Gut health and dietary sugars

This content is authored by Registered Dietitian, Juliette Kellow.

Understanding the gut

The gut refers to the entire gastrointestinal (GI) tract and includes the mouth, oesophagus, stomach, small intestine, large intestine and anus. Together with the liver, gallbladder and pancreas, the GI tract makes up the digestive system, which is responsible for digesting food and absorbing nutrients and water to support the body’s needs (1).

When most people talk about the “gut”, they are often speaking about one specific part – the large intestine. It is well established that trillions of microscopic organisms, known as microbes, live in the GI tract, particularly the large intestine (2). These microbes, which are mainly bacteria, together with the substances they produce, are collectively known as the gut microbiome (3).

The gut microbiome

The gut microbiome plays an important role in supporting the digestive system. Beneficial gut bacteria produce certain B vitamins (4) and vitamin K (5), and break down components of food that cannot be digested, such as fibre and resistant starch, through a process called fermentation. This helps to support normal bowel function and stool formation, and produces substances known as short-chain fatty acids (SCFAs), which help protect and strengthen the gut lining (6).

Scientific studies also suggest the gut microbiome may have many other functions that support health. For example, emerging research suggests SCFAs may help control inflammation within the body (7). Other research indicates the gut microbiome supports the immune system (much of the body’s immune system is associated with the gut). It does this in many ways, for example, by helping to prevent harmful bacteria from thriving, supporting immune cells so they develop and respond effectively to threats, and strengthening the body’s overall immune defences (8).

Research also suggests there is a two-way communication pathway between the gut and the brain, often referred to as the gut-brain axis. This has led to interest in how gut health may influence mental health, mood and neurological conditions such as Alzheimer’s disease and Parkinson’s disease (9).

Emerging research suggests links between changes in the gut microbiome and conditions such as type 2 diabetes, cardiovascular disease, inflammatory bowel disease, and some cancers (10, 11). However, much of this research is observational, meaning it cannot prove that the gut microbiome directly causes or prevents disease. More high-quality human studies are needed before firm conclusions can be made.

Factors that affect gut health

Thousands of different bacterial species have been identified in the human gut microbiome, although a much smaller number of species are typically present in an individual’s gut at any one time (12). The type of microbes in the gut are partly shaped by a person’s genetics, their mode of birth and their diet in early life (13). Many other factors, including age, where someone lives, pollution, stress, sleep, hygiene, illness, use of medications (e.g., antibiotics and laxatives), diet, alcohol intake, smoking and physical activity, also influence the type of microbes that live in the gut (14, 15).

Together, these factors mean every person’s gut microbiome is unique, and there is no single ‘healthy’ or ‘optimal’ blueprint (16). Instead, scientists believe that maintaining a diverse and balanced mix of microbes is important for maintaining gut health. If this balance is disrupted – for example, if potentially ‘harmful’ bacteria begin to dominate, known as dysbiosis – it may be linked with digestive symptoms such as bloating, diarrhoea and abdominal pain, and could also have broader effects on overall health (17).

How diet affects gut health

Diet plays an important part in influencing the diversity and balance of microbes living in the gut. This includes:

Fibre

Dietary fibre is an umbrella term for the diverse range of carbohydrates found in plant-based foods such as wholegrains, fruit, vegetables, pulses, nuts and seeds, that are not digested in the small intestine and therefore reach the large intestine. Here, some types of fibre are fermented by gut bacteria and are often referred to as prebiotics. Other types of fibre add bulk to stools, making them softer and easier to pass. Certain fibres also help slow the absorption of glucose into the blood or support healthy cholesterol levels (18).

Polyphenols

Emerging research shows that many polyphenols – found naturally in foods like fruit, vegetables, nuts, seeds, tea and cocoa – reach the large intestine, where they interact with the gut microbiome (19).

Water

Fluid is vital for the functioning of the digestive system. In the large intestine, it helps to prevent constipation by keeping the stools soft, so they are easier to pass (20) Some early studies also suggest that insufficient fluid may affect the balance of gut microbes (21).

