Side effects of MCT oil
MCT oil is often marketed as “clean fuel” for the brain and muscles, but for many people the first experience of taking it ends in abdominal discomfort. Our editorial team examined which MCT side effects are actually described in the scientific literature, why they occur, and how to keep them to a minimum.
Why MCT oil causes side effects at all
MCT oil consists of medium-chain triglycerides — mainly caprylic (C8) and capric (C10) acids. Unlike ordinary fats, they do not need to be packaged into chylomicrons: after hydrolysis in the intestine, the fatty acids are absorbed directly into the portal vein system and quickly reach the liver. It is precisely this speed that is both the product's main advantage and the cause of most adverse reactions.
When a large amount of medium-chain fatty acids enters the small intestine at once, they act as osmotically and locally irritating substances. Part of them fails to be absorbed in time, draws water into the intestinal lumen, and accelerates peristalsis. Hence the rumbling, cramps, and loose stools that users report most often.
The second mechanism is metabolic. In the liver, MCTs are rapidly oxidized, and the excess acetyl-CoA is converted into ketone bodies. For a healthy person this is safe and even desirable in ketogenic regimens, but in certain conditions discussed below, extra ketone production can be a problem.
The toxicological review by Traul and colleagues (2000) concluded that MCTs have low systemic toxicity and that the main tolerance limitations are precisely gastrointestinal. In other words, most side effects are not “poisoning” but a matter of dose, speed, and method of intake.
Gastrointestinal reactions: the most common problem
The best-documented adverse effects of MCTs are nausea, a feeling of stomach fullness, bloating, cramps, and diarrhea. They were described as far back as the classic review by Bach and Babayan (1982), as well as in studies of ketogenic epilepsy therapy, where MCTs are used as a source of ketones. In children on an MCT ketogenic diet, gastric complaints were one of the main factors limiting the dose.
Studies by Jeukendrup and colleagues (1998) are instructive for athletes: participants consumed a significant amount of MCTs together with carbohydrates during prolonged cycling, and some of them complained of gastric discomfort, while their results in the final test actually worsened. Similar complaints were recorded by the Goedecke group (2005) in an ultramarathon protocol.
The intensity of symptoms depends greatly on whether a person takes the oil on an empty stomach or with food. Mixing it with proteins, fiber, and other fats slows gastric emptying and spreads the intake of fatty acids over time, so tolerance improves noticeably.
Another factor is adaptation. Many people notice that symptoms that are pronounced in the first days weaken after one or two weeks of gradually increasing the portion. However, in some people even small amounts of MCT remain uncomfortable, and this is a variant of the individual norm rather than a “wrong” product.
Metabolic effects: lipids, ketones, the liver
The effect of MCTs on the lipid profile remains a subject of debate. Some studies show no substantial changes in cholesterol, while others observed a moderate rise in total cholesterol or triglycerides compared with unsaturated oils. The result depends on what MCTs are compared with and what share of the diet they make up.
The ketogenic effect of MCTs is neutral for most healthy people: beta-hydroxybutyrate levels rise moderately and temporarily. However, in people with insulin-dependent diabetes, especially with insufficiently controlled glycemia, any additional source of ketones requires caution and coordination with an endocrinologist.
Since the entire flow of medium-chain fatty acids first passes through the liver, MCT supplements should not be taken without a doctor in severe liver diseases, in particular cirrhosis. They are also contraindicated in rare congenital disorders of fatty acid oxidation, for example MCAD deficiency.
Finally, MCT is a high-calorie product: about 8.3 kcal per gram. A tablespoon of the oil adds roughly 100–120 kcal to the diet. If you add it “on top” of your usual diet instead of replacing other fats, the result may be a gradual gain in weight — although this is not a side effect in the classic sense but simply energy balance.
Other adverse reactions and interactions
In addition to digestive symptoms, users sometimes report headache, dizziness, or a feeling of “heaviness” in the first days of use, especially against the background of a low-carbohydrate diet. Such complaints are hard to attribute unambiguously to MCTs, because they often coincide with adaptation to ketosis in general.
Allergic reactions to purified MCT oil are rare, since the product contains almost no proteins. However, the raw material is usually coconut or palm kernel oil, so people with a confirmed coconut allergy had better check the manufacturer's specification.
