An amino acid that appears in lower that normal amounts in a particular plant protein is sometimes referred to as the limiting amino acid of that food source.
Soy is a widely used vegetable protein source and has a PDCASS score of 1, the equivalent of an animal protein, This rating makes it a good choice for vegetarians and individuals who are lactose intolerant. It has a high concentration of BCAAs and is a complete protein. As with whey, there are three soy protein types.
This can be natural or full fat – containing natural oils, lecithinated (lecthin added) or defatted (all oils removed). Soy flour is the least refined of the three types and has a 50% protein composition.
Soy concentrate is made from defatted soy beans and was developed in the early 1970s. It does not have as much soluble CHO as the flour but retains most of the beans protein content (approx 70%) making it a more palatable choice. Soy concentrate can be found in nutrition bars, yoghurts and cereals.
Soy isolates have the greatest concentration of protein at about 90% and are the most refined. They contain no dietary fibre, unlike the first 2 and are very digestible. These make them a good choice for additions to sports drinks and health beverages.
Nutritional Benefits of Soy
Populations that have high intakes of soy shared a lower incidence of certain cancers, improvements in menopausal symptoms, osteoporosis in women and decreased cardiac conditions. Many if the health benefits of soy are attributable to the physiologically active components found in soy. These include phytosterols, isoflavones, orotease inhibitors and saponins. These have all been shown to increase LDL oxidation, lower blood lipids and are beneficial in reducing hypertension.
Plant proteins are found in fruit, vegetables, nuts, seeds and beans. Although these food products are low in some of the essential amino acids, it is inaccurate to describe plant proteins as incomplete proteins. In fact, all common seeds, grains, nuts and legumes contain the essential amino acids.
Legumes tend to be high in tryptophan and lysine while grains are low in these. The deficiencies in particular types of plant proteins can of course be overcome by combining different sources, including meals such as beans on toast, and vegetables and nut roast.