"How to improve gut health" is one of the most common health queries in the UK — but what does the term actually mean?

Type "gut health" into Google and you will find an endless supply of advice: drink bone broth, take probiotics, avoid lectins, eat fermented foods, try L-glutamine. Some of it is evidence-based. Much of it is not.

The fundamental problem is that "gut health" is not a clinically defined term. There is no blood test for it, no diagnostic criteria, no agreed measurement. This makes it easy for marketers to claim their product "supports gut health" and nearly impossible for consumers to evaluate those claims.

What we can do is look at what researchers actually study when they investigate the gut, and what the best available evidence says about influencing it. That is what this article does — honestly, without making claims the science does not support.

What does the science actually mean by "gut health"?

Researchers studying the gastrointestinal system tend to focus on several measurable parameters rather than a single concept of "gut health":

This is an area of active, rapidly evolving research. The gut microbiome was barely studied before the 2000s. Many of the findings are observational, meaning they show associations rather than proving that intervention X causes outcome Y. Be cautious of anyone — including supplement companies — who speaks about gut health with more certainty than the science warrants.

Diet is the single most impactful intervention

If you are serious about your gut, diet is where to start. No supplement can compensate for a poor diet, and the evidence for dietary interventions is substantially stronger than for any supplement.

Fibre: the most evidence-based factor

Dietary fibre is the primary fuel source for beneficial gut bacteria. When bacteria in the colon ferment fibre, they produce short-chain fatty acids (SCFAs) — primarily butyrate, propionate, and acetate — which serve as energy sources for colonocytes (the cells lining the colon) and play roles in immune regulation and inflammation. Makki et al. (2018) reviewed the mechanisms by which dietary fibre impacts gut microbial ecology and host physiology, concluding that fibre intake is a primary driver of microbiome composition and metabolite production.

The UK government recommends 30g of fibre per day for adults. According to the National Diet and Nutrition Survey, the average UK adult consumes approximately 18g — barely 60% of the target.

Increasing fibre intake is probably the single most effective thing most people could do. Good vegan sources include:

If your current intake is low, increase gradually over two to three weeks to allow your gut bacteria to adapt. A sudden jump from 15g to 30g will likely cause bloating and gas — which is a fermentation response, not a sign that fibre disagrees with you.

Diversity of plant foods matters as much as total fibre

The American Gut Project — one of the largest microbiome studies ever conducted, analysing over 10,000 participants — found that people who consumed more than 30 different plant foods per week had significantly greater microbiome diversity than those consuming fewer than 10, regardless of whether they identified as vegetarian or omnivore (McDonald et al., 2018).

This finding suggests that variety, not just quantity, matters. Each type of plant food contains different types of fibre (cellulose, pectin, inulin, resistant starch) and different polyphenols, which feed different microbial communities. Eating the same three vegetables every day is less effective than rotating through a wider range.

Polyphenols: feeding gut bacteria through colour

Polyphenols are bioactive compounds found in colourful plant foods — berries, dark chocolate, green tea, red onions, olive oil, and spices like turmeric. Only about 5–10% of dietary polyphenols are absorbed in the small intestine; the remainder reach the colon where they interact with gut bacteria. Cheng et al. (2023) reviewed the mechanistic interactions between polyphenols and gut microbiota, finding evidence that polyphenols can modulate microbial diversity and the abundance of specific organisms.

In practical terms: eat a wide variety of colourful plant foods. The benefit comes from the whole dietary pattern, not from isolated polyphenol supplements.

Fermented foods: promising but not conclusive

A well-publicised Stanford trial by Wastyk et al. (2021) randomised 36 healthy adults to either a high-fermented-food diet (six servings per day of foods like yoghurt, kimchi, kombucha, and kefir) or a high-fibre diet for 10 weeks. The fermented food group showed steadily increasing microbiota diversity and decreased inflammatory markers, while the fibre group showed more variable responses depending on baseline microbiota.

This is a single, small trial. But it suggests that regularly including fermented foods — sauerkraut, kimchi, miso, tempeh, kombucha — alongside high fibre intake may be beneficial. Note that many fermented foods in UK supermarkets are pasteurised, which kills the live organisms. Look for unpasteurised versions or make your own.

Lifestyle factors that affect the gut

Diet gets most of the attention, but several lifestyle factors also influence gut microbial composition.

Sleep

Smith et al. (2019) found that gut microbiome diversity was positively correlated with sleep efficiency and total sleep time in healthy adults. This was an observational study — it cannot tell us whether poor sleep causes reduced diversity or vice versa — but it adds to a growing body of evidence linking sleep quality to gut microbial composition.

