Yup, you read that right! If you’ve made it here, I’m guessing you suspect that oatmeal and/or oat milk could be triggering your symptoms of diarrhea – and you want to see if this is actually possible.
As a gut health dietitian nutritionist, I’m here to confirm: it’s entirely possible for oats, oatmeal, and/or oat milk (and virtually ANY food, for that matter) to cause diarrhea!
This doesn’t make sense to most people, first and foremost because 1/2 cup of cooked oatmeal is considered low FODMAP, and oat milk is lactose-free and dairy-free.
And we’re also taught to believe that the low FODMAP diet is the solution to IBS.
- Spoiler alert: it isn’t! ~2/3 to 3/4 of individuals with IBS often DO feel better when reducing their FODMAP intake. (1, 2)
- Reading between the lines, this data is basically also telling us that the low FODMAP diet doesn’t help ~25-30% of folks with IBS. That’s pretty significant!
On another note, we’re also often told that oats are GOOD for gut health and irritable bowel syndrome (IBS). (After all, they contain fiber and prebiotics!)
And while oats are generally beneficial for gut health in most cases, it’s important to remember that gut health is complex – and one size never fits all! (Nothing about nutrition is black or white! It’s all relative.)
If oatmeal and/or oat milk are triggering symptoms of diarrhea, it’s not for no reason. (Everything happens for a reason!)
You’ll need to dig deeper, read between the lines, identify patterns, and ultimately figure out WHY these specific foods are triggering diarrhea – since it could indicate something else is out of alignment with your health.
In this article I’ll pull back the curtain and reveal 7 possible reasons oats, oatmeal, and oat milk could be giving you diarrhea – and what you can do about it.
Disclaimer: This article was written for general educational purposes, not to replace medical or nutritional advice. Make sure to consult with your doctor and a gut health dietitian nutritionist to receive custom guidance and recommendations tailored to your individual needs!
Table of Contents
What is considered diarrhea?
A lot of people have diarrhea, and they aren’t even aware of it. (We don’t know what we don’t know, and most people don’t really talk about poop!)
If you’re unsure whether or not you’re having diarrhea, you can refer to the Bristol Stool Chart.
In a nutshell, diarrhea means you’re having looser-than-”normal” bowel movements on a spectrum from mild to severe, with mild being a “Type 5” and severe diarrhea being a “Type 7” on the Bristol Stool Chart.
Possible diarrhea triggers in oatmeal and oat milk
Before we dive into all the possible reasons oats could trigger diarrhea, it’s important to keep in mind that these may or may not apply to you!
But if you suspect that oatmeal and/or oat milk are causing your diarrhea, it’s good to know how to decipher the clues.
(Please take what you need, and leave the rest!)
1: Gluten/wheat cross-contamination (in cases of celiac disease / non-celiac wheat sensitivity)
Oats are inherently a gluten free grain.
But unless you’re consuming oatmeal and oat milk that is certified gluten free, there’s a likelihood of cross-contamination with wheat, which is a grain that contains high amounts of gluten.
(To clarify, I’m NOT saying that gluten or wheat trigger diarrhea for everyone!)
But…celiac disease (an autoimmune disorder and gluten allergy which causes your intestinal cells to literally attack themselves when you eat gluten) is NOT a routine test in the United States.
So a lot of Americans are potentially walking around with undiagnosed celiac disease. (This is not ideal!)
If there’s any chance you’re walking around with undiagnosed celiac disease and/or a non-celiac wheat sensitivity, and you’re consuming gluten via cross-contamination in oats, this could explain why these foods are causing symptoms of diarrhea.
What you should do
First, check to see if the oatmeal and/or oat milk you consume are gluten free. (If so, proceed onto reason #2!)
But if the oats aren’t certified gluten free, consider keeping a detailed food-symptom journal (such as this IBS food diary), with clinical supervision from a dietitian as needed.
Look for patterns to see if other foods containing gluten (such as regular bread, pasta, and other foods made with all-purpose wheat flour) are triggering symptoms of diarrhea.
If you suspect gluten is the common denominator, consult your doctor about testing for celiac disease – BEFORE experimenting with a gluten free elimination diet.