Powdered forms of MCT contain carriers — maltodextrin, gum arabic, sodium caseinate, or other emulsifiers. It is precisely these components that can cause a reaction in people with milk-protein intolerance or sensitivity to certain fibers, although the triglycerides themselves have nothing to do with it.
No direct significant drug interactions have been described for MCTs, but when taking fat-soluble medications, anticoagulants, or glucose-lowering agents, a sharp change in the dietary pattern (for example, switching to a ketogenic diet with a large amount of MCTs) may affect their action. This is a reason to inform your doctor.
How to reduce the risk of side effects
Most problems with MCT oil are solved by a gradual approach. Our editorial team has summarized the practical principles that follow from the clinical experience of ketogenic therapy and tolerance studies.
- Start small:5 g (about a teaspoon) per day for several days in a row.
- Increase slowly:add 5 g every 3–7 days if there is no discomfort.
- Take with food:coffee with milk, porridge, and smoothies are tolerated better than oil on an empty stomach.
- Split the portion:two or three intakes instead of one large one.
- Do not use before competitions without testing:check tolerance during training.
| Symptom | Probable cause | What to do |
|---|---|---|
| Diarrhea, cramps | Too large a single dose, intake on an empty stomach | Reduce the portion, take with food |
| Nausea | Rapid arrival of fat in the stomach | Split the dose, choose a powdered form |
| Bloating | Carriers in the powder or rapid increase | Check the composition, slow the pace |
| Weight gain | Excess calories | Replace other fats rather than adding |
| Headache at the start | Adaptation to low-carbohydrate eating | Water, electrolytes, gradualness |
If symptoms do not go away even at the minimum dose, there is no point in continuing to “endure” it. MCT oil is not an essential nutrient, and for most purposes there are alternatives.
You should specifically see a doctor in case of severe abdominal pain, vomiting, signs of dehydration, and also with any alarming symptoms in people with diabetes or liver diseases.
It is also important to remember: MCT oil is not intended for frying at high temperatures because of its low smoke point, and the products of its breakdown can irritate the mucous membranes.
Editorial conclusions
MCT oil has a good safety profile for healthy adults, and its main side effects concern digestion: nausea, cramps, diarrhea, and bloating. They are almost always dose-dependent and manageable.
The key to comfortable use is small starting portions, intake with food, and a gradual increase. Large single doses before or during exercise can not only cause discomfort but also worsen performance, as endurance studies have shown.
People with diabetes, liver diseases, fatty acid oxidation disorders, as well as pregnant and breastfeeding women, should consult a doctor before starting.
We also recommend reading our articles “Who should not take MCT oil,” “How to take MCT oil: dosage and timing,” and “MCT oil: forms and which to choose.”
References
- Bach AC, Babayan VK. Medium-chain triglycerides: an update. Am J Clin Nutr. 1982;36(5):950–962.
- Traul KA, Driedger A, Ingle DL, Nakhasi D. Review of the toxicologic properties of medium-chain triglycerides. Food Chem Toxicol. 2000;38(1):79–98.
- Jeukendrup AE, Thielen JJ, Wagenmakers AJ, Brouns F, Saris WH. Effect of medium-chain triacylglycerol and carbohydrate ingestion during exercise on substrate utilization and subsequent cycling performance. Am J Clin Nutr. 1998;67(3):397–404.
- Goedecke JH, Clark VR, Noakes TD, Lambert EV. The effects of medium-chain triacylglycerol and carbohydrate ingestion on ultra-endurance exercise performance. Int J Sport Nutr Exerc Metab. 2005;15(1):15–27.
- St-Onge MP, Jones PJH. Physiological effects of medium-chain triglycerides: potential agents in the prevention of obesity. J Nutr. 2002;132(3):329–332.
- Liu YM. Medium-chain triglyceride (MCT) ketogenic therapy. Epilepsia. 2008;49(Suppl 8):33–36.
- Mumme K, Stonehouse W. Effects of medium-chain triglycerides on weight loss and body composition: a meta-analysis of randomized controlled trials. J Acad Nutr Diet. 2015;115(2):249–263.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
