Eating nothing but oatmeal for just two days might sound extreme, but it delivered a striking payoff in a new clinical trial. People with metabolic syndrome who followed a short, calorie-reduced oat…
Eating mostly oatmeal for just two days may significantly reduce cholesterol, according to a clinical trial from the University of Bonn published in Nature Communications. The study focused on people with metabolic syndrome, a cluster of conditions that includes excess body weight, high blood pressure, elevated blood sugar, and abnormal blood lipid levels. Participants followed a calorie restricted plan made up almost entirely of oatmeal for 48 hours.
Compared with a control group that also reduced calories but did not eat oats, those on the oat based plan saw a markedly greater improvement in their cholesterol levels. The reduction remained noticeable even six weeks later. Researchers also found that the diet changed the balance of bacteria in the gut. Substances produced by these microbes appear to play an important role in the health benefits linked to oats.
A Historic Diabetes Therapy Revisited
Oats have long been associated with metabolic health. In the early 20th century, German physician Carl von Noorden used oats to treat patients with diabetes, reporting strong results. "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."
The volunteers in the new study did not have diabetes, but they did have metabolic syndrome, which raises the risk of developing the disease. This condition is defined by excess weight, high blood pressure, elevated blood sugar, and disorders of lipid metabolism. "We wanted to know how a special oat-based diet affects patients," says 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
During the intensive phase, participants ate boiled oatmeal three times daily and could only add small amounts of fruit or vegetables. In total, 32 women and men completed the two day oat based intervention. Each person consumed 300 grams of oatmeal per day and cut their usual calorie intake roughly in half. The control group also reduced calories but did not consume oats.
Both groups experienced some benefits from eating fewer calories. However, the improvements were stronger among those who ate oats. "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."
Lowering LDL cholesterol is especially important for heart health. When LDL levels are too high, cholesterol can build up inside artery walls, forming plaques that narrow blood vessels. These plaques may rupture during physical strain, emotional stress, or spikes in blood pressure. A resulting blood clot can completely block blood flow or travel to the heart or brain, triggering a heart attack or stroke.
Gut Microbiome Changes May Explain the Effect
To understand why oats had this impact, researchers examined the gut microbiome. "We were able to identify that the consumption of oatmeal increased the number of certain bacteria in the gut," says Linda Klümpen, the study's lead author. Scientists increasingly recognize that gut bacteria are central to how the body processes food. These microbes generate metabolic byproducts that nourish intestinal cells and support their normal function.
Some of these bacterial products also enter the bloodstream, where they can influence other organs. "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, certain microbes help eliminate the amino acid histidine. Without this process, the body can convert histidine into a compound believed to promote insulin resistance, a hallmark of diabetes mellitus.
Short Intensive Plan Outperformed Longer Moderate Intake
The cholesterol lowering effects were still visible six weeks after the two day intervention. "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, the benefits were strongest when oats were consumed in high amounts alongside calorie restriction. In a separate six week phase, participants ate 80 grams of oatmeal per day without additional dietary limits. That approach produced only modest changes. "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," Simon adds.
How the Randomized Controlled Trials Were Conducted
A total of 68 people took part in the research. In the two day oat based study, 17 participants in the oat group and 15 in the control group completed the trial. Two individuals in the control group withdrew for personal reasons. In the six week intervention, 17 participants in each group finished the study. The researchers determined the group size of 17 per arm based on earlier interventional data.
Both the short and longer interventions were randomized controlled trials. In these "RCTs," participants are assigned at random to different groups. One group receives the intervention being tested, in this case oats, while the control group does not. Ideally, participants are "blind" and unaware of which group they are in, which reduces placebo effects.
In nutrition studies, full blinding is often difficult because people usually know what they are eating. That was true here. However, the laboratory teams analyzing blood and stool samples were unaware of which group the samples came from. The same applied to blood pressure and weight measurements, reducing the chance that expectations could influence the results.
Before any dietary changes, researchers collected blood and stool samples and measured blood pressure, weight, height, waist circumference, and body fat. Follow up assessments took place immediately after the two day oat phase and again at two, four, and six weeks. The same measurements and sample collections were repeated each time. The six week oatmeal group underwent identical testing procedures.
