A University of Bonn study suggests that eating mostly oatmeal for just two days can significantly lower harmful LDL cholesterol in people with metabolic syndrome. The short, intensive diet also reshaped gut bacteria in ways that may help protect against diabetes and heart disease.
Two days of eating mostly oatmeal may be enough to give cholesterol levels a meaningful nudge in the right direction, according to a new study from the University of Bonn in Germany.
In adults with metabolic syndrome — a cluster of conditions that raises the risk of diabetes and heart disease — a short, intensive oat-based diet lowered harmful LDL cholesterol, promoted modest weight loss and appeared to reshape the gut microbiome in beneficial ways. The findings are published in the journal Nature Communications.
Metabolic syndrome is defined by a combination of excess body weight, high blood pressure, elevated blood sugar and abnormal blood lipids. Together, those factors sharply increase the odds of developing type 2 diabetes, heart attack and stroke.
The Bonn team wanted to revisit an old idea with modern tools. More than a century ago, German physician Carl von Noorden treated patients with diabetes using an oat-heavy diet and reported strong results. But as medications improved, that approach faded from view.
“Today, effective medications are available to treat patients with diabetes,” Marie-Christine Simon, a junior professor in the Institute of Nutritional and Food Science at the University of Bonn, said in a news release. “As a result, this method has been almost completely overlooked in recent decades.”
Simon, who is also part of the university’s “Life & Health” and “Sustainable Futures” research areas, and her colleagues set out to test how a focused oat intervention would affect people with metabolic syndrome in a rigorous, randomized controlled trial.
Participants in the intensive oat group ate oatmeal boiled in water three times a day for two days, with only small amounts of fruit or vegetables allowed as additions. They consumed 300 grams of oatmeal per day — about 10.5 ounces — and roughly half their usual calories.
A control group followed a similar calorie-reduced diet for two days, but without oats.
Both groups saw health benefits from cutting calories. But the oat group stood out.
“The level of particularly harmful LDL cholesterol fell by 10 percent for them – that is a substantial reduction, although not entirely comparable to the effect of modern medications,” Simon added. “They also lost two kilos in weight on average and their blood pressure fell slightly.”
LDL cholesterol is often called “bad” cholesterol because high levels can lead to fatty deposits, or plaques, in artery walls. Over time, those plaques can narrow blood vessels or rupture, triggering clots that may cause heart attacks or strokes. Even modest reductions in LDL are considered important for long-term cardiovascular health, especially in people already at high risk.
The researchers followed participants for six weeks after the two-day diet and found that the cholesterol improvements largely persisted, suggesting that a brief but intense dietary intervention can have lasting effects.
To understand why oats might be so powerful, the team looked beyond blood tests. They collected stool samples to analyze the gut microbiome — the trillions of bacteria that live in the digestive tract and help break down food, produce vitamins and generate a wide range of metabolic byproducts.
“We were able to identify that the consumption of oatmeal increased the number of certain bacteria in the gut,” added lead author Linda Klümpen, a doctoral student in the Institute of Nutritional and Food Science under Simon’s supervision.
Those microbes do more than just help digest fiber.
“For instance, we were able to show that intestinal bacteria produce phenolic compounds by breaking down the oats,” Klümpen added. “It has already been shown in animal studies that one of them, ferulic acid, has a positive effect on the cholesterol metabolism. This also appears to be the case for some of the other bacterial metabolic products.”
The team measured levels of a key phenolic compound called dihydroferulic acid in blood samples, which is thought to be produced when certain gut bacteria process oat components. These phenolic metabolites are believed to support a more favorable cholesterol profile and may also influence how the body handles blood sugar and inflammation.
Other microbes in the gut appeared to help “dispose of” the amino acid histidine. In the body, histidine can be converted into a molecule suspected of promoting insulin resistance, a hallmark of type 2 diabetes. By shifting which bacteria thrive, the oat diet may indirectly reduce the buildup of such harmful compounds.
The study did not stop at the two-day intensive intervention. In a separate six-week trial, another group of participants with metabolic syndrome ate a more moderate dose of oats — 80 grams per day — without strict calorie limits or other restrictions. That longer, lower-dose approach produced only small health effects, suggesting that intensity and calorie reduction may be key.
Taken together, the results point to a potential strategy that is both simple and low-cost: using short, repeated bursts of an oat-heavy, calorie-reduced diet as a metabolic reset.
“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,” Simon added.
However, she emphasized that more research is needed before doctors can confidently recommend this as a routine preventive measure.
“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,” added Simon.
The trials were randomized controlled trials, considered the gold standard in clinical research. Participants were randomly assigned to oat or control diets. While it was not possible to keep people “blind” to what they were eating, the scientists who analyzed blood and stool samples, and those who measured blood pressure and weight, did not know which group each participant belonged to. That design helps reduce bias and strengthens confidence in the findings.
For now, the work adds to a growing body of evidence that what we eat can quickly and powerfully influence both our blood chemistry and our gut microbes — and that humble foods like oats may play an outsized role in protecting long-term health.
The study does not suggest that two days of oatmeal can replace cholesterol-lowering medications or other treatments. But it does hint that, under medical guidance, targeted dietary changes could become a valuable complement to standard care for people at high risk of diabetes and heart disease.
Source: University of Bonn