Humans require eight amino acids in their diet (possibly nine), and these are referred to as essential amino acids. These must be supplied in a minimum quantity and a deficiency of any of these can have detrimental effects on health. Generally, animal proteins supply all the essential amino acids required for human health in the correct ratios. However, plant proteins can be lacking in certain amino acids (legumes contain low amounts of methionine for example) and so consuming soley such plant material can leave the diet deficient in certain amino acids. However the problems associated with vegetable proteins have been over inflated because foods are rarely eaten in isolation, and combinations of plant foods generally provides a range of essential amino acids in high enough amounts. Some of the studies used to assess protein needs are also of questionable value as they have used particular sub-groups of the population or have been over short time periods. Isolated protein extracts from vegetables may therefore provide a good source of protein.
The relatively cheap nature of soy protein, compared to animal protein, has made it attractive to food manufacturers for addition to products (usually under the guise of textured vegetable protein). Isolated soy protein has become popular as the amount of soy entering the market has increased. The ability of isolated soy to provide the required essential amino acids for growth in humans has been investigated in a number of studies, some in comparison to animal proteins. For example, in one study1, researchers compared isolated soy and beef protein as the sole source of protein in young healthy adult males. The soy protein was fed at an intake of 0.8 g per kg of body weight per day over an 84 day metabolic balance investigation. In a second experimental period, beef protein was fed at an intake of 0.8 and 0.68 g per kg body weight per day from around 60 to 81 days. Both proteins maintained the nitrogen balance of the subject and use of radiolabelled potassium showed that body cell mass did not change between the protein treatments.
In another study by the same research group2, the nutritional value of soy was investigated by performance of a series of nitrogen balance studies using young healthy males. Graded intakes of soy protein were administered to the subjects (48, 64, 80 and 96 mg nitrogen per kg body weight), while a control group received egg protein (at 32, 48, 64 and 90 mg nitrogen per kg body weight). Analysis of the results showed no difference in the nitrogen retention between the soy and the egg protein groups. Because soy has lower levels of methionine than animal proteins, the researchers also looked at the effects of soy protein with additional methionine supplements on the nitrogen retention in healthy adult males. At lower levels of nitrogen intake (82 mg nitrogen per kg per day), some but not all intakes of methionine supplementation did improve nitrogen retention, but the pattern was not consistent. However, no beneficial effects of the methionine supplements were found between soy and egg when nitrogen intake levels were at 128 mg nitrogen per kg per day.
Evaluation of vegetable protein in human nutrition is important because much of the world’s population relies on vegetables for its protein needs. Only the developed nations have easy access to cheap animal protein, and some evidence suggests that the intakes in some individuals might now be too high. Another factor to consider is the environmental cost of producing protein from animals compared to vegetables. Generally vegetable protein production is less environmentally damaging and cheaper which makes such agriculture more sustainable. However, the lack of animal protein in the diets of those living in developing nations has been suggested to be a primary cause of the increase susceptibility of such individuals to infectious diseases, particularly of the respiratory system. The is relates to the requirement of the immune system of high quality protein sources. In reality, balance may be the key here, with a combination of vegetable protein and animal protein is probably the optimal way to satisfy protein needs.
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