Review Examines Gut Microbes in Preclinical Alzheimer's, With Timing Unresolved

The paper, published in Nutrients in 2026, is a review article rather than a trial. The authors, who include Ralph Martins and Colin Masters, pull together earlier research on gut bacteria, their by-products and diet in the earliest stage of Alzheimer's. The paper does not state a number of studies or people in the sections available to us.

The stage in question is called preclinical Alzheimer's. People have biological signs of the disease in the brain, such as a build-up of a protein called amyloid-beta, but their thinking is still normal. The review notes this is usually confirmed with brain imaging or spinal fluid tests.

According to the review, people with Alzheimer's and with mild cognitive impairment tend to have fewer gut bacteria that make short-chain fatty acids. The bacteria named are Faecalibacterium, Roseburia and Eubacterium. Short-chain fatty acids, including acetate, propionate and butyrate, are compounds made when gut microbes ferment food. The review says they help keep the gut lining intact, regulate immune responses and support communication between gut and brain.

The authors link these microbial changes to disrupted metabolism, a weaker gut barrier and more inflammation in the brain. They describe these as associations. They also say the pattern is not yet consistent across studies.

The central caveat is timing. Most of the evidence is cross-sectional, meaning it compares groups at one point in time. As the authors write, it remains unclear whether microbe changes come before Alzheimer's pathology or follow from early disease. Much of the evidence also comes from people who already have symptoms. Only a small number of studies have looked at cognitively unimpaired people whose preclinical status was confirmed by biomarkers.

Diet is the part most relevant to everyday life. The review says Mediterranean, DASH and so-called prudent dietary patterns are generally associated with beneficial microbial profiles and more short-chain fatty acid production. Western dietary patterns are linked to lower microbial diversity and more pro-inflammatory bacteria. Diet changes what the microbes have available to ferment, which shapes which species thrive and what they produce.

The authors also point to a wider body of evidence tying long-term eating patterns to cognitive impairment and Alzheimer's risk. They say diet may influence diseaserelated biology from the earliest stages. But they also say how diet, microbes and their functions interact during the preclinical stage is still poorly understood. The review does not show that eating a particular way changes gut microbes in a way that prevents Alzheimer's.

Their earlier work, the authors say, identified gradual gut microbial and metabolic changes during the preclinical stage. They suggest these may be early shifts that come before disease progression. That remains a suggestion, not a demonstrated cause.

What to watch next

Studies that follow cognitively healthy, biomarker-confirmed people over time, to test whether gut changes come first and whether diet alters them.

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