Blood samples were analyzed for LDL cholesterol levels and for dihydroferulic acid, a phenolic compound thought to be produced by beneficial gut bacteria. Stool samples were used to identify bacterial species by isolating 16S RNA, a molecule unique to bacteria that varies slightly between species, much like a fingerprint. Researchers also examined which metabolic byproducts were present.
The study received funding from the German Federal Ministry of Education and Research (BMBF), the German Diabetes Association (DDG), the German Research Foundation (DFG), the German Cereal Processing, Milling and Starch Industries' Association (VGMS), and RASO Naturprodukte.
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Comments
Fiber is the ultimate nutritional power tool.
Not only does fiber reduce cardiovascular mortality by 26% (by cutting cholesterol), surprisingly enough, fiber even reduces your risk of cancer by 22%: https://www.empirical.health/blog/dietary-fiber-reduces-all-...
(Oatmeal is high in fiber, among other things, which I think is part of what's going on here.)
By davidmurdoch 2026-03-012:361 reply Oatmeal is extra special because of it has high amounts of beta glucan, a soluble fiber.
By Romanulus 2026-03-013:28 [dead]
I wonder, do overnight oats wind up processing away a lot of the benefits? Do steel-cut oats have more of an effect on cholesterol than rolled?
By ac29 2026-03-0116:42 > I wonder, do overnight oats wind up processing away a lot of the benefits?
Overnight oats are just normal oats left in liquid overnight to skip the cooking step.
What processing were you thinking of? If anything they are less processed than normal oats since they aren't cooked.
By canadiantim 2026-03-012:52 I'd bet that oatmeal helps cancer due to increased elimination from the GI system, like any fibre
By buzzerbetrayed 2026-03-013:229 reply Oh boy. Now we’re entering the fiber era. We’re just leaving the protein era. Before that it was the intermittent fasting era. Before that it was the keto era. The low fat era was probably a few before that.
I hear about fiber constantly all of the sudden. You might be right about it, but how do we know it’s different than. All the past nutrition tends?
By zem 2026-03-0110:01 I'm in my 50s and I've been hearing about the benefits of fibre pretty much all my life. I doubt it's some sort of diet fad.
By bulbar 2026-03-016:43 Idk about cholesterol, fiber is well known to be very healthy. Same for protein.
Losing body fat will often have the biggest impact by far if one is overweight, though. It also stabilizes blood sugar and has a lot of benefits in general.
By midnitewarrior 2026-03-0113:56 Before manufactured insulin shots, the treatment for diabetes was a multi-day oatmeal fast. This has been around for many decades. The only thing that's changed is that you are finally hearing about it.
By rybosworld 2026-03-013:30 It is funny how you can break diet/nutrition into generations like this.
I think the trends are a reflection of poor education. Fiber/protein/whatever being important components of a diet isn't new information. But the information is new to folks that never had nutrition explained to them.
I feel like we're due for something really ridiculous next. I've been paying attention to macros, fibre, salt, and having a reasonably varied diet for years; we've done salt, fat, carbs, protein, and now we're doing fibre.
"Eat a varied diet" seems boring but maybe those influencers selling pills made from 500 vegetables were ahead of the curve all along.
It would probably be better to just eat all those different vegetables as part of actual meals to get a varied diet, rather than in pill form.
I was under the impression that more protein and less salt/fat/carbs are still kinda the trend? If more fiber gets added to the mix I guess it is essentially telling people to eat more plants, thus leading to more varied diets overall.
By rickydroll 2026-03-0115:39 > I was under the impression that more protein and less salt/fat/carbs are still kinda the trend?
kinda sorta. The low-carb, higher-protein diet is standard diet advice for T2D, and even more so if on a GLP-1 drug to reduce muscle loss.
By 7bit 2026-03-0111:07 I heard about fiber for the last 30 years. But I'm also not American.
By Hikikomori 2026-03-0110:19 Heard about fiber all my life, but also went in school in Europe.
By bionaut 2026-03-0112:30 Because the trends are bullshit and nutrition is just not that complicated.
The trends are a strange type of nutrition entertainment for people to read and then ignore in practice. There is some kind of psychological comfort in the knowing you can switch to oatmeal next week while gorging yourself at the Cheesecake Factory.
