Two-day oat diet shifts gut microbiome, cuts LDL cholesterol for up to six weeks

A two-day, calorie-restricted oat-based diet has produced sustained reductions in harmful cholesterol among adults with metabolic syndrome, with researchers identifying microbiome-derived metabolites as a likely mechanism 

A short-term oat-based diet has appeared to reduce cholesterol levels to a clinically meaningful degree, according to a randomised controlled trial conducted by researchers at the University of Bonn. The investigation targeted adults with metabolic syndrome, a cluster of risk factors that includes excess body weight, raised blood pressure, elevated blood glucose, and disturbed lipid metabolism, all of which are associated with an increased risk of type 2 diabetes and cardiovascular disease.

Participants followed a calorie-reduced diet that consisted almost exclusively of oatmeal for two consecutive days. During this period, they consumed approximately half of their usual daily caloric intake. Their cholesterol levels improved significantly when compared with those of a control group that also followed a calorie-restricted diet but without oats. Notably, the reduction in cholesterol persisted for at least six weeks after the intervention, suggesting a durable metabolic effect rather than a transient dietary response.

The metabolic benefits of oats have a long history. At the beginning of the twentieth century, the German physician Dr. Carl von Noorden treated people with diabetes using oat-based regimens, with reported success. Despite this historical precedent, the approach has largely fallen out of favour in recent decades as pharmacological treatments have become widely available.

“Today, effective medications are available to treat patients with diabetes,” said Dr. Marie-Christine Simon, a junior professor at the Institute of Nutritional and Food Science at the University of Bonn.

“As a result, this method has been almost completely overlooked in recent decades,” she added.

Although none of the participants in the present trial had diabetes, all exhibited metabolic syndrome, a condition that substantially increases the likelihood of future disease.

“We wanted to know how a special oat-based diet affects patients,” said Simon, who is also a member of the Transdisciplinary Research Areas Life and Health and Sustainable Futures at the University of Bonn, in Germany.

Participants assigned to the oat-based intervention were instructed to eat boiled oatmeal three times per day, with the option to add small amounts of fruit or vegetables. Thirty-two women and men completed the two-day intensive phase, each consuming 300 grams of oats per day. The control group followed a calorie-reduced diet of comparable energy content but without oats.

Both groups experienced metabolic improvements, but the effects were markedly greater among those who consumed the oats.

“The level of particularly harmful low-density lipoprotein (LDL) cholesterol fell by 10 per cent for them – that is a substantial reduction – although not entirely comparable to the effect of modern medications,” said Simon.

Participants in the oat group also lost an average of two kilograms in body weight, alongside modest reductions in blood pressure.

The reduction in LDL cholesterol is of particular clinical relevance. Excess circulating LDL cholesterol can accumulate within blood vessel walls, where it forms plaques that narrow the vessels and impair blood flow. Plaques may rupture under physical or emotional stress, which can lead to clot formation and vessel occlusion. Alternatively, fragments may dislodge and travel through the circulatory system, potentially triggering a heart attack or stroke.

To understand the biological basis of these effects, the researchers examined changes within the gut microbiome.

“We were able to identify that the consumption of oatmeal increased the number of certain bacteria in the gut,” said doctoral candidate Linda Klümpen, the study’s lead author.

Research over recent decades has established that intestinal bacteria play a central role in food metabolism and release bioactive metabolic by-products that influence host physiology. Some of these microbial products enter the bloodstream and exert effects beyond the gut.

“For instance, we were able to show that intestinal bacteria produce phenolic compounds by breaking down the oats. It has already been shown in animal studies that one of them – ferulic acid – has a positive effect on the cholesterol metabolism,” said Klümpen.

The researchers reported that related microbial metabolites also appeared to contribute to the observed lipid-lowering effects.

At the same time, other members of the gut microbiome reduced the availability of the amino acid histidine by metabolising it. In the absence of this microbial activity, histidine can be converted by the body into a compound that has been implicated in the promotion of insulin resistance, a defining feature of diabetes mellitus.

The beneficial effects of the two-day oat-based intervention remained detectable six weeks later.

“A short-term oat-based diet at regular intervals could be a well-tolerated way to keep the cholesterol level within the normal range and prevent diabetes,” said Simon.

However, she cautioned that the strongest effects were observed only at high level of oat intake in combination with calorie restriction. A separate six-week intervention, in which participants consumed 80 grams of oats per day without additional dietary limits, produced only modest improvements.

“As a next step, it can now be clarified whether an intensive oat-based diet repeated every six weeks actually has a permanently preventative effect,” she said.

In total, 68 participants took part across both interventions. For the two-day intensive phase, 17 participants in the oat group and 15 in the control group completed the study, with two control participants withdrawing for personal reasons. In the six-week intervention, 17 participants in each group completed the trial. The sample size was calculated on the basis of data from a previous interventional study.

Both dietary interventions followed a randomised controlled design. Participants were allocated to groups at random, with one group receiving the oat-based intervention and the other serving as a control. Full blinding was not feasible, as participants were aware of what they consumed, a common limitation in nutritional research. However, laboratory analyses of blood and stool samples, as well as measurements of weight and blood pressure, were conducted without knowledge of group allocation, which reduced the risk of observer bias.

Blood and stool samples were collected before any dietary changes and at multiple follow-up visits, including immediately after the two-day intervention and at two, four, and six weeks. Researchers measured LDL cholesterol and the concentration of dihydroferulic acid, a phenolic compound thought to originate from microbial metabolism and to have health-promoting properties.

Analysis of stool samples supported this hypothesis. The team isolated 16S ribonucleic acid, a bacterial molecule that varies between species and enables microbial identification, alongside assessments of metabolic by-products present in the gut. Together, these analyses have strengthened the evidence that short-term, intensive oat consumption can reshape the gut microbiome in ways that support improved cholesterol metabolism.

For further reading please visit: 10.1038/s41467-026-68303-9