Exogenous ketones or MCT oil: what is the difference

Ketone drinks and MCT oil are often marketed as interchangeable means of "ketone energy." In reality, one is ready-made fuel and the other only its precursor. The editorial team examined how each product works in the body, how much it raises ketone levels, and what the research says about it.

What exogenous ketones and MCT oil are

On sports-nutrition shelves these two categories often stand side by side and are sold under similar slogans: "energy for the brain," "keto-diet support," "clean fuel." Yet by their nature they are fundamentally different. Exogenous ketones are ready-made ketone bodies that enter the blood "from outside." MCT oil is a fat from which the liver may only form ketones, or may simply burn it as ordinary fuel.

Exogenous ketones exist in two main forms. Ketone salts are beta-hydroxybutyrate (β-HB) bound to sodium, potassium, calcium or magnesium. Ketone esters are β-HB attached to an alcohol; the best-known example is (R)-3-hydroxybutyl-(R)-3-hydroxybutyrate, whose safety was studied by Clarke and colleagues (2012). In the intestine and liver esters are broken down, releasing β-HB.

MCT (medium-chain triglycerides) are triglycerides with medium-chain fatty acids: caproic (C6), caprylic (C8), capric (C10) and lauric (C12). Commercial MCT oil is obtained by fractionating coconut or palm-kernel oil, leaving mainly C8 and C10. Ordinary coconut oil is not the same: it is dominated by lauric acid, which behaves more like long-chain fats.

So the main difference is set already at the level of the definition: ketones are the end product, while MCT is the raw material. Everything else depends on this: speed of action, how high blood ketone levels rise, effect on digestion, calorie content, and the purposes for which each product makes sense.

The path through the body: from intake to the bloodstream

MCTs are absorbed differently from ordinary dietary fats. As Bach and Babayan (1982) summarize, medium-chain fatty acids do not need to be packaged into chylomicrons: they are absorbed into the portal vein and go directly to the liver. There they enter the mitochondria without the carnitine transporter, are rapidly oxidized, and the excess acetyl-CoA is converted into ketone bodies.

The amount of ketones the liver forms from MCTs depends on the composition of the oil and the dietary context. In the study by Vandenberghe and colleagues (2017), it was pure tricaprylin (C8) that produced the largest rise in ketones, noticeably higher than mixtures with C10 or coconut oil. And if MCTs are taken together with carbohydrates, insulin inhibits ketogenesis and the response becomes even more modest.

Exogenous ketones do not require hepatic synthesis: β-HB is simply absorbed in the intestine and appears in the blood. According to the review by Stubbs and colleagues (2017), ketone esters raise β-HB levels far higher than salts at an equivalent dose, and the peak usually occurs within the first hour. After that, ketones are used by the muscles, heart and brain or excreted in urine and breath.

An important detail concerns stereoisomers. The ketone esters studied at Oxford contain D-β-HB, the form the body produces. Many ketone salts are a racemic mixture of D- and L-forms, and L-β-HB is metabolized more slowly and its physiological role is far less studied. That is why a reading on a blood ketone meter, which measures precisely the D-form, may underestimate total intake.

Ketone levels and speed of effect

If the differences are boiled down to a single characteristic, it is the "strength" of the ketosis that can be achieved. MCTs can only moderately raise ketone levels, and this effect depends heavily on what the person ate beforehand. Ketone salts give a short-lived and modest rise, while esters can within an hour bring a person into a range that on a diet is reached only after several days of strict carbohydrate restriction.

At the same time, ketone levels obtained from outside are not equivalent to dietary ketosis. On a ketogenic diet the body reorganizes: fat oxidation increases, insulin falls, enzymes adapt. After a single intake of exogenous ketones this adaptation does not occur. Moreover, ketones themselves somewhat reduce the release of fatty acids from adipose tissue, meaning that "fat burning" is not enhanced.

The duration of action also differs. β-HB from esters or salts stays elevated for several hours, after which it returns to baseline. MCTs act more gently, but with regular consumption throughout the day they can maintain a small, stable level of ketones, especially against the background of a low-carbohydrate diet.

In practice this means that MCTs and ketones are not interchangeable. A person who wants precisely a high level of ketones "here and now" - for example, for an endurance experiment - will not get it from MCTs. And someone looking for a convenient energy source on a keto diet is unlikely to benefit from expensive esters.

Effect on training: what the research shows

The study that has drawn the most attention in sport is that of Cox and colleagues (2016) in the journal Cell Metabolism. High-level cyclists who drank a beverage with a ketone ester together with carbohydrates covered, on average, a slightly greater distance in a 30-minute test after an hour of exercise than when taking carbohydrates alone. The authors attributed this to a shift in fuel selection in the muscles and to glycogen sparing.