(Otherwise you’d get a false negative and may walk around with undiagnosed celiac disease unbeknownst to you!)
If you don’t have celiac disease, but notice you feel better on a gluten free diet, you can try switching to gluten free oats and gluten free oat milk.
You may also want to rule out a non-celiac wheat sensitivity with help from a certified LEAP therapist.
(More on food sensitivities in Reason #3!)
2: Fiber / roughage intolerance, in cases of inflammatory bowel disease
“Just eat more fiber!”
…This is what most of my clients with what they believe is diarrhea-predominant irritable bowel syndrome (IBS-D) hear from their doctors before they make it to my functional nutrition clinic.
The problem with this is that while fiber is generally beneficial for gut health, it’s not well tolerated by a lot of people with inflammatory bowel disease (IBD) such as:
- Celiac disease
- Crohn’s disease
- Ulcerative colitis
If you have IBD, and it’s undiagnosed, the fiber and roughage in oatmeal may be less of a friend and more of a “foe” for you!
What you should do
Again, it’s important to keep a detailed food-symptom journal (with clinical supervision from a registered dietitian, as needed).
What are the patterns? What are the common denominators?
- For example, if you notice that other high-fiber foods like whole grains, salads, and raw veggies all seem to trigger diarrhea, you should consult your doctor about a colonoscopy to rule out IBD.
3: Food sensitivity to oats or other ingredients in oatmeal / oat milk
Most people aren’t aware that diarrhea is a common sign and symptom of food sensitivity reactions.
When you have a leaky gut, it’s entirely possible to become “sensitive” to any type of food – even the “healthy” stuff like oats!
I’ve also seen some of my clients with a corn sensitivity react to some of the fillers that often get added to oat milk. (You’d be surprised how many other ingredients go into mainstream conventional oat milk! Not all oat milk is “healthy.”)
The tricky thing about food sensitivities is they’re more difficult to identify, since reactions can sometimes be more delayed and dose-dependent.
What you should do
If you suspect a food sensitivity to oats, I recommend running a food sensitivity test such as the Mediator Release Test or the ALCAT, which are the most valid and accurate since they measure the end-point reaction of all food sensitivities: mediator release.
(Don’t waste your time or money on an IgG test, as these are less accurate!)
You may also want to scroll down and check out reason #7 in this article, in case you’re having a food sensitivity reaction to something else you’re consuming alongside the oatmeal or oat milk.
4: Sucrase-isomaltase deficiency
A significant percentage of the population has an underlying sucrase-isomaltase deficiency (aka “sucrose intolerance”).
- This may be caused by a gene mutation (such as in cases of genetic or congenital sucrase-isomaltase deficiency, aka “CSID”) or it may be a acquired, secondary to collateral damage to your intestines from small intestinal bacterial overgrowth (SIBO) and/or IBD left unchecked for too long.
If you have a sucrase-isomaltase deficiency, it means your body is missing the enzymes required to break down sugars and certain starches, which are naturally occurring in both oatmeal and oat milk.
35-40% is pretty significant! And unfortunately a lot of healthcare providers aren’t yet familiar with this condition.
Long story short – if it’s pretty clear that oatmeal and oat milk are triggering diarrhea, there’s a chance it could be caused by a sucrase-isomaltase deficiency.
What you should do
Keep a food-symptom journal to look for patterns. (A lot of foods naturally contain sucrose and starch.)
Is sugar giving you diarrhea as well? What about maple syrup? Potatoes? These are just a few examples that could warrant a sucrose breath test, or perhaps a disaccharide intestinal biopsy (the gold standard for confirming sucrase-isomaltase deficiency).
Consulting your doctor and a CSID-informed gut health dietitian to help you investigate through proper testing whether or not the sucrose and starches in oatmeal and oat milk could be giving you diarrhea.
