I spent several years of my life (a while ago) investigating a potential role of MCT for athletes and because there appears to be a renewed interest in it, I want to discuss what evidence is available today. Medium chain triglycerides or MCT are sold as a supplement. It is usually a synthesized fat that is made from coconut oil. Often it is used to replace other fats in the diet because it is argued that MCT are not stored in the body. Therefore MCT could help athletes to loose body fat. It has been a reasonably popular supplement among body builders and other athletes have also become interested in it. There was a lot of talk about MCT in the early 1990s but some of this seems to have returned. MCT is also used as an extra energy source in various energy bars. This blog will discuss what MCT is and what is true and isn’t true about its use.
What is MCT?
MCT is normally present in our diet in very small quantities and there are few natural sources that contain MCT. Most fats are long chain triglycerides (LCT). MCT is usually synthesized from coconut oil. MCT consists of a glycerol backbone with three medium chain fatty acids (MCFA) attached to it.
MCFA contain 8 to 10 carbons, whereas long chain fatty acids contain 12 or more carbons. Unlike most LCT, MCT are liquid at room temperature. This is in part the result of the small molecular size of MCT. MCT are more soluble in water. This greater water solubility and smaller molecular size have consequences for its metabolism.
Effects of MCT
Here are some of the possible advantages of MCT:
MCT do not slow down gastric emptying. Whereas most fats are knows to keep food in the stomach longer and therefore delay delivery of nutrients, MCT do not seem to have this effect.
MCT are also rapidly absorbed in contrast to most other fats.
When long chain fatty acids reach the blood stream they are incorporated into large particles called chylomicrons. These chylomicrons travel through the body and deliver fatty acids to the tissues. This process is very slow and it is believed that these chyomicrons are not a great energy source for the muscle. When medium chain fatty acids reach the blood stream they do this as medium chain fatty acids and are transported very rapidly to the muscle. Some of the medium chain fatty acids are converted to ketones in the liver and ketones are then used as a fuel by the muscle.
MCT are rapidly used as a fuel. Within minutes after ingestion it is possible to see that the MCT has been oxidized in the muscle.
Carbohydrate plus MCT
When we started to look into MCT we wanted to see if MCT could be an additional energy source during exercise. Carbohydrate intake was important but could we add MCT to provide more energy and perhaps increase fat metabolism and spare muscle glycogen?
First we observed that MCT added to CHO drinks did not inhibit gastric emptying. Then, in a subsequent we studied the oxidation rates of orally ingested MCT were investigated we concluded that more MCT is oxidized better when ingested in combination with CHO compared with ingestion of MCT only. Data confirmed the hypothesis that oral MCT might serve as an energy source in addition to glucose during exercise. However, in these and other studies we found that the maximal amount of oral MCT that could be tolerated in the gastrointestinal tract is small (for most athletes about 30 g). If you can only ingest 30 grams of MCT this limits the contribution of oral MCT to total energy expenditure to values between 3 and 7%. We did not see changes in fat burning and we did not observe any glycogen sparing. When we measured performance with or without MCT, there were no differences, even when we provided up to 80 grams of MCT.
MCT is rapidly emptied from the stomach, absorbed and oxidized, and
the oxidation of exogenous MCT was enhanced when co-ingested with CHO.
Ingestion of 30g MCT did not affect muscle glycogen breakdown and
contributed only up to 7% to energy expenditure. Thus MCT even when oxidized well, cannot add much energy.
Ingestion of larger amounts of MCT resulted in gastrointestinal distress.
Therefore MCT does not appear to have the positive effects on performance that are sometimes claimed.
MCT and weight loss
Can MCT be a useful addition to the general diet or for those individuals who are trying to achieve weight loss? Perhaps. A recent meta-analysis suggested that “Replacement of LCTs with MCTs in the diet could potentially induce modest reductions in body weight and composition without adversely affecting lipid profiles.” But they also noted that the quality of research is not great and therefore “further research is required by independent research groups using large, well-designed studies to confirm the efficacy of MCT and to determine the dosage needed for the management of a healthy body weight and composition”. So I think the jury is still out on MCT, but at least in terms of performance, the evidence is pretty straightforward.
Jeukendrup AE. Modulation of carbohydrate and fat utilization by diet, exercise and environment. Biochem Soc Trans. 2003 Dec;31(Pt 6):1270-3.