Type 2 diabetes is a complex disease. Increasingly, the relationship between gut health and diabetes is becoming recognised as an area of interest. Diets with low nutritional value, in addition to a sedentary lifestyle, are factors that can not only impact the gut microbiome, but also influence type 2 diabetes risk. The two are explicitly linked; low nutritional value diets promote microbiome dysregulation, and this dysregulation promotes metabolic disorders such as insulin resistance.
In this article, we explain the emerging relationship between the gut microbiome and type 2 diabetes, including real evidence from human clinical trials on the potential links with glucose metabolism, insulin resistance, inflammation, and diet, and the habits that may help lower type 2 diabetes risk.
Of the 537 million cases of diabetes worldwide, type 2 diabetes accounts for 90%. The condition is characterised by insulin resistance, followed by an inability of pancreatic β cells to produce insulin. This insulin dysfunction ultimately impacts glucose regulation, as the primary function of insulin is to transport glucose from the blood into cells. Consequently, those with type 2 diabetes experience elevated blood glucose, known as hyperglycaemia 1.
The impact of type 2 diabetes is not just isolated to elevated glucose. Type 2 diabetes is known to have wider metabolic health implications for the cardiovascular system and the liver. The development of this condition has several risk factors, including being overweight or obese and having poor metabolic health 1. A nutritious diet and active lifestyle are therefore crucial to mitigating risk.
The past two decades of research have revealed a key link between the gut microbiome and metabolic health. We now know that a healthy microbiome is characterised by high microbial diversity, microbial stability, and the production of short-chain fatty acids (SCFAs) by gut microbiota 2. These SCFA’s play a critical role in supporting and maintaining the function of the gut barrier, a selective filter that allows essential nutrients, water, and minerals to pass into the bloodstream while keeping harmful bacteria, toxins, and undigested food particles out 2.
Disruption to the gut microbiome can therefore cause a substantial impact. Without microbial diversity, stability, or the production of SCFA’s, the gut barrier becomes more permeable, resulting in low-grade inflammation across the body as foreign bodies pass through the gut barrier. These immune reactions have been associated with disrupted metabolic signalling, including insulin signalling 3.
By comparing the gut microbiomes of healthy individuals to those with type 2 diabetes, researchers can understand how the gut microbiome may play a role, or become affected by, type 2 diabetes. For example, in a systematic review analysing 58 observational studies, it was found that the type 2 diabetes gut microbiome exhibited altered beta diversity and microbial abundance, which included a higher occurrence of Lactobacillus, Escherichia-Shigella, and Enterococcus, but a lower occurrence of SCFA-producers such as Bifidobacterium and Bacteroides 4.
In an experimental trial, transfer of the gut microbiota from healthy individuals into those with insulin resistance increased insulin sensitivity 5. So, the relationship between the gut microbiome and insulin resistance is evident. However, the interaction between gut microbiota and type 2 diabetes is complex; whether gut microbiome dysfunction is a cause or a consequence of type 2 diabetes is unclear. What’s more, human studies frequently include individuals with type 2 diabetes who are taking multiple medications. Medications can confound results as they may independently impact the gut microbiome 2.
It is well established that SCFA’s are an important factor in maintaining gut barrier function. SCFA’s are produced when anaerobic gut bacteria ferment undigested dietary fibres and non-digestible dietary polysaccharides 2. These nutrients can be obtained by eating plants such as whole grains and legumes, for example, which contain a high proportion of dietary fibre 6,7.
In terms of gut health and diabetes, high fibre diets are known to positively impact blood sugar, namely, blood glucose. In a trial involving 192 participants with type 2 diabetes, those allocated a fibre-enriched formula versus a fibre-absent formula were found to have reduced blood glucose, as well as a higher abundance of SCFA-producing bacteria Roseburia faecis and Anaerostipes hadrus 8.
However, while dietary fibre is important, it is equally important to consider plant diversity. Diverse plant types provide a different variety of fibres and nutrients that nourish a greater variety of beneficial gut bacteria, thus promoting microbial diversity. In trials examining plant-based diets, those with higher plant diversity were associated with greater SCFA production and higher microbial diversity 9. These studies not only highlight the importance of eating dietary fibre to support gut bacteria and blood sugar levels, but also emphasise how maintaining a diverse diet of plants can support a healthier microbiome overall.
Diet plays a central role in access to fibre and the association between gut health and diabetes. In addition to fibre, however, probiotics could also play a role in supporting metabolic health.
Human clinical trials show that the introduction of probiotics over several weeks can provide metabolic benefits. Probiotics, including Lactobacillus casei 10, Lactobacillus paracasei HII01 11, Lactobacillus reuteri ADR-1 12, Lactobacillus acidophilus La5, and Bifidobacterium lactis BB12 13 have all been found to lower blood glucose levels compared with a placebo in individuals with type 2 diabetes.
Probiotics have also been found to work beyond blood sugar; they may also contribute to supporting wider metabolic health. When administered over eight weeks, Lactobacillus casei was found to decrease insulin resistance in participants with type 2 diabetes 10. Similar results have been observed in trials using a combination of eight probiotics, including Bifidobacterium bifidum, Bifidobacterium lactis, Lactobacillus acidophilus, Lactobacillus casei, and Lactococcus lactis, for 12 weeks 14. Similarly, in a trial of 68 patients with type 2 diabetes, Lactobacillus reuteri ADR-1 increased Bifidobacterium levels and additionally improved blood cholesterol levels 12, and Lactobacillus paracasei HII01 substantially decreased markers of inflammation over a 12-week period, as well as increasing SCFA levels and beneficial bacteria such as Lactobacillus, Faecalibacterium, and Bifidobacterium 11. These results suggest a broader impact on whole-body health.
This current body of research indicates that probiotics may have a role in supporting metabolic health. However, it is important to note that human clinical trials vary; the use of strains, doses, and treatment periods differ between studies, and this makes it difficult to compare and form consistent conclusions. While further research is needed to understand the long-term relationship between probiotics and type 2 diabetes, probiotics may be one of the many ways to support a healthier gut microbiome in those with metabolic dysregulation, along with a high-fibre diet, appropriate lifestyle choices, and required medical interventions.
There isn’t a single solution for supporting the gut microbiome in type 2 diabetes. It is a combination of approaches that yield the best benefits, but adjusting dietary habits is one step in the right direction on a journey to improved gut health. Steady increases in fibre can provide substantial benefits to gut health 15,16.
Diversity in the diet is equally important, along with appropriate physical activity and lifestyle choices. Diverse plants promote a healthier microbiome by supporting diversity in the gut. Likewise, in trials where physical activity was encouraged in those with type 2 diabetes, gut microbiome composition improvements were observed 17,18.
Type 2 diabetes is a complex condition. A one-size-fits-all approach is not always appropriate, as patients can vary in weight and fitness, or may have dietary restrictions or other issues, such as gastrointestinal problems, that limit their ability to adapt to a diet. For these patients, personalised nutritional considerations should be recommended.
The gut microbiome plays an important role in type 2 diabetes and metabolic health. A diverse, fibre-rich diet, including wholegrains, legumes and plant foods, supports beneficial gut bacteria and blood sugar regulation. Certain probiotics may improve glucose control, insulin resistance, and inflammation, although long-term evidence remains limited. Combining dietary diversity, physical activity, and personalised medical care remains essential for diabetes management.
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