein is equivalent to animal protein in quality.

Soybeans are higher in protein than other legumes. About 35 to 38 percent of the calories in soybeans are derived from protein, compared to 20 to 30 percent in most other beans. Soymilk, regular tofu and tempeh contain seven, nine and 16 grams of protein per serving (although values vary considerably among brands).⁽¹⁾

Evaluating Protein Quality

The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is the official method by the World Health Organization, the U.S. Food and Drug Administration and the U.S. Department of Agriculture.

The PDCAAS is the amino acid score (amino acid pattern of a protein relative to amino acid needs) with a correction factor for digestibility. The PDCAAS uses human amino acid requirements to calculate the amino acid score. Specifically, the amino acid requirements used are those developed by the World Health Organization (WHO) for two to five-year-old children because it is the most demanding group other than infants. Those proteins that meet the WHO guidelines are given a rating of 1.0.

Soy protein contains enough of all the essential amino acids to meet biological requirements when consumed at the recommended level of protein intake. Children below two years of age have the highest amino acid requirements; therefore, infant formulas are often supplemented with methionine for added assurance.

Most soy products, with the exception of whole soybeans, are very well digested. Values for digestibility of steamed soybeans, full-fat soy flour, defatted soy flour, tofu and soy protein isolate are 65, 75 to 92, 84 to 90, 93 and 93 to 97 percent respectively.^(2,3) The high digestibility of soy protein products contributes to the overall high protein quality and PDCAAS score.

Conclusion

Meeting protein needs is relatively easy as long as caloric needs are met. However, many rich sources of protein are also high in cholesterol and saturated fat. Soyfoods provide high-quality protein, generally while staying cholesterol-free and low in saturated fat. The PDCAAS for soy protein isolate is 1.0, which makes soy equivalent to animal proteins in quality and higher than other plant proteins. Soyfoods can contribute significantly toward meeting protein needs, and they can make an important addition to a healthy diet for a variety of reasons.



REFERENCES
1. Pennington JAT. Bowes and Church's Food Values of Portions Commonly Consumed. 16th Edition. J.B. Lippincott Company, Philadelphia, 1994.

2. Young VR. J Am Diet Assoc 91:828, 1991.

3. Synder HE, Kwon TW. Soybean Utilization. Van Nostrand Reinhold Company, New York, 1987.

This information was adapted from Mark Messina, PhD, and Virginia Messina, MPH, RD, authors of The Simple Soybean And Your Health.