Plants are the primary source of proteins for all heterotrophic animals, and plant proteins are thus termed primary proteins. When proteins are consumed, the proteins are digested, which means that they are broken down into their amino acid components, and these amino acids are then absorbed and used to construct the various proteins required by the organisms. If animal proteins are consumed as dietary protein, then the protein is obtained from a secondary source and animal proteins are thus termed secondary proteins. In view of human amino acid needs, proteins are also classified as complete and incomplete proteins.
A protein that contains all the essential amino acids in balanced proportions is called a complete protein whereas a protein that is limited in one or more essential amino acids is called an incomplete protein. Plant proteins are mostly incomplete proteins and animal proteins are mostly complete proteins. This has led to the general assumption that animal proteins are superior to plant proteins in terms of their nutritional value, and has led many to believe that a diet consisting solely of plants would be inferior to one which included animal products. This would certainly be true, if all plants had similar amino acid shortages, but as this is not the case, there is no sound reason for arguing that animal products supply a better protein than plants. Of course, if one were to follow a restrictive vegetarian diet, which included only a limited variety of plants, then a plant-based diet would certainly be inadequate.
A plant-based diet will supply all the essential amino acids if a variety of foods is utilized, and they will be as effective in meeting the body’s needs as proteins from animal sources.4 Plant proteins contain more branched chain amino acids than do animal proteins and they are easier to digest than animal proteins. Animal proteins, on the other hand, are rich in the sulphur-containing amino acids cysteine and methionine, and also have a greater proportion of the aromatic amino acids phenylalanine and tyrosine. Excesses of these two groups of amino acids have been associated with various degenerative diseases, in view of their degradations to cresol and phenol which are promoters of skin and colon cancer.5
The ratio of the various amino acids to each other may be equally as significant as the presence of essential amino acids in determining the value of a protein. Plant proteins produce higher levels of arginine and glycine in the blood than do animal proteins and the higher levels of these amino acids is associated with protection against the clogging of arteries and arteriosclerosis.6 In our own laboratory, we have found that legumes as well as grains produce high plasma levels of arginine in all experimental animals tested thus far, which include rats, rabbits and vervet monkeys (unpublished data). The mechanism whereby high levels of arginine in particular, affords protection against degenerative diseases such as arteriosclerosis and osteoporosis is not clear at this stage, but might be related to its role in the elimination of nitrogenous waste products via the urea cycle. High protein diets will necessitate large-scale deamination of amino acids as dietary protein excesses have to be converted carbohydrates or fats before they can be stored by the body. Deamination produces ammonia, which is toxic to the system, and high levels of arginine would aid the rapid conversion of the highly toxic ammonia to the less toxic urea, thus limiting the effects of ammonia.
Animal proteins tend to have higher proportions of essential amino acids, except for arginine, than do plant proteins. The higher values in themselves need not necessarily be regarded as a positive attribute, as some amino acids are required in higher concentrations than others. High levels of essential amino acids that are used in only small amounts, will also require the conversion of the excess, and thus lead to increased toxic loading. More important than the absolute quantity of essential and non-essential amino acids in the food we consume, is the
ratio in which these amino acids occur. The more closely the ratio is attuned to our needs the fewer conversions will be required and the lower the toxic load will be. In this regard, it has been suggested that the ratio of lysine to arginine could be important in determining the ability of a protein to induce arteriosclerosis.7,8 Early studies showed, that if animal protein is fed to rabbits, they develop arteriosclerosis and have elevated cholesterol levels even if their diet is cholesterol free. If they are fed plant proteins, such as soya, these effects are not observed. Moreover, the plant protein source was shown to decrease the degree of sclerosis even in those animals that were fed cholesterol.7 Models of cholesterol metabolism based on experiments with rabbits have been criticised, in view of the rabbits unique hypersensitivity to dietary cholesterol, but the fact that a plant protein source could decrease sclerosis is significant.
Recent studies have shown conclusively that animal proteins increase cholesterol levels, whereas plant proteins tend to reduce the levels of cholesterol in animals and humans.9,10 Apparently, the ratio of lysine to arginine plays a significant role in this hypocholesterolaemic effect, and the concentrations of various other amino acids are also implicated. It is therefore not surprising, that various national bodies and expert groups recommend an increase in the consumption of plant foods to improve long-term health.11 Plant foods that are rich in proteins, also come pre-packed with other macronutrients, vitamins and minerals which enhance the digestion and assimilation of these foods. The modern trend of refining plant protein sources, so as to obtain concentrated forms of protein as substitutes for animal products, thus strips them of these additional components. The use of unrefined plant protein sources, such as grains, legumes and nuts has added advantages, in that many of these foods contain phytochemicals that protect against cancer.12
Diets high in animal protein are normally low in carbohydrates, particularly fibre, and in typical Western diets up to 12g of partially digested protein (approximately 2g nitrogen) enters the colon daily in the form of protein, peptides and amino acids.13,14 When carbohydrate levels are low the bacteria in the colon will utilize these protein residues, particularly peptides, to meet their metabolic demands and liberate ammonia, short chain fatty acids and a variety of other products including phenols and branched chain fatty acids. Urinary phenol levels increase when a high meat diet is followed and decrease when more fibre is present.15 Phenols have been implicated as promoters of bowel cancer and ammonia increases cell proliferation and has also been linked to colon cancer.16.