Medium-chain triglycerides (MCTs) are a type of lipid (fat) composed of three medium-length fatty acids (6 to 12 carbon atoms in length) attached to a glycerol backbone.^[17] There are four main types of MCTs, each with a different fatty acid chain length.^[17]

• Caproic acid (hexanoic acid) has a fatty acid length of 6 carbon atoms.
• Caprylic acid (octanoic acid) has a fatty acid length of 8 carbon atoms.
• Capric acid (decanoic acid or decylic acid) has a fatty acid length of 10 carbon atoms.
• Lauric acid (dodecanoic acid) has a fatty acid length of 12 carbon atoms.

MCTs are found in high amounts (per serving size) in coconut oil, palm kernel oil, and dairy products like butter. MCTs are also found in specialized MCT oil supplements.^[17]

MCTs are rapidly digested and converted into energy, bypassing the usual fat digestion process (see How are medium-chain triglycerides absorbed?).^[17] However, people with a rare genetic condition called medium-chain acyl-CoA dehydrogenase (MCAD) deficiency cannot break down MCTs effectively, which can lead to serious health issues.^[18] See What is medium-chain acyl-CoA dehydrogenase (MCAD) deficiency? for more info.

Muscle strength and function

Evidence from some randomized controlled trials shows that daily supplementation with MCTs can boost exercise-induced increases in muscle strength and function in adults with or at risk of sarcopenia and frailty.^[1][2][3]

Cognitive performance

Some observational studies have shown that a higher serum caprylic acid (a.k.a. octanoic acid) concentration in the body is associated with a lower risk of mild cognitive impairment.^[19][20] Some small randomized controlled trials have shown that daily supplementation with MCTs can enhance cognitive performance (working memory and executive function) in adults without known health conditions,^{[4][11][5][6]} and some evidence shows that daily supplementation with MCTs can improve cognitive performance in people with mild to moderate Alzheimer's disease.^[7] However, this has only been observed in people with an APOE4-/- genotype, i.e., in people who do not have any ε4 alleles (versions) of the apolipoprotein E (APOE) gene that predispose people to a high risk of Alzheimer's.^[7] This means that the current evidence does not show a benefit of MCTs on cognitive performance in people with a high genetic risk of Alzheimer’s disease (people with ε4 alleles of the APOE gene).^[7] Furthermore, meta-analyses of studies that examined the effects of MCTs on dementia show large heterogeneity in the effect size between the studies and do not currently support the use of MCTs in the treatment or prevention of Alzheimer’s disease.^[8][9][10] In general, studies in this field also have a high risk of bias because of poor study designs and conflicts of interest.^[8][9][10]

Fat oxidation

Small randomized controlled trials have also shown that daily supplementation with MCTs can increase fat oxidation rates during a high-fat meal^[12] and during low-intensity exercise in men without known health conditions.^[13][14] However, these studies typically compare MCTs to carbohydrates or a calorie-free placebo rather than other types of fats (e.g., long-chain triglycerides). Consequently, the observed effects might not be specific to medium-chain triglycerides.

Body weight

Some research has also examined the effects of MCTs on body composition and body weight. Some meta-analyses have found that supplementation with MCTs can lower body weight,^[15][16] but the effect is very small and variable among the studies.^[15][16][21] Furthermore, the studies are small and few in number, and the majority of studies are affected by conflicts of interest from commercial bias.^[15][16][21]

Energy intake

Although MCTs do not appear to affect the appetite or circulating concentrations of gut hormones,^[22][23] the potential effect of MCTs on body weight might be driven by the moderate decrease in daily energy intake that one meta-analysis found.^[22] However, this effect was demonstrated by pooling diverse study types involving acute and chronic ingestion of MCT supplements as well as diets that contained different amounts of MCTs. Consequently, further high-quality trials and meta-analyses are needed to understand the effects of MCTs on body weight regulation.

Research into the long-term safety of and optimal dosing for MCTs in humans is limited, which emphasizes the need for further research.^[17] However, MCTs are generally considered safe to consume, and toxicity is unlikely.^{[17][24][25][26][27]} That said, some supplementation studies have reported side effects like nausea, diarrhea, and abdominal discomfort.^[28][17]

The no-observed-adverse-effect level (NOAEL) for MCTs ranges from 3 to 5 grams per kg of body weight per day.^[28] However, people who received MCTs via intravenous infusion (e.g., people who received total parenteral nutrition (TPN)) have tolerated dosages of 3 to 9 grams per kg of body weight (gram/kg) per day for several months without adverse effects.^[28] To put this in context, a standard 2,500-calorie-per-day diet in which 30% of calories come from dietary fat would include about 80 grams of fat per day. If all 80 grams were derived from MCTs, then the daily MCT intake in a 70-kg person would be approximately 1.1 grams/kg, which is far lower than the NOAEL range. However, a typical daily intake of MCTs in the diet is only around 2 to 9 grams per day, and someone who takes a daily MCT supplement will likely only ingest 5 to 20 grams per day; these ranges are also much lower than the NOAEL of 210 to 350 grams per day for a 70 kg (154 lb) person.

It is also important to note that, in general, the evidence underpinning the benefits of MCTs has a moderate to high risk of bias because there are few studies with small sample sizes and/or poor methodological quality (e.g., lacking a control group), and they were funded/authored by manufacturers of MCT-containing supplements.

Cognitive performance

Under fasting conditions, ingesting MCTs rapidly increases the serum concentrations of ketone bodies (beta-hydroxybutyrate, acetoacetate, and acetone).^[29] Similarly, regular supplementation of MCTs also increases serum ketone body concentrations.^[30] Because ketone bodies serve as an alternative energy source for the brain when glucose availability is low,^[31] it is plausible that providing the brain with a source of ketone bodies — as shown in some studies^{[4][11][5][6][7]} — is the mechanism by which MCTs might enhance cognitive function. Preclinical studies in rodents support this concept,^[32][33][34] and some clinical trials have indeed found that supplementation with exogenous ketones can improve aspects of cognitive function in people under conditions of metabolic stress (e.g., fasting, hypoxia, or exercise)^{[35][36][37][38][39][40]} and in people with neurodegenerative conditions.^[41] However, the effects are highly variable, and several clinical trials do not find that exogenous ketones have an effect on cognitive function.^[42][43][44]

Muscle strength and function

The mechanism by which MCTs can augment exercise-induced increases in muscle strength^[1][2][3] is not completely understood but might involve blocking the expression of the enzyme MuRF-1 (and thereby stimulating muscle protein synthesis while inhibiting muscle protein breakdown),^[45] increasing the concentration of plasma acyl-ghrelin (thereby stimulating growth hormone secretion),^[46] or activation of the sympathetic nervous system and upregulation of peroxisomal proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha, which promotes mitochondrial biogenesis).^[46]

Body weight

The mechanism by which MCTs influence body weight^[15][16] might involve changes in appetite hormone secretion,^[47] reductions in energy intake,^[47][22] or increases in fat oxidation rates,^{[47][12][13][14]} but not all the studies confirm these effects.^[47][22][23] Evidence from rodent studies and in vitro experiments also shows that MCTs might promote the browning of white adipose tissue, which would enhance lipolysis (fat breakdown) and energy expenditure.^[48] However, such mechanisms are poorly understood and require more research.