If sugar and starch are indeed the “culprits” of your oatmeal-induced diarrhea, you may find the following resources helpful:
- Navigating the Sucrose Intolerance (Sucrase-Isomaltase Deficiency) Diet
- Sucrose Intolerance Foods List PDF (free download)
- 7-Day Sucrase-Isomaltase Deficiency Meal Plan & 21 Recipes (PDF)
5: Food poisoning
According to the United States Department of Agriculture (USDA), diarrhea is one of the most common symptoms of food poisoning. (4)
Food poisoning occurs when a food has been at an unsafe temperature (above 40 degrees Fahrenheit for cold food, or below 140 degrees Fahrenheit for hot food) for long enough that infectious bacteria can grow.
If you eat or drink anything harboring infectious bacteria, this will most likely lead to food poisoning – which manifests as pretty severe, sudden-onset gastrointestinal symptoms like nausea, vomiting, and diarrhea.
While oatmeal and oat milk are less perishable compared to animal-based foods such as meat, poultry, seafood and dairy, it’s still never a good idea to eat or drink something that has been sitting out on the counter for too long.
What you should do
If your diarrhea is acute and sudden-onset, and you suspect it was caused by spoiled / expired oatmeal or oat milk, consider tracing back your steps. back on the exact time and place you ate/drank it.
Was it sitting out? Did it expire? What was the expiration date listed on the package?
Did you consume anything else around the same time that could have been expired or spoiled?
If your symptoms are severe, and/or they last more than a few days, make sure to consult your doctor or go to your nearest emergency room!
6: FODMAP stacking
Oatmeal and oat milk contain small quantities of fermentable oligosaccharides, disaccharides, monosaccharides and polyols (FODMAPs) – even though oatmeal is a low FODMAP grain.
Only up to ~1/2 cup of cooked/soaked oatmeal at one time is considered low FODMAP, and only up to 1/8 cup of oat milk is low in FODMAPs according to the Monash University app.
So if you’re “FODMAP stacking” (consuming these foods and/or other foods in quantities that exceed the low FODMAP diet parameters), it’s possible that the FODMAPs in oats, oatmeal and oat milk can potentially trigger IBS symptoms like diarrhea.
What you should do
If you react to FODMAPs, consider keeping an IBS food diary to look for patterns.
Are you consuming large portions of oatmeal / oat milk that exceed the low FODMAP serving sizes?
If so, consider reducing your portions of oatmeal / oat milk and take a look at the FODMAPs in the rest of that meal/snack/beverage, to avoid FODMAP stacking.
7: You’re reacting to something else
If none of the other stuff spoke to you, it’s possible that you’re reacting to something else that you consumed with your oatmeal and/or oat milk.
For example, are you making your oatmeal with regular milk? If so, maybe you have a lactose intolerance / dairy sensitivity.
Or if you’re pouring the oat milk into a big cup of coffee, maybe the coffee is what’s giving you diarrhea and not the oat milk. (Read more about the correlations between coffee and IBS here!)
Or maybe you’re sweetening your oatmeal or oat milk latte with a high FODMAP sweetener that your body doesn’t like.
(I could go on, but hopefully you’re picking up what I’m putting down here!)
Lastly, there are additives in certain types of oat milk which may potentially disrupt your gut microbes (which affect pretty much everything!).
Are you consuming a good quality oat milk or is it filled with a bunch of junk? (Get my two cents on the best milks and milk substitutes for IBS sufferers here!)
What you should do
Keep a detailed food-symptom journal, and do an “oatmeal audit”.
What kinds of stuff are you putting in the oatmeal / oat milk? Do you have an allergy, intolerance, or food sensitivity to any of those things?
And as always, call me biased – but you should probably also consult a dietitian. 😉
Frequently asked questions
Is oatmeal good for diarrhea?
In some cases, people with diarrhea feel better when they eat oatmeal because of the soluble fiber.
However, due to bio-individuality, I’ve noticed in my private nutrition practice that this varies a lot from one person to another.
For example, if you feel better on a low fiber diet or a “BRAT diet” when you’re suffering from diarrhea, oatmeal probably wouldn’t be the best choice since it is high in fiber.
Is oatmeal good for IBS?
Oatmeal is usually generally safe and beneficial for most, but not all, folks with IBS. But it varies greatly case-by-case.
Consider working with a dietitian for help navigating food allergies, food intolerance, and/or food sensitivities. (These are three very similar but separate types of adverse food reactions!)
Can oat milk cause diarrhea in adults?