Probiotic foods, drinks and supplements

Sometimes described as ‘good’ or ‘friendly’ bacteria, probiotics are live microorganisms that reach the large intestine and add to the community of microbes already living there (22).

Fermented foods and drinks

Fermented foods and drinks, such as sauerkraut, miso, kefir, kombucha and sourdough bread, are often described as being sources of live microbes. However, the types and amounts are not usually measured, and research into their health benefits is limited (22).

Sugars and gut health

In the popular press, high intakes of dietary sugars are often blamed for ‘feeding’ harmful gut bacteria and causing various gut health complaints such as bloating, diarrhoea and excess gas. However, these claims are often oversimplified and not well supported by scientific evidence.

The idea that dietary sugars ‘feed’ harmful gut bacteria in the large intestine is misleading because, in healthy people, most sugars are digested and absorbed in the small intestine (23). During digestion, sugars are broken down into the monosaccharides glucose, fructose and galactose, which are then absorbed from the small intestine into the bloodstream. As a result, the vast majority of dietary sugars don’t reach the large intestine and therefore are not a typical fuel source for gut bacteria. Instead, the composition of the gut microbiome is primarily influenced by dietary fibre and other non-digestible carbohydrates such as resistant starch.

However, the absorption of some dietary sugars in the small intestine can be impaired, for example, in individuals with certain bowel diseases or by consuming excessively large amounts of fructose in one sitting (24, 25). When this happens, unabsorbed dietary sugars reach the large intestine where gut bacteria break them down, producing gases such as hydrogen, methane and carbon dioxide. In some people, these gases can contribute to symptoms such as flatulence, bloating, and abdominal discomfort. Unabsorbed sugars also draw water into the large intestine, which can make stools looser and move them through the gut more quickly, leading to diarrhoea (26).

When do dietary sugars reach the large intestine?

Lactose intolerance

Lactose – the natural sugar found in milk and dairy products – is digested by an enzyme called lactase. If the body does not produce enough lactase, lactose cannot be properly digested or absorbed in the small intestine and instead passes into the large intestine. Lactose intolerance occurs when this causes symptoms. It can happen naturally in some people, as their levels of lactase reduce after early childhood. In others, the condition may be temporary – such as after a stomach bug (gastroenteritis) – or be associated with chronic health conditions such as inflammatory bowel disease or coeliac disease. In rare cases, people are born with little or no ability to make lactase. Many people with lactose intolerance can still tolerate small amounts of lactose, meaning symptoms can often be managed by reducing rather than eliminating lactose-containing foods (27).

Fructose malabsorption

Fructose is a natural sugar found in fruit and fruit juices, but processed forms are also added to some foods and drinks for sweetness, especially in the USA (25). Fructose malabsorption occurs when the small intestine does not absorb fructose efficiently, allowing some to pass into the large intestine. Research suggests this is more likely to happen when large amounts of fructose are consumed, especially when it is present in greater amounts than glucose.

However, fructose absorption is complex and not fully understood. The ability to absorb fructose varies greatly between individuals and may be influenced by factors such as gut function, how quickly food moves through the gut, usual dietary intake, the amount consumed, and other dietary components including glucose, sorbitol and fibre.

Symptoms are also most likely to occur after consuming excessively large amounts of fructose in one sitting, rather than from usual dietary intake. Identifying amounts required for the development of symptoms is further complicated by the fact that many studies give large, single doses of fructose that may not reflect typical eating patterns, and fructose malabsorption is also difficult to reliably diagnose. Further research is needed to better understand individual differences, especially in the context of the overall diet (24, 28, 29, 30, 31, 32).

A note on polyols

Polyols are a type of carbohydrate, also known as sugar alcohols (33). Even though they are not actually sugars or alcohols, they have a sweet taste and a chemical structure similar to both. Examples include sorbitol, mannitol, xylitol, erythritol and maltitol. Some occur naturally in varying amounts in fruit and vegetables, such as sorbitol in plums, erythritol in grapes or xylitol in mushrooms. They are also used to provide sweetness in products such as sugar-free chewing gum or confectionery, or reduced-calorie products. Many polyols are only partially absorbed in the small intestine and therefore pass into the large intestine, where gut bacteria break them down. This can produce symptoms such as bloating, flatulence, abdominal discomfort and diarrhoea. Within some countries, including the UK and EU, foods containing more than 10% added polyols are required to state on the packaging that excessive consumption may produce laxative effects (34).

