Project Plant Protein: the P3 Study in Humans

A diet or a meal higher in dietary protein has been reported to promote satiety and suppress food intake in humans in several previous studies. It was suggested that the presence of amino acid in the blood as a result of higher protein intake served as signals that suppress appetite in humans. More recently, it has been observed that incomplete or lower quality proteins, i.e. those missing one or more essential amino acids, may suppress hunger more than complete proteins, as the ingestion of incomplete proteins in the long term may lead to deficiency of essential amino acids. Plant proteins are generally considered incomplete protein when compared to animal proteins, which mean that plant protein may have higher appetite-suppressing effects.

Dietary carbohydrates are the most potent stimuli of insulin secretion. However, the addition of dietary protein in a carbohydrate-rich meal has been shown to further stimulate insulin release.The insulinotropic effect of dietary protein is thought to be attributed to the branched-chain amino acid (BCAA) content of the protein-rich meal. Interestingly, there is a hierarchy of effects induced by protein from different sources. For example, milk protein has been shown to be superior to fish or plant protein in stimulating insulin secretion following a meal.Even within milk protein, whey protein stimulated insulin secretion more than casein protein. One of the factors that distinguish whey from casein proteins was the digestibility or the digestion rate of these proteins. Protein that is digested faster appear to increase blood amino acid levels faster, and increase insulin release faster.This implies that plant proteins that have lower digestibility rate relative to animal proteins may have lower insulinotropic effects, and the effective dose required to achieve optimal insulin secretion may be different from animal protein and warrants further investigation. In addition, the amino acid profile (in addition to lower digestibility) of plant proteins may differ from animal protein, especially the BCAA content that is associated with the stimulation of insulin secretion.

Dietary patterns and cuisines vary between countries and cultures. Unlike in the Western countries, plant-based foods are more commonly consumed in an Asian cuisine. Based on the statistics released by Food and Agriculture Organization, cereals and meat are the top two protein sources in both Western (29.1% and 26.4% respectively) and Asian (58.8% and 8.6% respectively) countries. The third major source of protein is dairy (16.7%) in Western countries, and pulses in Asia (7.4%). In addition, vegetarianism is widely practiced in Asian countries. Studies investigating the appetitive and the blood glucose regulating effects of dietary protein have been focused predominantly in animal protein, with a number of studies included plant-based protein such as soy protein. Since cereals and pulses are an important part of an Asian cuisine, it is important to understand and compare the effects of proteins from these sources on blood glucose and appetite regulation. Since the amino acid profile and the digestibility of these plant-based proteins differ from animal protein, it is important to also understand the effective dose that exerts optimal effects.