Practical advice: prioritise consistent sleep timing and aim for 7–9 hours. This is good general health advice regardless of gut-specific effects.

Stress

The gut-brain axis is bidirectional: the brain influences gut function, and the gut influences brain function. Rusch et al. (2023) reviewed how the hypothalamic-pituitary-adrenal (HPA) axis and gut microbiota interact, with chronic stress and elevated cortisol associated with changes in microbial composition and intestinal permeability.

This is one of the most active areas of microbiome research, but also one where the evidence is primarily observational and mechanistic. Practical stress management — regular movement, adequate sleep, social connection — is worth pursuing on its own merits.

Exercise

Clauss et al. (2021) reviewed the interplay between exercise and the gut microbiome, finding that regular moderate exercise is associated with increased microbial diversity and higher production of short-chain fatty acids. The effects appear to be independent of diet, though most studies are observational.

The NHS physical activity guidelines recommend 150 minutes of moderate-intensity activity per week. Meeting this target may benefit your gut alongside its well-established cardiovascular, metabolic, and mental health benefits.

Alcohol

Engen et al. (2015) reviewed the effects of alcohol on intestinal microbiota composition, finding that chronic alcohol consumption is associated with dysbiosis (an imbalanced microbial community) and increased intestinal permeability. Even moderate alcohol consumption can alter microbial composition, though the clinical significance of moderate intake remains debated.

What about probiotic supplements?

This is where regulatory honesty matters most. The European Food Safety Authority (EFSA) has never approved a single health claim for any probiotic product. Every application — and there have been hundreds — has been rejected for insufficient evidence. No probiotic supplement can legally claim to "support gut health" or "improve digestion" in the UK.

That does not mean the clinical evidence is nonexistent. It means the evidence has not met the specific regulatory standard EFSA requires for authorised health claims. Clinical trials on specific strains do exist, particularly for conditions like antibiotic-associated diarrhoea, IBS symptoms, and functional bloating.

The critical principle is that probiotic evidence is strain-specific. A clinical trial on one strain of Lactobacillus acidophilus tells you nothing about a different strain of the same species. If a probiotic product does not identify the exact strain on its label, you cannot evaluate whether it has been studied.

We cover this topic in depth in our guide to the best probiotics in the UK, including strain comparison, survival mechanisms, and how to evaluate clinical evidence.

PARTICULAR includes Bacillus coagulans Unique IS-2 — a spore-forming strain with published clinical trials. Spore-forming bacteria survive stomach acid naturally and do not require refrigeration, which addresses two of the biggest practical problems with conventional probiotic supplements.

Which nutrients have authorised health claims relevant to gut function?

While no health claims exist for probiotics, certain vitamins and minerals do have authorised claims under EU Regulation 432/2012 that are relevant to the tissues and processes involved in digestion.

Calcium

Calcium contributes to the normal function of digestive enzymes. This is a specifically authorised health claim. Digestive enzymes break down food in the stomach and small intestine, and calcium is a required cofactor for several of these enzymes, including pancreatic lipase.

Zinc

Zinc contributes to the maintenance of normal skin. This is an authorised claim, and it is relevant because the gut lining is epithelial tissue — biologically similar to skin. Wan and Zhang (2022) reviewed the impact of zinc on the intestinal mucosal barrier, finding that zinc plays a role in maintaining tight junction protein expression and epithelial cell integrity in the intestinal lining.

Zinc also contributes to normal protein synthesis and has a role in the process of cell division — both relevant to the rapid turnover of intestinal epithelial cells, which are replaced every 3–5 days.

Vitamin A

Vitamin A has a role in the process of cell specialisation — an authorised claim. The intestinal epithelium is one of the most rapidly renewing tissues in the body, and cell specialisation is fundamental to its function. Cantorna et al. (2019) reviewed how vitamin A regulates microbial complexity, barrier function, and mucosal immune responses to maintain intestinal homeostasis. Vitamin A also contributes to the normal function of the immune system, much of which resides in the gut-associated lymphoid tissue (GALT).

Vitamin D

Vitamin D contributes to the normal function of the immune system — an authorised claim. Emerging research suggests a role in gut barrier function specifically. Fleet (2022) reviewed evidence that vitamin D enhances gut barrier function and inhibits intestinal inflammation through the vitamin D receptor (VDR), which is expressed throughout the intestinal epithelium. Fakhoury et al. (2020) further reviewed vitamin D's role in intestinal homeostasis, including barrier maintenance and microbiota modulation.

These are not "gut health supplements" — that claim would not be permitted. They are nutrients with authorised claims for functions that are relevant to the tissues and processes involved in digestion.