Dietary sugars and irritable bowel syndrome

Irritable bowel syndrome (IBS) is associated with digestive symptoms such as flatulence, abdominal pain or cramps, bloating, diarrhoea and constipation. While the exact cause is unknown, it has been linked to stress, gastroenteritis, oversensitive gut nerves and food passing through the digestive system too quickly or too slowly. While there is no cure, dietary changes have been shown to help manage symptoms (35).

Research suggests some people with IBS are sensitive to certain carbohydrates and sugar alcohols known as FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols). If these are poorly absorbed in the small intestine, they pass into the large intestine, where they are fermented by gut bacteria, potentially triggering symptoms. Studies show a low FODMAP diet – which temporarily restricts foods high in these carbohydrates, including certain grains, fruits, vegetables, dairy products and pulses – can help to ease symptoms in some people with IBS. Foods are then gradually reintroduced to identify which can be tolerated without causing symptoms. However, as the diet is so restrictive and limits many nutritious foods, it should only be followed under dietetic supervision (36, 37, 38).

Low-and no-calorie sweeteners and gut health

Current research on low-and no-calorie sweeteners and the gut microbiome remains inconclusive. Recent studies on sweeteners such as aspartame, sucralose, saccharin, acesulfame-K and polyols have reported some associations, but findings from both animal and human studies are inconsistent. While some research shows little or no effect on gut bacteria, other studies suggest certain sweeteners may alter the type of microbes in the gut. These differences may be due to differences in study design, dose, and individual factors such as diet and the composition of the gut microbiota at the start of the research.

At present, there is no scientific consensus on whether sweeteners meaningfully alter gut health in humans or on the best methods to measure their effects. Further high-quality human research that includes longer study durations, larger numbers of participants and more realistic intakes of sweeteners is needed before clear conclusions can be made (39, 40).

The impact of dietary sugars on the gut microbiome

There is currently no convincing evidence that dietary sugars alone have an impact on the gut microbiome, largely because most are absorbed in the small intestine, with only small amounts reaching the large intestine. When dietary sugars do reach the large intestine, this is usually due to very high intakes or related to health conditions that impair their digestion and absorption. In both cases, other aspects of diet and health may also influence the gut microbiome.

Even when dietary sugars do reach the large intestine, they are only one part of a mixed and varied diet, making it difficult to isolate their specific effects on the gut microbiome or gut-related symptoms in human studies (41).

Image of the gut surrounded by healthy food

Overall diet quality is likely to be a far more important consideration for gut health than dietary sugars alone. Indeed, the strongest dietary evidence for supporting a diverse gut microbiome is the regular consumption of a wide variety of fibre-rich plant-based foods (42).

It is also important to recognise that the gut microbiome responds rapidly to diet change, often in as little as a few days (43). This makes it challenging to draw firm conclusions from studies that examine the effects of individual dietary components on the gut microbiome composition (41). This is further complicated by the fact that many other factors – such as stress, physical activity levels, sleep, illness and medications – also influence the gut microbiome.

Finally, it is important to encourage individuals to look beyond dietary sugars if they suffer with symptoms such as bloating, abdominal pain or cramp, or diarrhoea. Such symptoms may reflect an underlying medical condition. As a result, persistent symptoms should always be discussed with a doctor.

Summary

The area of gut health is complex and influenced by many factors. As all consumed food and drink interact with the GI tract, many aspects of the diet likely contribute in some way to the composition and diversity of the gut microbiome. Despite rapid growth in research on diet and gut health in recent years, this area of science is still in its infancy, and many gaps in knowledge remain, with many aspects of the relationship between diet and the gut microbiome still not fully understood.

However, current evidence suggests broader dietary patterns – particularly diets rich in a variety of fibre-rich plant-based foods – are likely to play a far greater part in supporting a healthy and diverse gut microbiome than individual components such as dietary sugars.

Last reviewed June 2026. Next review due June 2029.