However, subsequent work produced contradictory results. In the study by Leckey and colleagues (2017) with professional cyclists, a ketone diester worsened performance in a time trial and often caused digestive upset. The review by Evans, Cogan and Egan (2017) summarizes: the effect of exogenous ketones on endurance is inconsistent and probably depends on dose, form, combination with carbohydrates and individual tolerability.

For MCTs the evidence base is even more modest. The review by Jeukendrup and Aldred (2004) concluded that the amount of MCT a person can consume during exercise without stomach problems is too small to significantly affect energy supply. Large doses during training often cause cramps and diarrhea, which negates any potential benefit.

A separate area is the effect of ketones on recovery and protein synthesis. There are individual studies in which taking an ester after exercise enhanced mTORC1 signaling, but this did not translate into a proven gain in muscle mass. The editorial team believes that for strength sports neither of these products has convincing arguments today.

• Endurance:for ketone esters - mixed data; for MCTs - practically no confirmed effect.
• Strength and hypertrophy:there is no evidence of benefit for either product.
• Keto diet:MCTs are convenient as a source of fat, ketones as a short-term "bridge" during the adaptation phase.

Tolerability, safety and product composition

Both groups of products most often let down digestion. MCT oil on an empty stomach or in a large single serving causes nausea, bloating and diarrhea - which is exactly why manufacturers advise starting with small portions and increasing gradually. Ketone salts and esters also produce gastrointestinal symptoms, especially during intense exercise.

Ketone salts have a specific problem: along with β-HB a person receives a considerable amount of minerals. Several servings a day can impose a noticeable sodium load, which is undesirable in arterial hypertension or kidney disease. Before buying, it is worth checking the label for how much sodium, potassium, calcium and magnesium is in one serving.

Ketone esters have a sharp bitter taste that is hard to mask, and they cost far more than the other options. In the study by Clarke and colleagues (2012) the ester was generally well tolerated by healthy adults, although gastrointestinal complaints were observed at high doses. There is still little long-term data on daily intake over years.

MCT oil is high in calories - roughly like any fat. A tablespoon or two of oil a day imperceptibly adds significant energy value, which is worth keeping in mind for those watching their weight. People with diabetes, especially type 1, or with liver or pancreatic disease should discuss both products with a doctor, as they affect the metabolism of ketones and fats.

Editorial conclusions

Exogenous ketones and MCT oil are similar only on the surface. Ketones are ready-made fuel that quickly and noticeably raises blood β-HB levels. MCT is a fat from which the liver forms ketones only partially and only under favorable conditions, primarily on a low-carbohydrate diet.

Ketone esters produce the highest ketosis, but they are expensive, taste unpleasant and have contradictory data on athletic performance. Ketone salts are a compromise option with a moderate effect and a mineral load. MCT oil is the most affordable product, useful primarily as a convenient fat in the diet on a keto diet.

None of these products is a "fat burner" and none replaces sensible nutrition and training. If the goal is simply to try ketosis, it is more logical to start with nutrition and to regard supplements as an auxiliary tool.

If you already understand the difference and want to decide what to choose for yourself, we recommend reading our article "Exogenous ketones vs MCT oil: what to choose and for whom," as well as articles on the ketogenic diet for athletes and on the role of carbohydrates in endurance sports.

References

1. Cox PJ, Kirk T, Ashmore T, et al. Nutritional ketosis alters fuel preference and thereby endurance performance in athletes. Cell Metab. 2016;24(2):256–268.
2. Clarke K, Tchabanenko K, Pawlosky R, et al. Kinetics, safety and tolerability of (R)-3-hydroxybutyl (R)-3-hydroxybutyrate in healthy adult subjects. Regul Toxicol Pharmacol. 2012;63(3):401–408.
3. Stubbs BJ, Cox PJ, Evans RD, et al. On the metabolism of exogenous ketones in humans. Front Physiol. 2017;8:848.
4. Evans M, Cogan KE, Egan B. Metabolism of ketone bodies during exercise and training: physiological basis for exogenous supplementation. J Physiol. 2017;595(9):2857–2871.
5. Leckey JJ, Ross ML, Quod M, Hawley JA, Burke LM. Ketone diester ingestion impairs time-trial performance in professional cyclists. Front Physiol. 2017;8:806.
6. Bach AC, Babayan VK. Medium-chain triglycerides: an update. Am J Clin Nutr. 1982;36(5):950–962.
7. Vandenberghe C, St-Pierre V, Pierotti T, et al. Tricaprylin alone increases plasma ketone response more than coconut oil or other medium-chain triglycerides: an acute crossover study in healthy adults. Curr Dev Nutr. 2017;1(4):e000257.
8. Jeukendrup AE, Aldred S. Fat supplementation, health, and endurance performance. Nutrition. 2004;20(7–8):678–688.