Animal protein vs plant protein: Which is best for the gut?

Animal protein vs plant protein, which is better for your gut? Research seems to show that replacing animal protein with plant protein has a beneficial effect on general health. This relationship is especially strong for cardiometabolic health(type 2 diabetes and cardiovascular disease).

However, these studies are often epidemiological, so they have their limitations. Furthermore, it likely depends. Animal protein is more bioavailable, so it may be better at preventing frailty in the elderly. That’s a pretty big benefit.

But, what about in the gut? In animal protein vs plant protein, who wins in the gut? A somewhat recent paper put this to the test.

They wanted to find out which created more hydrogen sulfide, a diet high in animal protein or a diet high in plant protein. They found that, for the most part, the animal protein diet created more hydrogen sulfide in the feces of participants.

However, it wasn’t straight forward. For some people, 1 out of 11, the plant protein diet created the same hydrogen sulfide than the animal protein diet. In another individual, the plant-based protein diet created more.

Which was better, based on hydrogen sulfide production, came down to their microbiome composition. Let’s unpack that a little bit, starting with why we should care about hydrogen sulfide.

Animal protein vs plant protein: Why hydrogen sulfide matters

Hydrogen sulfide(H2S) is a gas made endogenously within our cells. At the low levels produced in our cells, H2S has protective effects to our cardiovascular system and elsewhere. Generally speaking, it limits inflammation and regulates blood pressure.

In our gut, our microbiome contributes to exogenous hydrogen sulfide production. Sulfate-reducing bacteria(SRB) create H2S primarily by fermenting the sulfur-based amino acids cysteine and methionine.

As is the case with most things in physiology, H2S is a double-edged sword. At low levels it is generally protective, but at high levels it can be toxic, especially in the gut.

Excess H2S generated by our microbiome erodes the mucus layer, prevents butyrate oxidation, creates inflammation, and kills commensals. Furthermore, H2S can also inhibit autophagy, the recycling of damaged proteins.

Other toxic effects include:

Pathological conditions with high H2S production by sulfate-reducing bacteria include inflammatory bowel disease & colon cancer, IBS-D & SIBO, and Parkinson’s disease.

The effects of animal protein vs plant protein on H2S production

So why would animal protein be worse for H2S production than plant protein. It all comes down to the amino acid and fiber content.

In most cases, animal protein contains more of the sulfur-based amino acids cysteine and methionine, soy being an exception. Furthermore, our ability to absorb sulfur-based amino acids is finite.

As a result, when we consume more than we can absorb, the excess makes it to the colon. There, SRB can generate H2S from the excess.

Generally speaking, plant sources of protein tend to come packaged with fiber. Additionally, those who focus more on plant-based foods are going to have a higher fiber intake than those who focus on animal-based foods.

Fermentable dietary fiber has an inhibitory effect on H2S production. Both fructooligosaccharide and resistant starch inhibited H2S production by over 80% in feces in vitro.

This jibes with the current data that composition of the microbiome matters when it comes to which is better for an individual: animal protein vs plant protein. Consequently, the lower pH generated by the fermentation of fiber into short chain fatty acids inhibits the production of H2S by SRB.

What this tells us about your response to an animal-based diet

Interestingly, this information is useful for those undertaking an animal-based diet such as the Carnivore diet. How this type of diet affects your bowel habits indicates some tweaking you may have to do.

The fermentation of fiber by our microbiome creates hydrogen gas(H+). This hydrogen gas acts as a sort of precursor for one of 2 gases: Methane or H2S.

Interestingly, these 2 gases have opposing effects on our bowel habits. High methane production by methanogens promotes constipation. Conversely, high H2S production by sulfate-reducing bacteria promotes diarrhea.

If an animal-based diet causes diarrhea, this indicates a high prevalence of sulfate-reducing bacteria in the microbiome. On the other hand, if it promotes constipation, you likely have a high prevalence of methanogens.

Therefore, if you experience diarrhea on a Carnivore diet, adding fiber can help control H2S production. However, it’s a game of find the right dose. Excess H+ may go down the methanogen route, causing constipation.

But where does the hydrogen come from in a no-fiber diet? The proton pump in your stomach and fermentation of amino acids.


As is the case with most factors associated with diet, the animal protein vs plant protein debate depends on the individual. Not surprisingly, the microbiome of the individual.

For the most part, plant-based proteins lead to lower H2S production in most individuals. From a gut perspective, this is a good thing.

But, that doesn’t mean that you should totally eschew animal-based proteins. Animal protein is more bioavaliable and may have beneficial effects over plant protein for some purposes.

In fact, since plant proteins are less bioavailable and contain less cysteine, they are likely inferior to animal protein for endogenous H2S production.

All this brings us to the most important conclusion for the vast majority of people: Eat both. It’s a stupid argument for most people eating an omnivorous diet.

Conversely, if you go Carnivore, pay attention to your bowel habits. If you get diarrhea, add fiber as the Paul Saladinos and Joe Rogans of the world have done. But find the amount that doesn’t induce constipation.

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