Eating oats for every meal for two days cut 'bad' cholesterol by up to 10 percent in a small clinical trial.
Surprisingly, the effect was still visible in participants' blood six weeks after they returned to their normal diets.
The most interesting part of the study wasn't just the cholesterol drop (which is smaller than what high-dose statins can achieve) – it was what might be causing it.
The researchers linked molecules produced by gut bacteria as they break down oats to reductions in low-density lipoprotein (LDL), a type of cholesterol that raises the risk of heart disease.
The trial, published in Nature Communications, involved 15 men and 17 women with metabolic syndrome.
Seventeen participants (six men and 11 women) were fed a calorie-reduced diet made up almost entirely of oatmeal. Three times per day, across 48 hours, the participants ate 100 grams of rolled oat flakes boiled in water. They were only allowed to add select fruit and vegetables to their oatmeal – no salt, sugar, or sweeteners permitted.
A control group consisting of the remaining 15 participants was also fed a calorie-reduced 'healthy' diet, but wasn't fed oats.
Both groups had their calories cut by around half for the two days before returning to their normal diets.
Immediately after the diet, total cholesterol levels had declined 8 percent in the oat group, and LDL levels were down around 10 percent – significantly more than those in the control group. Participants on the 48-hour oat diet also experienced weight loss and slight reductions in blood pressure.
"The level of particularly harmful LDL cholesterol fell by 10 percent for [the oat group] – that is a substantial reduction, although not entirely comparable to the effect of modern medications," says senior author Marie-Christine Simon, a food scientist at the University of Bonn in Germany.
By analyzing fecal samples and blood plasma, the study found evidence that the oat diet increases gut bacteria that produce compounds linked to improved cholesterol metabolism.
"We were able to identify that the consumption of oatmeal increased the number of certain bacteria in the gut," lead author Linda Klümpen explains in a press release.
"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. This also appears to be the case for some of the other bacterial metabolic products."
To take the results further, the team tested the effects of dihydroferulic acid – a byproduct produced by gut bacteria breaking down oats – where it appeared to reduce cholesterol storage.
It's important to note that the results may not apply to everybody in the general population. Volunteers participating in the trial all had metabolic syndrome, a condition associated with excess weight, high blood pressure, and elevated blood sugar that can be a precursor to diabetes.
But given that LDL concentrations remained below starting levels six weeks after the diet had ended, the researchers think they've found a biological process worthy of further investigation.
The researchers also ran a second trial in which 17 volunteers ate 80 grams of oats daily for six weeks without other dietary restrictions, compared to 17 controls who did not eat oats.
While there were some benefits, the longer, less extreme oat diet didn't produce the same rapid cholesterol drop.
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While the study received support from the German Cereal Processing, Milling and Starch Industries' Association and a food company, the researchers state the funders had no role in the study's design or analysis and didn't declare any conflicts of interest.
Larger studies are needed to confirm the findings and further explore how oats influence gut bacteria, cholesterol, and maybe even weight loss.
"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 Simon.
"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."
The research has been published Nature Communications.