Novel method developed to measure protein digestion

Removing the hull of mung bean before ingestion increases the average digestibility of amino acids

By labelling spirulina and two legumes (chick pea and mung bean pea) with stable, nonradioactive isotopes of carbon (13C) and hydrogen (2H) respectively, researchers at St. John’s Research Institute have found a new way to accurately measure the digestibility of dietary proteins.

In the six adults who consumed the labelled proteins, 85% of the spirulina protein was absorbed while only about 57% was absorbed in the case of both chickpea and mung beans. The results were published in the American Journal of Clinical Nutrition.

The research team led by Anura V Kurpad from the Institute’s Department of Physiology found that removing the hull or skin of mung bean before ingestion increases the average digestibility of essential amino acids by 10%. There are two ways by which the hull reduces digestibility. While anti-proteases present in the hull block the action of intestinal digestive enzymes, the polyphenols in it reduce the activity of the digestive enzyme.

“We knew the hull has anti-nutrition factors that hinder digestion and that dehulling will increase amino acid digestibility. But the 10% increase in digestibility when dehulled was not expected,” says Sarita Devi at the Institute’s Division of Nutrition and first author of the paper.

“The hull has less effect in the case of chick pea whereas it is a substantial problem in mung bean,” says Prof. Kurpad.

In another study, which will soon be published in the American Journal of Clinical Nutrition, the team used stable isotopes to label and measure protein digestion in meat (chicken) and egg. At 92%, the essential amino acid digestion was highest in cooked chicken, followed by whole boiled egg at 89% and egg white protein at 86%.

“The amount of essential amino acid digested and absorbed is far higher in animal-source protein than plant-based protein,” says Prof. Kurpad. “In contrast, one has to consume about 40% more dal to get the same amino acid into the body as animal source protein. On the other hand, legumes are good for the environment and land and offer many other beneficial nutrients and so a balance has to be reached when deciding on the source of high quality protein foods in the diet.”

Of the 20 amino acids that we need, the body cannot synthesise nine; dietary protein is the only source of these essential amino acids. But till now there was no way to know how well the protein consumed is digested and absorbed by the body.

The crude method adopted so far was to feed protein-rich food and measure the nitrogen content in the faeces. “But protein is only digested and absorbed in the small intestine. In the large intestine the microbiome takes away or adds nitrogen. So measuring nitrogen in the faeces as an index of what has been digested and absorbed can be misleading,” says Prof. Kurpad. “If you have wrong information about digestion, we will end up with wrong recommendation for the quantity of specific food-based protein to be consumed.”

The team has been working for over 15 years on a stable carbon isotope (13C) method that they developed to measure the true amino acid requirement in humans. These studies have defined the requirement of amino acid has to be consumed every day to meet the protein requirement, and formed the primary evidence for the 2007 WHO/FAO/UNU Expert Committee Recommendations. These recommendations were very important since they overturned the earlier values for amino acid requirements set by the WHO/FAO/UNU in 1985, which was based on the nitrogen balance measurements from urine and faeces, and were inaccurate.

“Since even digestion measurement was not correct, owing to difficulties in accessing small intestinal events, we devised the dual-isotope method to measure digestion in the small intestine,” he says. These results, while setting up a new method, will inform policies on feeding populations and individuals the appropriate amounts and types of food.

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