Protein is a nutrition buzzword, but a new study shows that not all proteins behave the same way in the body.
Researchers at North Carolina State University found that some proteins are digested less completely than others, and the leftovers that make it to the large intestine can fuel gut microbes in ways that may influence health.
The study, published in Food & Function, compared purified proteins from six sources, soy, casein, brown rice, yeast, pea and egg white, in mice. Using high-resolution mass spectrometry, the team tracked which proteins were broken down in the small intestine and which persisted into the colon. They discovered that even proteins thought to be highly digestible, such as egg white, left behind fragments that reached the gut. Brown rice protein, in particular, showed poor digestibility, making up about half of the protein found in fecal samples.
“Protein that isn’t fully digested makes its way to the colon, where it can interact with gut microbes — and those interactions may not always have the effect you’re aiming for in your diet,” said lead author Ayesha Awan, a PhD candidate at NC State.
Normally, most dietary protein is broken down into amino acids and absorbed in the small intestine. But when protein fragments escape that process, they become food for microbes in the large intestine. Sometimes this is helpful, leading to the production of short-chain fatty acids that support gut health. Other times, the byproducts can fuel inflammation or disrupt metabolism. The NC State team found that the source of protein strongly influences which way the balance tips.
Certain “antinutritional” proteins, such as the trypsin inhibitor in soy and antimicrobial proteins in egg white, were more likely to survive digestion and reach the gut microbes intact. These proteins may play a role in shaping the gut environment, with effects that could ripple into digestion, immunity and metabolic health.
“Oftentimes what people think about is animal protein versus plant protein,” said co-author Manuel Kleiner, PhD, associate professor of plant and microbial biology at NC State. “What we are finding is really it’s much more about the specific protein source and not about an animal-plant dichotomy.”
This study was supported by the National Institute of General Medical Sciences of the National Institutes of Health.