Oatmeal is good for you. News at a 11. We have known this for at least that last 50 years.
By Rakshith 2026-03-0111:55 [dead]
By happytoexplain 2026-03-013:031 reply Wait, how does fiber cut cholesterol?
The article is a little densely worded.
By jjtheblunt 2026-03-013:061 reply iirc, from older articles, which differ from this nice result, bile acids contain cholesterol(s) and they're generally reabsorbed in the intestines, so the fiber is conjectured to bind with some before reabsorption, expelling the bound fraction of circulating cholesterol in feces.
this result in the paper is very interesting in the conjecture is that the gut microbiome is altered in a beneficial way, and that the effect (with the resulting lowering of cholesterol) persists for weeks after even 2 days of oats.
By kulahan 2026-03-013:19 We know almost nothing about how digestion works, but fiber has the added benefit of lining your intestines, preventing the absorption of some nutrients. It also helps push things through, so they spend less time sitting around being absorbed.
In related fashion, you might be interested in reading about the impact of soy protein.
I'll recommend starting here https://nutritionsource.hsph.harvard.edu/soy/ (link of studies at the bottom of the page)
Takeways related to parent:
- "cardiovascular mortality ": > eating approximately 50 grams of soy protein a day (no small amount as this translates to 1½ pounds of tofu or eight 8-ounce glasses of soy milk!) in place of animal protein reduced harmful LDL cholesterol by 12.9 percent. [1] Such reductions, if sustained over time, could mean a greater than 20% lower risk of heart attack, stroke, or other forms of cardiovascular disease.
- "risk of cancer": many studies shows breast and prostate cancer reduction, but that is probably more related to isoflavones (Phytoestrogen) than fibers.
> in place of animal protein
makes it sound like this is unrelated to soy specifically and more about displacing less healthful things (like higher saturated fat and caloric animal sources)
By aziaziazi 2026-03-027:16 That’s fair. Re-reading the citation: tofu and soy milk contains very low amount of fibers so it’s probably not a greet exemple to illustrate "soy protein" if the fibers are at play. Or they aren’t. A dive into the source seems reasonable.
Note that saturated fat is also present in plants based food like peanut butter, although that one also contains tons of fibers (absent in animal sources). Coconut oil on the other hand is a tasty evil.
By lanfeust6 2026-03-0216:44 Read further research, there are some meta reviews. Soy protein yields results that other plant proteins don't.
I routinely used to eat an oat based breakfast, and would then feel as though my blood pressure and energy levels were seesawing around for the rest of the morning. Turns out I have celiac disease with sensitivity to the protein in oats.
Dropping this here in case anyone else has a mysterious and unpleasant reaction to oats.
By Sverigevader 2026-03-0110:131 reply Wait, I don't understand. I thought oatmeal was gluten free, but because of where it's grown and processed, there's a lot of cross contamination with wheat. I buy gluten free oats because of this.
By dtj1123 2026-03-0114:13 That's why I specified the protein found in oats, which is similar to but different from gluten. This paper adds some detail: https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=oat+...
By ndsipa_pomu 2026-03-0115:09 I'm somewhat gluten sensitive (tends to make my psoriasis flare up) but used to have gluten-free oats for breakfast. Then the porridge seemed to increase my uric acid levels, leading to gout attacks, so I've had to stop eating them (oats are usually classified as mid-level purine content and thus should be only eaten once or twice a week for those prone to gout).
By 3eb7988a1663 2026-03-012:361 reply I did this after it was first posted. My cholesterol is great, but it is a simple enough intervention, I mostly did it for the lols.
Not terribly difficult, you never feel hungry. The worst part was how sore my jaw felt from the excessive chewing of the bulk mass. Which is funny for something so mushy, but my instinct was to chew it a good amount. Also, it is a lot of fiber. Feel like the effect might just have to do with cleaning out every surface of my digestive tract.
By mrbombastic 2026-03-013:27 I did it as well, 2 days of oatmeal (plus some chicken and toppings) and then oatmeal most days just one meal. Didn’t expect much…my cholesterol dropped 25% over a period of 3 months. One data point, will do another 2 days and see in another 3 months.
