Carnivore and keto: less gas, but at what cost?

Healthy state

On a ketogenic (keto) diet you eat so few carbohydrates that the body makes ketone bodies Ketone bodies Energy carriers that the liver makes from fat during fasting or a very low-carbohydrate diet. The brain and muscles can use them instead of sugar. More in the chapter “Longer fasting: water fasting, therapeutic fasting and fasting-mimicking” from fat and uses them as fuel. On a carnivore diet you eat almost only animal foods: meat, fish, eggs, sometimes dairy. Both contain little or no fibre.

Why many people feel better on them is easy to explain. Gas in the colon arises mainly when bacteria ferment carbohydrates the small intestine did not absorb, i.e. FODMAPs FODMAP Short for fermentable sugars and sugar alcohols that are poorly absorbed in the small intestine, such as fructans, lactose, fructose and sorbitol. More in the chapter “Doing low-FODMAP right” and fibre. Take away this food and there is less gas and less stool. In a small study without a control group, all 13 people with diarrhoea-predominant IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis” who ate only 20 g of carbohydrate a day for four weeks reported adequate relief. Stool frequency fell from 2.6 to 1.4 a day. That is real relief, but it is symptom control by starving the bacteria, not healing.

In an online survey of 2029 people who had eaten carnivore for at least six months, most reported high satisfaction and few complaints. However, respondents were recruited via social media groups. People who did badly and stopped were hardly included, there was no comparison group, and the values were self-reported.

Ketogenic diets do have a real medical use: epilepsy that does not respond well enough to medication. A Cochrane review found more seizure freedom and markedly fewer seizures in children, with low to very low certainty of evidence; in adults the benefit is uncertain. This therapy is supervised by doctors and dietitians. It says nothing about gut health.

Disrupted state: causes and effects

The gut microbiota Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” reacts within days. In a study of volunteers who ate only animal foods for five days, bile-tolerant bacteria such as Alistipes, Bilophila and Bacteroides increased. Bacteria that break down plant fibre, such as Roseburia, Eubacterium rectale and Ruminococcus bromii, decreased. Bilophila wadsworthia has been linked to gut inflammation, so far mainly in mice.

With the fibre degraders, butyrate Butyrate A short-chain fatty acid and the main energy source for the cells of the colon wall. It strengthens the barrier and dampens inflammation. More in the chapter “Butyrate and other short-chain fatty acids” falls, the main fuel of colon cells. In a study of 19 people with obesity, 24 g of carbohydrate a day for four weeks lowered faecal butyrate from 18 to 4 mmol/l. The share of Roseburia and Eubacterium rectale fell from 11 to 3 percent, and bifidobacteria also declined.

Keto also acts via the ketone bodies themselves. In an eight-week inpatient study, the gut microbiota changed markedly on keto, and in the lab ketone bodies selectively inhibited the growth of bifidobacteria.

Without fibre, bacteria can switch to the mucus layer Mucus layer A gel layer of mucins on the gut wall that keeps bacteria at a distance. In the colon, the inner layer is almost free of microbes. More in the chapter “The mucus layer and additives” of the gut wall. This is an animal experiment: mice with a model microbiota of human bacteria were given fibre-free food. The bacteria degraded the protective layer, a pathogen reached the gut wall more easily and caused severe inflammation. Whether the same happens in humans has not been shown, but it is a clear warning sign.

There are also question marks beyond the gut. In a meta-analysis Meta-analysis An analysis that combines the results of several studies. It is only as good as the studies it includes. of 11 trials with 1369 people, LDL cholesterol rose somewhat more on very low-carbohydrate diets than on low-fat diets, even though weight dropped more. In the carnivore survey, average LDL was clearly raised at 172 mg/dl. A 2026 review lists possible deficiencies in vitamins C and D, calcium, magnesium, iodine and fibre, and advises against long-term carnivore eating. Processed meat is classified as carcinogenic and red meat as probably carcinogenic for colorectal cancer. People eating 25 to 29 g of fibre a day have a lower risk of colorectal cancer and early death in large observational studies.

Then there is eating behaviour. Very strict rules with allowed and forbidden foods can make it hard to eat broadly again later. Many report more gas at first when they go back to carbohydrates and take this as proof that plants are harmful. More plausibly, the gut microbiota first has to get used to that food again. There are no long-term studies on the gut and health for either diet.