Gut Health Without Fiber

A carnivore diet contains essentially no fiber. For decades we have been told that fiber is essential for good digestion and long-term health, so this is one of the biggest questions people have. The evidence is more interesting, and more uncertain, than either side usually admits.

What fiber does

Dietary fiber is the part of plant food that human enzymes cannot digest. Some of it adds bulk to stool; some of it (fermentable fiber) is broken down by gut bacteria into short-chain fatty acids such as butyrate, which nourish the cells lining the colon and have wider effects on metabolism and immunity.

In large observational studies, people who eat more fiber tend to have lower rates of heart disease, type 2 diabetes, colorectal cancer and death. A major 2019 series of systematic reviews and meta-analyses commissioned by the World Health Organization found that the highest fiber intakes were associated with roughly 15% to 30% lower risk of a range of these outcomes compared with the lowest intakes^[1]. This is mostly observational evidence, and high-fiber eaters differ from low-fiber eaters in many ways, but it is a consistent finding and should be taken seriously.

It is worth noting what those comparisons involved: mostly people eating typical Western diets with more or less whole grain, fruit and vegetables. They do not tell us much about people eating a whole-food, zero-fiber carnivore diet, which was not a group represented in these studies.

The 2014 Nature study: an animal-based diet reshapes the microbiome fast

The most-cited study on this topic was published in Nature in 2014 by David and colleagues^[2]. A small group of volunteers ate either an entirely plant-based diet or an entirely animal-based diet (meats, eggs and cheeses) for five days, and the researchers tracked their gut bacteria.

What they found

• The gut microbiome changed within a day or two of starting the diet and returned to baseline within a few days of stopping.
• The animal-based diet increased bile-tolerant bacteria (such as Alistipes, Bilophila and Bacteroides) and decreased bacteria that ferment plant fiber.
• Fecal short-chain fatty acids from fiber fermentation fell, while markers of protein fermentation rose.
• One bile-tolerant organism that increased, Bilophila wadsworthia, has been linked to intestinal inflammation in mouse studies, which the authors raised as a possible concern.

How to interpret it

This study shows that diet powerfully and quickly shapes the gut microbiome, and that an animal-based diet produces a distinctly different microbial community. It does not show that this community is harmful to humans. The study was tiny, lasted only five days, and did not measure health outcomes. Different is not automatically worse; the question is what the long-term consequences are, and we do not know.

Diversity and "extinctions"

Microbial diversity is often treated as a marker of gut health. In a mouse study published in Nature in 2016, Sonnenburg and colleagues found that a diet low in fermentable fiber reduced microbial diversity, and that some lost species did not return when fiber was reintroduced; the losses compounded across generations^[3]. This is animal research and cannot be directly applied to humans, but it is a reason for caution about the idea that the microbiome will simply "bounce back" after years without fiber.

Constipation: the counterintuitive evidence

Many people assume a zero-fiber diet will cause constipation. In practice, many carnivore eaters report the opposite: less bloating, and smaller, less frequent, but comfortable bowel movements.

There is a small study that supports the idea that fiber is not always helpful. In a 2012 study of 63 people with idiopathic constipation, those who stopped or reduced fiber intake reported marked improvements in bowel frequency, bloating and straining, while those who continued high fiber did not improve^[4]. The study was small, not randomized (patients chose their group), and focused on a specific clinical population, so it does not prove that zero fiber is good for everyone. But it does challenge the assumption that more fiber is always better for bowel function.

In the 2021 carnivore survey, gastrointestinal complaints were the most commonly reported adverse symptoms, but they were reported by a small minority^[5].

Other gut-related questions

TMAO

Gut bacteria convert carnitine and choline, abundant in red meat and eggs, into a compound that the liver turns into trimethylamine N-oxide (TMAO). High TMAO levels have been associated with cardiovascular disease in observational studies. In a controlled feeding study, a diet high in red meat raised TMAO levels substantially compared with white meat or non-meat protein, and levels fell again when red meat was removed^[6]. Whether TMAO causes heart disease or is mainly a marker is still debated.

Colorectal cancer

The International Agency for Research on Cancer classifies processed meat as carcinogenic to humans (Group 1) and red meat as probably carcinogenic (Group 2A), primarily based on colorectal cancer associations in observational studies^[7]. The absolute risk increase is modest, and these studies involved people eating mixed diets. However, carnivore diets can be very high in red meat, have no fiber, and may include substantial processed meat, so this is a legitimate area of uncertainty. Favoring fresh meat and fish over processed meats such as bacon, salami and hot dogs is a sensible precaution.

Practical suggestions

• Expect bowel habits to change; less frequent is not necessarily constipation.
• If you are constipated, try more fluid and fat, less cheese, and consider magnesium after discussing with your doctor.
• Keep processed meats as occasional foods rather than staples.
• Stay up to date with colorectal cancer screening according to your age and risk; this matters on any diet.
• If you have a history of inflammatory bowel disease, involve your gastroenterologist.
• If you follow an animal-based version, small amounts of fruit add some fermentable fiber.

The bottom line

Removing fiber rapidly changes the gut microbiome, and many people's digestive symptoms improve. Fiber's benefits in large populations are consistent but mostly observational, and they were not measured in people eating whole-food carnivore diets. The long-term effects of a zero-fiber diet on gut health and colorectal cancer risk are genuinely unknown. That uncertainty is a reason to monitor, favor unprocessed foods, and keep up with screening.