Abstract:
The gastrointestinal (GI) tract harbours a large and diverse microbial community, collectively called
the microbiome, which is inextricably linked with host health. Diet is considered a chief influencing
factor of gut microbiome structure and activity, and a key modifiable lifestyle factor which impacts
health and disease risk. Dietary protein is an essential macronutrient in the human diet however the
impact of dietary protein on the gut microbiome is not well described. Protein quantity and quality
are important considerations for the effect of protein on the gut microbiome and production of
microbiota-derived metabolites.
High protein diets have been shown to alter gut microbiota community and structure and increase
production of detrimental proteolytic metabolites, however these studies are often confounded by
low fibre intake or energy deficits. High protein foods may also lead to increased production of a
co-metabolite trimethylamine-N-oxide (TMAO) a putative biomarker for cardiovascular disease
(CVD) risk. Here, we demonstrate that a high protein dietary intervention combined with adequate
fibre intake does not alter the gut microbiota composition or increase proteolytic metabolites after
10 weeks in healthy elderly men. However, TMAO, formed from gut microbial fermentation of
nutrients choline and L-carnitine, does increase after a high protein diet. As this was not
accompanied with biomarkers for CVD risk, TMAO concentrations may more accurately reflect a
role as a protein intake indicator rather than a CVD risk biomarker. However, the habitual diet and
health status of the participants recruited to this study are not representative of the typical elderly
New Zealand male.
Diets rich in red meat are linked with increased risk for chronic disease, by increasing the production
of detrimental proteolytic metabolites and increasing circulatory TMAO. Replacing red meat for
plant-based protein such as soy may mitigate this risk, through modifications in microbiota
community and activity. We demonstrated that a diet containing 500g per week of red meat
compared to a diet containing a portion-matched soy protein alternative does not increase
proteolytic metabolites or TMAO concentrations after 8 weeks. Therefore at this level of intake,
red meat consumption does not contribute to increased levels of detrimental metabolites associated
with chronic disease.
A protein which cleaves into different post-digestion peptides may, by its structure rather than
fermentative potential, influence gut microbiota community and function. The peptide bovine β-
casomorphin 7 (BCM7) is released after digestion from the A1 β-casein protein but not the A2 β-
casein in dairy, and is implicated in reduced GI motility and inflammation. We demonstrated that a
diet containing conventional dairy products (comprising both A1 and A2 β-casein) compared to a
diet containing A2 β-casein only dairy products did not induce changes in the gut microbiota
composition or function indicative of reduced GI motility or inflammation. Additionally, the increased dairy protein load compared to baseline (no dairy) also did not induce change in the
microbiota structure, although an increase in microbial lactose metabolism was observed.