The fact that oats have a beneficial effect on the metabolism is nothing new. German medic Carl von Noorden treated patients with diabetes with the cereal at the beginning of the 20th century – with remarkable success. “Today, effective medications are available to treat patients with diabetes,” explains Marie-Christine Simon, 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.”

Although the test subjects in the current trial were not diabetic, they suffered from a metabolic syndrome associated with an increased risk of diabetes. The characteristics include excess body weight, high blood pressure, an elevated blood sugar level, and lipid metabolism disorders. “We wanted to know how a special oat-based diet affects patients,” explains Simon, who is also a member of the Transdisciplinary Research Areas “Life & Health” and „Sustainable Futures“ at the University of Bonn.

300 grams of oatmeal per day

The participants were asked to exclusively eat oatmeal, which they had previously boiled in water, three times a day. They were only allowed to add some fruit or vegetables to their meals. A total of 32 women and men completed this oat-based diet. They ate 300 grams of oatmeal on each of the two days and only consumed around half of their normal calories. A control group was also put on a calorie-reduced diet, although this did not consist of oats.

Both groups benefited from the change in diet. However, the effect was much more pronounced for the participants who followed the oat-based diet. “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,” stresses Simon. “They also lost two kilos in weight on average and their blood pressure fell slightly.”

The effect on LDL cholesterol, in particular, is likely to be relevant to health. If the blood contains too much of this, it is deposited in the vessel walls. These deposits, known as plaques, narrow the blood vessels. In addition, the deposits can rupture, for instance due to an increase in blood pressure following physical exertion, anger, or stress. As a result, a blood clot can form at the affected site, completely blocking the blood vessel. Alternatively, parts of the plaque can be washed away by the blood and cause a heart attack or stroke.

Oats promote the growth of “healthy” intestinal bacteria

But how does oatmeal exert its beneficial effect? “We were able to identify that the consumption of oatmeal increased the number of certain bacteria in the gut,” explains Simon’s colleague Linda Klümpen, the lead author of the trial. The microbiome has increasingly been the focus of research in recent decades. After all, it is now known that intestinal bacteria play a decisive role in metabolizing food. They also release the metabolic by-products that they create into their environment. They supply, among other things, the cells of the gut with energy, enabling them to better perform their tasks.

In addition, the microbes send some of their products around the body in the blood stream, where they can have various effects. “For instance, we were able to show that intestinal bacteria produce phenolic compounds by breaking down the oats,” says Klümpen. “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.” At the same time, other microorganisms “dispose of” the amino acid histidine. The body otherwise turns this into a molecule that is suspected of promoting insulin resistance. This insensitivity to insulin is a key feature of diabetes mellitus.

A large amount of oats for two days better than a small amount for six weeks

The positive effects of the oat-based diet tended to still be evident 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,” says Junior Professor Simon. However, in the current study, the cereal above all exerted its effect at a high concentration and in conjunction with a calorie reduction: A six-week diet, in which the participants consumed 80 grams of oats per day, without any other restrictions, achieved small effects. “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,” continues Simon.

Test method:

A total of 68 participants took part in the trial. For the two-day short-term oat-based diet, all 17 participants on the oat-based diet and 15 participants on the control diet successfully completed the study phase. Two participants in the control group withdrew for personal reasons. For the six-week long-term oat-based intervention, 17 participants in the study group and the same number in the control group took part until the end. The sample size of 17 participants per group was calculated by the researchers on the basis of data from an earlier interventional trial.

Both the two-day intensive diet and the six-week trial with a moderate dose of oats were randomized controlled trials. In these “RCTs,” the test subjects are divided into two groups at random (i.e. randomized). One of them receives the potential active ingredient – in this case the oats –, but the other (the control group) does not. Ideally, the test subjects are “blind”: They do not know to which group they belong. This rules out any placebo effects.

In nutritional experiments, blinding is often not possible – those involved ultimately generally know what they are eating. This was also the case in these studies. However, the evaluation of the blood and stool samples was indeed “blind”: The researchers in charge of this were not informed whether the material had been taken from members of the test group or the control group. The same also applied to the blood pressure and weight measurements. This ruled out the possibility of the scientists’ expectations falsifying the results.

Blood and stool samples were taken before the participants made any changes to their diet. Their blood pressure, weight, height, waist size, and body fat were also measured. A second examination took place immediately after the two-day oat-based diet, followed by three others after two, four, and six weeks. The same analysis were conducted on these four visits as during the initial examination and further blood and stool samples were collected. The researchers took the same approach during the second nutritional study, in which the subjects consumed 80 grams of oatmeal a day for six weeks.

The blood samples were examined in the lab for their LDL cholesterol content, among other things. The researchers also measured the concentration of a key molecule, dihydroferulic acid. This phenolic compound is presumably formed by certain intestinal bacteria, which are known to have a health-promoting effect.

By examining the stool samples, the researchers were able to confirm this hypothesis. They isolated what is known as 16S RNA from the samples. This is a molecule that exclusively occurs in bacteria, but differs somewhat between different species. A 16S RNA molecule can thus be used to identify the bacterium from which it originates, just like a fingerprint. The researchers also analyzed which metabolic products were present in the stool.