Why MCT oil makes me Diarrhea?
Medium-chain triglycerides (MCTs) are natural fats present in foods such as coconut oil and palm oil. They are not metabolized like long-chain triglycerides but have different mechanisms. MCT oil is a supplement having a high amount of these fats with several health benefits. MCTs have 6 to 12 carbon atoms
In contrast, the medium-chain fatty acids in MCTs have 6–12 carbon atoms. Most common MCTs include
- Hexanoic acid or caproic acid containing 6 carbon atoms
- Octanoic acid or caprylic acid containing 8 carbon atoms
- Capric acid or decanoic acid containing 10 carbon atoms
lauric acid or dodecanoic acid containing 12 carbon atoms 
Medium-chain fatty acids may cause diarrhea because MCT has triglycerides that are metabolized without being transported to the liver. When a person consumes any food that contains MCT oils like coconut oil, he/she is intaking different macronutrients that require enzymes for their metabolism. Hydrogen bonds are present in the molecular structure of coconut and other oils, require enzymatic degradation. When these are broken down into eight or ten carbon fatty acid molecules, they get oxidized by the cells of the human body without allowing them to reach the liver. So, whenever you consume MCT oil in any form, including medication or a high quantity of a compound, it disturbs the physiological mechanism and irritates the GIT tract, resulting in diarrhea.
The human body naturally contains enzymes that digest long-chain triglycerides, which is commonly found in food items. Natural enzymes required to digest MCT oils are deficient in the human body, and additives that are usually present in the human body harm body organs, especially the gastrointestinal tract. A high amount of MCT oil acts as a stressor and irritant, and the body needs to flush it out quickly, which results in diarrhea. A high amount may also cause other side effects like stomach ache, bloating, and vomiting.
Most of the brands use lauric acid that is a 12 carbon compound. Most cosmetic industries are using this compound. Supplement companies usually use C8 and C10 MCT oils. Lauric acid is not for oral consumption, and cosmetic industries use harmful chemicals to extract oils. One of these chemicals is hexane, which is harmful to the gut lining. Caprylic acid (C8) is an antimicrobial that eliminates bacteria and yeasts in contact. When bacteria and yeast are exposed to caprylic acid, they undergo lysis and create an immune response that irritates the lining of the GIT tract. 
Health Benefits of MCT Oils:
MCT oils have a lot of benefits that include:
Better Brain Functions
MCT oils have benefits in patients suffering from Alzheimer's disease. MCT oils increase ketosis in the body, which favors thinking ability in patients suffering from Alzheimer’s disease. Ketosis is a metabolic state in which the brain uses ketones instead of glucose as a source of energy. In people with Alzheimer's disease, glucose uptake is compromised while the ability to take up ketone is the same as in healthy people. Many researches have suggested that MCT oils enhance memory and other brain functions. 
Weight Loss and Healthy Weight Management
MCT oil help in weight loss and maintain a healthy weight. It also increases the number of calories that obese people burned at rest and prevented obesity in normal people. MCT oils increase the hormones that decrease appetite and promote satiety. In some studies, it was found that MCT fats are better than longer-chain fats in losing weight. But, it is significant to notice that researches have determined MCTs as a type of dietary fat rather than MCT oil supplements. 
Controls body Cholesterol
MCTs also play a significant role in lowering blood cholesterol that lowers the risk of heart and vascular diseases. Consuming MCT oils helps to increase the good type of cholesterol that is HDL and reduce the bad type of cholesterol that is LDL and VLDL. This result was obtained after comparing soybean oil to a calorie-controlled diet containing a sufficient amount of MCT oils. Food containing MCT oils, if consumed regularly, also prevents high cholesterol levels and its complications. 
Lowers Blood Sugar levels
MCT oils reduce glucose levels in the blood and decrease the risk of diabetes. It reduces glucose levels by increasing insulin sensitivity in target tissues and reducing insulin resistance. Increased insulin sensitivity also causes lipolysis, which can help in fat loss.
MCT oils affect fat loss, but at the same time, it boosts energy and endurance. Athletes and bodybuilders consume more items containing MCT oils to increase the stamina and energy during physical activities and hard workouts. Consuming food rich in MCTs, rather than longer-chain fats, increase the time that athletes could endure high-intensity exercise. 
 St-Onge, M., & Jones, P. (2002). Physiological Effects of Medium-Chain Triglycerides: Potential Agents in the Prevention of Obesity. The Journal Of Nutrition, 132(3), 329-332. https://doi.org/10.1093/jn/132.3.329
 Patrone, V., Minuti, A., Lizier, M., Miragoli, F., Lucchini, F., Trevisi, E., Rossi, F., & Callegari, M. L. (2018). Differential effects of coconut versus soy oil on gut microbiota composition and predicted metabolic function in adult mice. BMC genomics, 19(1), 808. https://doi.org/10.1186/s12864-018-5202-z
 Rebello, C. J., Keller, J. N., Liu, A. G., Johnson, W. D., & Greenway, F. L. (2015). Pilot feasibility and safety study examining the effect of medium chain triglyceride supplementation in subjects with mild cognitive impairment: A randomized controlled trial. BBA clinical, 3, 123–125. https://doi.org/10.1016/j.bbacli.2015.01.001
 St-Onge, M. P., Mayrsohn, B., O'Keeffe, M., Kissileff, H. R., Choudhury, A. R., & Laferrère, B. (2014). Impact of medium and long chain triglycerides consumption on appetite and food intake in overweight men. European journal of clinical nutrition, 68(10), 1134–1140. https://doi.org/10.1038/ejcn.2014.145
 Eyres, L., Eyres, M. F., Chisholm, A., & Brown, R. C. (2016). Coconut oil consumption and cardiovascular risk factors in humans. Nutrition reviews, 74(4), 267–280. https://doi.org/10.1093/nutrit/nuw002
 Thomas, D. D., Stockman, M. C., Yu, L., Meshulam, T., McCarthy, A. C., Ionson, A., Burritt, N., Deeney, J., Cabral, H., Corkey, B., Istfan, N., & Apovian, C. M. (2019). Effects of medium chain triglycerides supplementation on insulin sensitivity and beta cell function: A feasibility study. PloS one, 14(12), e0226200. https://doi.org/10.1371/journal.pone.0226200
 Wang, Y., Liu, Z., Han, Y., Xu, J., Huang, W., & Li, Z. (2018). Medium Chain Triglycerides enhances exercise endurance through the increased mitochondrial biogenesis and metabolism. PloS one, 13(2), e0191182. https://doi.org/10.1371/journal.pone.0191182