Soy Protein - Nutrition

Nutrition

Soybean protein is a "complete protein" since it provides all of the essential amino acids for human nutrition. Soybean protein is essentially identical to that of other legume pulses (that is to say, legume proteins in general consist of 7S and 11S storage proteins), and is one of the least expensive sources of dietary protein. For this reason, soy is important to many vegetarians and vegans.

Soy flour contains 50% protein.

The digestibility of some soyfoods are as follows: steamed soybeans 65.3%, tofu 92.7%, soy milk 92.6%, and soy protein isolate 93–97%. Some studies on rats have indicated the biological value of soy protein isolates is comparable to animal proteins such as casein if enriched with the sulfur-containing amino acid methionine.

Lafayette Mendel and Morris S. Fine of the Sheffield Laboratory of Physiological Chemistry at Yale University made the observation in the September 1911 edition of the Journal of Biological Chemistry that soybeans produce a positive nitrogen (N) balance in a human subject when they conducted a study to determine the utilization of legume proteins. The treatment called for five days of a 2,400 calories (10,000 kJ) diet consisting of meat, eggs, nut butter, potatoes, and fruit, followed by six days where 90.5% of total nitrogen was supplied by soybeans, and then another five days of the first diet, minus the nut butter. They discovered the soybean nitrogen is "distinctly (if only slightly) less well utilized than that of the preceding and succeeding mixed diets".

When measuring the nutritional value of protein, the original protein efficiency ratio (PER) method, first proposed by Thomas Burr Osborne and Lafayette Mendel in 1917, was the most widely used method until 1990. This method was found to be flawed for the biological evaluation of protein quality because the young rats used in the study had higher relative requirements for sulfur-containing amino acids than did humans. As such, the analytical method universally recognized by the FAO/WHO (1990), as well as the FDA, USDA, United Nations University and the National Academy of Sciences when judging the quality of protein is the protein digestibility-corrected amino acid score, as it is viewed as accurately measuring the correct relative nutritional value of animal and vegetable sources of protein in the diet. Based on this method, soy protein is considered to have a similar equivalent in protein quality to animal proteins. Egg white has a score of 1.00, soy concentrate 0.99, beef 0.92, and isolated soy protein 0.92. In 1990 at an FAO/WHO meeting, it was decided that proteins having values higher than 1.0 would be rounded or "leveled down" to 1.0, as scores above 1.0 are considered to indicate the protein contains essential amino acids in excess of the human requirements.