While many people can drink oat milk without any issues, there are cases where oat milk CAN cause diarrhea in adults for some of the above reasons mentioned.
For example, if you have undiagnosed celiac disease, and you’re drinking oat milk that is not certified gluten free, you could be reacting to traces of gluten in that oat milk, which would come from cross-contamination with wheat.
It’s also possible for oat milk to cause diarrhea if you have an underlying sucrase-isomaltase deficiency (sucrose intolerance), and/or a food sensitivity to oats (or something else in the oat milk).
On the other hand, if you feel better on a low FODMAP diet, note that only 1/8 cup (2 tablespoons) of oat milk is low FODMAP.
- While most people with IBS tolerate oat milk, keep this in mind if you’re drinking it by the glass or pouring it over cereal.
In many cases, it turns out it is not the oat milk people are reacting to but rather something they’re mixing it with.
- If you put oat milk in your coffee everyday, perhaps it’s the coffee or a sweetener you’re adding to your morning brew that is the culprit of your diarrhea.
- Or if you’re pouring oat milk over cereal, consider switching to a simple IBS-friendly cereal and see if this makes a difference!
When do I need to see a doctor for diarrhea?
If you’re experiencing moderate to severe diarrhea for more than a day or two, consult with your doctor.
If you notice symptoms of dehydration or electrolyte imbalance (dizziness, vertigo, extreme fatigue, nausea, light-headedness, brain fog, fainting, or weakness), go to your nearest emergency room immediately.
Oatmeal, oat milk, and diarrhea: explained
If you suspect oatmeal and/or oat milk are culprits of your diarrhea, it may not be all in your head.
The 7 most common explanations for this include the following:
- Gluten/wheat cross-contamination. This is relevant if you have celiac disease/non-celiac wheat sensitivity. Make sure you’re sticking to oat products that are certified gluten free.
- Fiber and roughage intolerance. This would be relevant if you have inflammatory bowel disease such as colitis, Crohn’s disease, diverticulitis, or celiac disease.
- Food sensitivities. You could be reacting to oats or to something else that you’re eating/drinking alongside the oatmeal / oat milk.
- Sucrose intolerance / starch intolerance. Oatmeal and oat milk contain sucrose and starch, which aren’t inherently bad. But these molecules can’t be easily digested broken down if you have a sucrase-isomaltase deficiency.
- Food poisoning. This is more likely to be applicable if your symptoms are acute and sudden-onset.
- FODMAP stacking. This means you’re exceeding the threshold of oatmeal or oat milk within low FODMAP diet parameters at one time. This may be applicable if you’re someone who feels better on a low FODMAP diet.
- You’re reacting to something else. What else are you eating and drinking alongside your oatmeal and/or oat milk? (Think milk/milk substitutes, fillers, sweeteners, nuts, cookies, coffee, etc.)
Regardless of what could be going on, the safest and quickest way to address the root-causes of your diarrhea is by keeping a food-symptom journal (such as this IBS food diary* created and self-published by yours truly!) and by consulting a doctor and a holistic/functional dietitian nutritionist who specializes in gut health.
On the other hand, it’s important to remember that oats aren’t inherently bad or “unhealthy”. In fact, oatmeal is often beneficial for many people with IBS and diarrhea. Remember one size never fits all.
- Is Oatmeal Low FODMAP?
- Wheat vs Gluten: What’s the Difference?
- 7 Possible Causes of Diarrhea After Eating Sugar
- The Best Milks & Milk Substitutes for IBS Sufferers
- Best Low FODMAP Yogurts for IBS – Comprehensive Brand/Product List & Guide
- What’s the Best Sweetener for IBS?
- Honey for IBS: Benefits & Contraindications
- IBS and Sugar – A Holistic Perspective
- Low FODMAP Nuts, Nut Butters, & Peanut Butter (Oh My!)
- Is Peanut Butter Low FODMAP? (Expert Guidance on Peanut Butter and IBS)
- How Long Can Oatmeal Sit Out: A Comprehensive Guide
Want to learn more about what I have to say on all things gut health, holistic & functional nutrition? If so, the best next step is to download my free gut health nutrition guide: