As protein-rich diets become increasingly popular, a new study suggests that categorizing dietary proteins as either animal- or plant-based fails to effectively capture the source-specific differences in their composition, digestive efficiency and accessibility to the gut microbiota.
The North Carolina State University study shows that not all proteins are digested the same way. Some are digested less completely than others, instead moving to the large intestine where their interactions with the gut microbiota – the microscopic life within the gut – can often have significant effects.
Using high-resolution mass spectrometry, the study examined the fate of purified protein from six different sources – soy, casein, brown rice, yeast, pea and egg white – in both germ-free mice, which don't have a gut microbiota, and mice with a conventional microbiota. The methodology allowed researchers to discern which proteins escape host digestion and become available to gut microbes.
"We wanted to not only track how much protein is digested by the host, but also which specific proteins escape digestion to interact with the gut microbiota in the colon and ultimately which proteins make it out of the gut," said Ayesha Awan, an NC State Ph.D. candidate, lead author and co-corresponding author of a paper describing the study. "This is especially important at a time when everyone is incorporating more protein in their diets.
"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."
Strikingly, dietary proteins from all sources were detected in fecal samples of both mouse groups. This suggests that even proteins presumed to be easily digestible can reach the colon and serve as nutrition for the gut microbiota.
"Egg white is often thought of as a highly digestible protein source, but our study showed that a notable portion escapes digestion," Awan said. "Also, brown rice protein constituted about 50% of the fecal proteins and was not very efficiently digested by the host or by the gut microbiota."
Manuel Kleiner, an NC State associate professor of plant and microbial biology and co-corresponding author of the paper, said the study shows that not all protein sources act the same way.
"Oftentimes what people think about is animal protein versus plant protein," he said. "What we are finding is really it's much more about the specific protein source and not about an animal-plant dichotomy."
Additionally, the study found that the gut microbiota strongly influenced which proteins persisted through the intestinal tract. Specific proteins within each of the sources either degraded more in mice with gut microbes compared to mice without gut microbes or were conversely enriched. Notably, several diet-derived proteins with functions relevant to host health were among those differentially abundant. For example, antinutritional factors such as the Kunitz trypsin inhibitor in soy and several antimicrobial egg white proteins, including lysozyme and avidin, escaped digestion and were accessible to the gut microbiota.
"Dietary proteins have a major impact on host physiology," Kleiner said. "We still need to understand if these proteins are intact or active when they make it to the colon."
The study is unique in that it examined multiple regions of the digestive tract, rather than just the feces. The researchers found that digestion in the small intestine was largely unaffected by the presence or absence of gut microbes. Differences in the composition of dietary proteins only occurred in the large intestine and feces. These findings highlight the significant role of the gut microbiota in shaping the fate of dietary proteins in the large intestine.
"This work follows proteins throughout the gut, not just at the very end," Kleiner said. "Most of the digestion is happening the same at the start in the small intestine, whether the mice have a microbiota or not."
"That means that in the small intestine, whether or not you have gut microbiota may not have much of an effect on how you're processing that protein," Awan said. "This makes sense since there are fewer microbes in the small intestine, and they don't have much time to interact with the dietary protein. The main differences we're seeing are in the large intestine, where the microbiota has more interaction with the protein and can modify or degrade it."
Such interactions may influence the production of metabolites, such as short-chain fatty acids or indoles, which can affect host health, the researchers say.
Furthermore, the inefficient digestion of functional proteins in the gut – including enzyme inhibitors, lectins and antimicrobial proteins – could suggest possible roles in modulating gut physiology and microbial composition. These findings suggest that the source of dietary protein is an important consideration when trying to understand diet-associated health outcomes, including inflammatory bowel conditions and metabolic disorders.
"Future work will focus on how different sources of dietary proteins and their interactions with the gut microbiota affect host health," Kleiner said.
Alexandria Bartlett, J. Alredo Blakeley-Ruiz, Tanner Richie and Casey M. Theriot, all from NC State, co-authored the paper, which appears in Food & Function.
This work was supported by the National Institute of General Medical Sciences of the National Institutes of Health under award number R35GM138362.
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