Protein, alcohol and more: How does diet affect gut health?

Sugar, fibre, protein, fat, alcohol; many components of our diet affect our gastrointestinal tract lining. A new review explains what this means for gut health.

Key facts at a glance on diet and gastrointestinal health:

Diets high in sugar and fat (particularly saturated fatty acids), as well as those extremely high or low in protein, were associated with a loss of the intestinal barrier and inflammatory processes.
In contrast, diets rich in fibre and omega-3 had a stabilising effect on the mucosa and a beneficial effect on the microbiome.
Alcohol impaired the structure of the mucosa and was linked to increased permeability, inflammation and carcinogenesis.

Sugar: A risk factor for intestinal permeability and inflammation

In animal models, high-sugar diets led to increased intestinal permeability, reduced tight junction expression, increased cytokine production and inflammation. At the same time, changes in the gut microbiome were observed, including an increase in pathogenic bacteria such as E. coli.

Such changes to the mucosa could contribute to the development or worsening of intestinal diseases:

In animal models, a high-sugar diet was associated with more severe cases of colitis.
In the oral cavity, sugar promoted demineralisation and tooth decay.
In the long term, a persistently high sugar intake could promote structural damage to the mucosa and be associated with an increased risk of chronic inflammatory bowel diseases such as colitis, as well as functional disorders such as irritable bowel syndrome.

Dietary fibre: benefits for the microbiome and mucosa

Dietary fibre promotes the energy supply to epithelial cells via breakdown products such as butyrate and propionate, increases mucus production in the intestine and improves the barrier function. The microbiota shifts in favour of protective species such as Bifidobacterium and Lactobacillus. Structural changes, such as altered crypt depth and villus length, have also been observed – depending on the type and amount of fibre consumed.

This could have a positive effect on chronic conditions:

Studies have associated a high-fibre diet with a lower risk of diverticulitis, ulcerative colitis, colorectal cancer and functional symptoms such as dyspepsia or haemorrhoids.
In irritable bowel syndrome and active chronic inflammatory bowel disease, however, the effect depends heavily on the type of fibre and individual tolerance – for example, soluble and insoluble fibres differ in their effects.

Proteins: Too much, too little – both can be harmful

Both excessive and insufficient protein intake can impair intestinal structure: in animal models, a very high protein intake led to impaired barrier function, reduced expression of tight junction proteins, increased formation of toxic metabolites and dysbiosis. Conversely, with a significantly reduced protein content, villous atrophy, impaired wall integrity and inflammatory changes in the mucosa were observed.

Some of the possible clinical consequences are:

In an animal model, a very high-protein diet (53% of energy intake) exacerbated inflammation in colitis; interestingly, however, mucosal healing was promoted by a moderately increased intake (30%).
The source of protein also played a role: animal protein exacerbated inflammation in ulcerative colitis, whereas plant protein did not exhibit this effect.
Furthermore, a high-protein diet has been linked to accelerated carcinogenesis in the colon – mediated by an increase in inflammatory processes.

Fats: Positive or negative effects – depending on the type of fatty acid

A high-fat diet, particularly one rich in saturated fatty acids, has been associated with increased intestinal permeability, dysbiosis and inflammatory processes in animal models. Omega-3 fatty acids counteracted these effects: They improved tight junction expression, stabilised the microbiota and reduced the number of inflammatory cells in the intestinal wall.

A long-term high-fat diet also altered the bile acid profile in the gut: the proportion of secondary bile acids such as deoxycholic acid increased – which was associated with increased epithelial proliferation and inflammatory changes.

Of particular clinical significance is the link between a high-fat diet and colorectal cancer:

A high-fat diet was associated with an increased risk of colorectal cancer – particularly where there was a high proportion of industrial trans fats, saturated fatty acids and an unfavourable ratio of omega-6 to omega-3 fatty acids.
In contrast, medium-chain and odd-chain saturated fatty acids, as well as highly unsaturated fatty acids such as omega-3 fatty acids, had a protective effect.

Alcohol: Inflammation and Cancer Risk

Studies have shown that alcohol consumption damages the mucous membranes throughout the gastrointestinal tract: ethanol penetrates cell membranes, increases the permeability of the intestinal wall and reduces the expression of tight junction proteins. At the same time, there was a reduction in saliva and mucus production, increased oxidative stress and changes to the microbiome.

Such disturbances in the intestinal environment can promote inflammatory and malignant processes in the long term:

Alcohol was associated with an increased risk of oesophageal dysplasia and colorectal neoplasms; furthermore, it weakened the immune system, thereby increasing susceptibility to gastrointestinal infections.
In the liver, toxic metabolites such as acetaldehyde led to the activation of inflammatory processes, with potential consequences such as fatty liver, fibrosis and cirrhosis.

Source

Vītola ME, Eisāne RA, Iļičuka S, Kļaviņa KA, Junga A, Pilmane M. Effects of Nutrients and Alcoholic Beverages on Gastrointestinal Tract Morphology. Gastroenterology Insights. 2025; 16(4):42. https://doi.org/10.3390/gastroent16040042