Vitamin C

Vitamin C contributes to the normal function of the immune system and contributes to normal collagen formation for the normal function of skin. As with zinc, the relevance to intestinal tissue integrity exists through the collagen and epithelial connection, though the authorised claims are not gut-specific.

What about L-glutamine, slippery elm, and bone broth?

These are among the most commonly recommended "gut health" interventions online. Here is what the evidence actually shows.

L-glutamine

Glutamine is the most abundant amino acid in the body and a primary fuel source for enterocytes (intestinal cells). The theoretical basis for supplementation is sound. However, Abbasi et al. (2024) conducted a systematic review and meta-analysis of clinical trials on glutamine supplementation and gut permeability in adults. The overall finding was that glutamine supplementation did not significantly affect intestinal permeability across the studies analysed. A subgroup analysis showed effects only at very high doses (above 30g daily) for short durations — far above what most supplements provide.

This does not mean glutamine is useless in all contexts — it may have specific applications in clinical settings such as critical care or post-surgical recovery. But the evidence does not support routine supplementation for general "gut health" in healthy adults.

Slippery elm

Slippery elm bark (Ulmus rubra) is a traditional herbal remedy widely recommended for digestive symptoms. It contains mucilage, which forms a gel-like substance when mixed with water. The theoretical mechanism — that this mucilage coats and soothes the intestinal lining — is plausible but largely untested in rigorous clinical trials. There are very few randomised controlled trials in humans, and none sufficient to support health claims under EU or UK regulations.

Bone broth

Bone broth is frequently marketed as a gut healing food, typically attributed to its collagen and amino acid content. Two problems: first, the evidence base consists almost entirely of mechanistic speculation rather than clinical trials. Second, it is an animal product — not relevant if you follow a plant-based diet, and PARTICULAR is a vegan product. The amino acids found in bone broth (glycine, proline, glutamine) are also available from plant-based dietary sources.

The honest position on all three: not harmful, but the evidence for meaningful gut health benefits in healthy adults is limited. Diet, fibre, and the nutrients discussed above have stronger evidence and — in the case of vitamins and minerals — actual authorised health claims.

Your microbiome is as personal as your nutrient needs

One of the most consistent findings in microbiome research is individual variation. Your gut microbial composition is influenced by your genetics, birth method, early-life antibiotic exposure, diet, lifestyle, geography, and dozens of other factors. Two people eating identical diets will have different microbiomes.

The Stanford fermented foods trial referenced above illustrated this directly: the same dietary intervention produced markedly different immune responses depending on participants' baseline microbiota. What works for one person may not work for another.

This is one of the reasons one-size-fits-all supplement formulations are fundamentally limited. Your nutrient needs depend on your diet, lifestyle, health history, and goals — and so does the right combination of vitamins, minerals, and probiotics.

PARTICULAR's personalised approach uses a detailed questionnaire to build a blend tailored to your individual profile. Each nutrient is delivered in independently coated microgranules — one daily scoop from your pouch — so that every component is protected and released where it can be most effective.

If your interest in gut health has led you here, the evidence-based starting point is clear: eat more fibre, eat a wider diversity of plant foods, sleep well, move regularly, and manage stress. If you want to ensure your vitamin and mineral intake supports the tissues involved in digestion, a personalised approach based on your individual needs is more rational than a generic "gut health" supplement making claims the regulations do not permit.

Key takeaways

  1. "Gut health" is not a clinically defined term. Researchers study measurable parameters: microbiome diversity, barrier integrity, inflammation, and digestive function. Be cautious of products claiming to "support gut health" — no such claim is authorised in the UK
  2. Diet is the most impactful intervention. Aim for 30g of fibre daily (most UK adults get ~18g), eat 30+ different plant foods per week, and include polyphenol-rich and fermented foods
  3. Lifestyle matters. Sleep quality, stress management, regular exercise, and moderate alcohol consumption all influence gut microbial composition in observational studies
  4. No probiotic health claim has ever been approved by EFSA. Strain-specific clinical trial evidence exists, but choose probiotics based on identified strains with published human trials, not marketing language. See our full probiotic guide
  5. Some nutrients have authorised claims relevant to digestive tissues. Calcium contributes to normal digestive enzyme function. Zinc and vitamin A support the epithelial tissue that lines the gut. Vitamin D contributes to normal immune function, with emerging evidence for gut barrier roles
  6. Popular remedies like L-glutamine and slippery elm have limited clinical evidence for gut health benefits in healthy adults. Diet and authorised nutrients have stronger support
  7. Your microbiome is unique. One-size-fits-all supplements ignore individual variation. A personalised approach to nutrition is more rational than a generic "gut health" stack

Sources cited in this article: