Plants are important sources of nutrients in human diets, yet despite some studies of cereal grains, legumes, tubers and nuts, data on the content of amino acids in these plant source are incomplete and highly inconsistent, possibly because different analytical techniques were used among the different laboratories.
In a recent study (Hou et al., June 13, 2019. Amino Acids. doi:10.1007/s00726-019-02751-0), researchers in the department of animal science at Texas A&M University used high-performance liquid chromatography methods to analyze the content of free and peptide-bound amino acids in major staple foods of plant origin: corn grain, peanuts, pistachio nuts, potatoes, sweet potatoes, wheat flour, soybeans and polished white rice. This work followed a previous report of amino acid and peptide composition in beef (Wu et al., 2016. Journal of Animal Science. 94:2603-2613).
Compared with beef, plant-source foods were generally found to contain a much lower content of glycine, proline, lysine, methionine plus cysteine, threonine and tryptophan, the amino acids that are very important in human nutrition and health.
Of note, taurine and creatine (a metabolite of arginine, glycine and methionine), carnosine (a dipeptide) and anserine (a dipeptide), which are all highly abundant in beef, were found to be absent from the plant-source foods. Taurine, creatine and the dipeptides have an important antioxidative effect and are crucial for the health and functions of organs in humans, particularly the eyes, heart and skeletal muscle, the researchers noted.
Furthermore, 4-hydroxyproline, which is highly abundant in meat and has a potent antioxidative effect in the intestine, was barely detectable or negligible in the plant-source foods.
Thus, based on the current concept of amino acid nutrition, the findings that show a diet of proper proportions of plant- and animal-source foods is likely most desirable for optimizing human nutrition and health.