Is a vegan diet good for your gut health and microbiome?

The objection

“Gut bacteria adapt to whatever you feed them, and microbiome science is far too young to say which mix of them is the healthy one. A 2025 study of more than 21,000 people found fewer bacterial species in vegans’ guts than in meat-eaters’, the American Gut Project found that being vegan explained next to nothing about anyone’s microbes, and when Stanford researchers doubled people’s fibre, their gut bacteria grew no more diverse. Some people with constipation even get better when they cut fibre out altogether. Meat, fish, eggs and dairy feed you perfectly well, and nobody has shown that a vegan gut is a healthier one.”

The answer

Each of those findings is real, and together they show that going vegan is no shortcut to a more diverse microbiome. What they leave standing is fibre, which a plant-based diet supplies in quantity, which many gut bacteria feed on and which is tied to lower rates of several major diseases. Dietary fibre, found in plant foods and in fungi such as mushrooms, is carbohydrate that the body can’t fully digest [1][2][3]. It passes through the small intestine largely intact and is fermented by bacteria in the large intestine [1], and meat, fish, eggs, cheese and yoghurt contain none of it [3]. Fermenting it produces short-chain fatty acids, which the body’s cells can use for energy and which act on cell receptors and enzymes in ways that may contribute to health and disease (Koh et al., 2016) [4].

Most people in the UK eat far less fibre than they are advised to. The government’s target, based on the Scientific Advisory Committee on Nutrition’s 2015 report, is an average of 30 g a day for adults [5][6], yet in the National Diet and Nutrition Survey for 2019 to 2023, published in 2025, adults aged 19 to 64 averaged 16.4 g and only 4% of adults reached 30 g [5]. The survey moved to an online recall method in 2019, so these figures can’t be compared with earlier survey years [5]. Other bodies set the bar a little lower: the WHO’s 2023 guideline asks adults for at least 25 g of naturally occurring fibre a day [7], and the European Food Safety Authority judged in 2010 that 25 g is adequate for normal bowel function [8].

Leaving out animal foods tends to push fibre intake up. In EPIC-Oxford, a study of 65,429 men and women in the UK, vegans had the highest fibre intake of the four diet groups (Davey et al., 2003) [9], and a 2021 systematic review of 141 studies found fibre intake higher in vegetarians and vegans than in meat-eaters, with meat-eaters the group at risk of falling short; both of its authors work for Unilever, which sells fortified foods and supplements (Neufingerl and Eilander, 2021) [10].

The microbes respond within days. In a small experiment published in 2014, American volunteers aged 21 to 33, ten on each diet, spent five days eating only meat, eggs and cheese, with almost no fibre, and five days eating grains, pulses, fruit and vegetables (David et al., 2014) [11]. The animal-based diet raised bile-tolerant bacteria such as Alistipes and Bilophila and cut the Firmicutes that break down plant carbohydrates, such as Roseburia and Eubacterium rectale, and in stool the products of carbohydrate fermentation fell while those of protein fermentation rose [11]. Two days after the meat diet ended, the volunteers’ microbes were back to their usual mix [11].

The strongest reason to eat more fibre comes from what happens to the people who eat it. A 2019 series of systematic reviews and meta-analyses in the Lancet, partly funded by the WHO, drew on 185 prospective studies and 58 clinical trials (Reynolds et al., 2019) [12]. In the prospective studies, the people who ate the most fibre had 15 to 30% lower rates of death from any cause and from cardiovascular disease, and of coronary heart disease, stroke, type 2 diabetes and colorectal cancer, than those who ate the least [12]. Risk fell most at intakes of 25 to 29 g a day, and the dose-response curves suggested that higher intakes could bring even greater protection against cardiovascular disease, type 2 diabetes and colorectal and breast cancer [12]. In the trials, higher fibre intakes went with lower systolic blood pressure and total cholesterol [12]. The disease figures are relative differences from observational studies, and the authors graded the certainty of the fibre evidence as moderate, though they judged its strong dose-response pattern a sign that the links could be causal [12]. A 2025 umbrella review of 33 meta-analyses of observational studies, covering more than 17 million people, found the same pattern and graded the evidence as convincing, its top grade, for lower rates of death from cardiovascular disease, of pancreatic cancer and of diverticular disease, which affects the bowel itself (Veronese et al., 2025) [24]. It rated only six of those meta-analyses as of high methodological quality [24].

Does a vegan diet make your gut microbiome more diverse?

A vegan diet has not been shown to make the gut microbiome more diverse, and a 2025 study of 21,561 people in five cohorts in the UK, the US and Italy found slightly fewer species of gut bacteria in vegans than in omnivores in its British and American cohorts (Fackelmann et al., 2025) [13]. There the vegans had a median of 209 to 266 species-level groups, against 217 to 299 for the omnivores [13]. Diet explained only a small share of the differences between people, but enough that their microbes could tell vegans, vegetarians and omnivores apart [13]. The vegans’ signature bacteria included known butyrate producers that specialise in breaking down fibre and went with better cardiometabolic markers, while omnivore signature species tied to red meat, such as Bilophila wadsworthia, went with worse ones [13]. The study is cross-sectional, so it shows associations, and several of its authors work for or advise the company ZOE, whose PREDICT cohorts supplied most of its data [13].

A 2020 systematic review of 16 studies found no consistent difference between the gut bacteria of vegans or vegetarians and those of omnivores, and some of the studies contradicted each other (Trefflich et al., 2020) [14]. In the American Gut Project, a citizen-science study of more than 10,000 volunteers, mostly in the US, the UK and Australia, what tracked the diversity of people’s gut bacteria was the number of different plants they ate, while labels such as vegan had little or no explanatory power (McDonald et al., 2018) [15]. Comparing 85 people at the two extremes, 41 who ate more than 30 types of plant a week and 44 who ate 10 or fewer, the study found more diverse gut bacteria in the high-plant group, and among people eating over 30 plant types it found more of the bacteria thought to ferment fibre into short-chain fatty acids and lower levels of certain antibiotic-resistance genes [15]. The volunteers chose to take part and reported their own diets, so these too are associations [15].

Adding fibre may not be enough on its own, at least in the short run. In a 2021 Stanford study, 39 healthy adults were assigned, 37 of them at random, to eat more fibre or more fermented foods for ten weeks, four building up and six at the full amount, and 36 were included in the analysis (Wastyk et al., 2021) [16]. By the end of those ten weeks the fibre group had more than doubled its intake, from 21.5 g to 45.1 g a day, and its microbes made more of the enzymes that digest plant carbohydrates, but the diversity of its gut bacteria stayed the same and more undigested carbohydrate turned up in stool [16]. Its main immune measure did not change, and people’s immune responses to the extra fibre varied with how diverse their gut bacteria had been at the start [16]. The fermented-food group, eating yoghurt, kefir and fermented cottage cheese as well as kimchi, kombucha and vegetable brine drinks, gained diversity and showed lower markers of inflammation [16]. The authors suggest that a rise in diversity from fibre may take longer than the six weeks at the full amount that their study allowed [16].

A 2018 meta-analysis of 64 randomised trials in 2,099 healthy adults found the same pattern for fibre: it made Bifidobacterium and Lactobacillus bacteria more abundant in stool, especially fibres called fructans and galacto-oligosaccharides, and brought a small rise in stool butyrate that only just reached statistical significance, without changing diversity (So et al., 2018) [23]. The British Nutrition Foundation points out that what gut microbes do, such as turning fibre into short-chain fatty acids, may matter more than which species are there [1].

Do humans need fibre at all?

People can go without fibre for months, but no one has shown that a diet without it is safe over the long term [17][18], and UK, European and WHO advice is for adults to eat 25 to 30 g a day [5][7][8]. The evidence linking fibre to lower rates of heart disease, type 2 diabetes and bowel cancer is rated moderate in certainty [12]. The constipation study the objection has in mind is real: in a study published in 2012, 63 patients whose constipation had no known cause, enrolled between 2008 and 2010, were told to eat no fibre for two weeks and then as much as they found acceptable, and after six months the 41 who had stayed fibre-free had gone from one bowel movement every 3.75 days to one a day (Ho et al., 2012) [18]. It enrolled only people with that kind of constipation, the patients chose their own diets after the first fortnight, and there was no randomised comparison [18]. Randomised trials point the other way: a 2022 meta-analysis of 16 of them, in 1,251 adults with chronic constipation, found that of the participants whose response was reported, 66% responded to fibre supplements against 41% given a control treatment, with psyllium, doses above 10 g a day and courses of at least four weeks appearing to work best, though fibre also brought more wind and results varied widely between trials (van der Schoot et al., 2022) [22]. A 2026 scoping review of the carnivore diet found nine human studies, which reported some short-term benefits, such as feeling fuller and possible improvements in inflammatory or metabolic markers, alongside risks of running short of fibre, vitamins C and D, calcium, magnesium and iodine and of raised LDL cholesterol (Lietz et al., 2026) [17]. It judged the evidence very limited, with small samples, short durations and no control groups, and concluded that long-term adherence to the diet cannot be recommended [17].

Which foods are highest in fibre?

Of the foods in the British Nutrition Foundation’s table, beans, lentils and wholegrain pasta give the most fibre per portion, and a single portion of kidney beans or lentils provides more than a quarter of the adult target of 30 g [1]. The foundation advises eating a variety of fibre-rich foods, because different fibres affect digestion, cholesterol and the gut microbiome in different ways [1]:

Food	Portion	Fibre
Red kidney beans	120 g	9.4 g
Wholegrain pasta	180 g	8.8 g
Baked beans	200 g	8.5 g
Lentils	120 g	8.1 g
Quinoa	180 g	6.3 g
Chickpeas	120 g	6.2 g
Wholemeal bread	80 g	5.8 g
Bran flakes	40 g	5.4 g
Baked potato	220 g	4.6 g
Oats	45 g	3.5 g
Berries	80 g	3.3 g
Carrots	80 g	3.1 g
Almonds	20 g	2.0 g
Meat, fish, eggs, cheese or yoghurt [3]	any	0 g

The exact amounts vary with the brand and the portion [1]. In the NHS’s examples, two slices of wholemeal toast with peanut butter and a medium orange give around 8.3 g, and wholemeal spaghetti with a lentil and tomato sauce around 11 g [19].

Raise your intake gradually and drink enough fluids, because adding fibre too quickly can bring bloating, wind and stomach cramps, and if symptoms continue, see your GP or a dietitian [1]. Children need less fibre than adults: about 15 g a day at 2 to 5 years, 20 g at 5 to 11 and 25 g at 11 to 16, the NHS says [19]. High-fibre foods can fill small stomachs before a child has had enough calories and nutrients, so the NHS advises against giving only wholegrain foods before the age of 2, or only high-fibre foods before 5 [21]. People with irritable bowel syndrome, Crohn’s disease or another gut condition may need individual advice on fibre [3], so ask your GP or a dietitian first, and don’t change or stop any medicine without speaking to your GP. Fibre powders are no substitute for food, and relying on them can cause bloating and reduce the absorption of iron and calcium, especially without enough water [3]. A change in your bowel habit that isn’t usual for you, or blood in your poo, is a reason to see a GP whatever you eat [20].