The ketone bodies beta-hydroxybutyrate and acetoacetate are produced in the body as a byproduct of fat metabolism. Essentially ketone bodies are formed in the body when there is an excess of fatty acids available. Ketones have received a lot of attention in the media recently. It has been reported that ketone body supplements are routinely used by professional cyclists. Have we found a new magic substrate (fuel) for the muscle or is it all hype?

Ketone bodies as a fuel

The existence of ketone bodies is known for a long time and their role has been described more than 50 years ago with extensive studies. Ketone bodies can serve as an important energy substrate in certain conditions, and can change the use of your body's carbohydrate and fat stores. Dietary strategies to increase blood concentrations of ketone bodies require a diet high in fat and extremely low in carbohydrate (ketogenic diets). However, reducing carbohydrate availability may impair performance as carbohydrate forms a key fuel source for intense exercise.

Ketone bodies are a starvation fuel

Ketones are mostly known as a substrate for the brain during prolonged periods of starvation. The brain normally function on glucose as the only fuel. During starvation glucose is not available and therefore the brain develops the ability to use an alternative fuel: ketones. This is generally seen as the main function of ketone bodies and since starvation is very uncommon in the Western world, it has received less attention. However, already in the 1960s it was observed that ketones are readily used by the muscle.

Delivery of carbohydrate during exercise is limited

Ingesting carbohydrate during prolonged exercise has been shown to improve exercise performance and there are indications that more exogenous (ingested) fuel can be beneficial.

However, it has been demonstrated that carbohydrate from drinks, gels or solid foods can only be delivered at a rate of 1 g/min to the muscle. This is the equivalent of 240 kcal/h, not a large amount considering that cyclists may expend as much as 1000 kcal/h for several hours. Are there fuels that can be ingested in addition to carbohydrate that will help deliver energy to the muscle?

Are ketones the solution?

Ketones, at least in theory, could form an alternative or additional fuel. Ketone salts have been used but this requires the intake of large amounts of sodium or potassium which causes all sorts of problems (stomach problems, absorption problems). At the University of Oxford in the UK under the leadership of expert Professor Kieran Clarke, a ketone ester was developed that seemed to prevent some of these issues.

So in theory we can give a fuel that adds additional energy, is rapidly used by the muscle and this methods does not require athletes to go on a ketogenic diet. Athletes could eat carbohydrate and still add ketones, a win-win situation.

The downsides

There are a number of downsides, the recently developed ketone esters are extremely expensive (reportedly £2000 or $3000 per liter). Taste is another issue. Therefore, in reality low doses of the ketone ester are used to prevent bad (bitter) taste and high cost.

With small amounts of fuel added the question is whether this is worth doing as adding a few grams or a few calories per hour is unlikely to make any difference at all.

MCT and ketone bodies

This idea is also not that new. In 1992, I started my PhD with the goals to find an additional fuel to glucose. Ketones were on the list, but the delivery mechanism was slightly different. We used medium chain triglycerides which are rapidly absorbed and converted to ketone bodies. MCT are readily used by the muscle (a significant portion converted into ketones first). However, the main limiting factor was how much MCT we could give to athletes. When more than 30 grams of MCT was ingested athletes ran very fast….. to the toilet. It caused significant gastro-intestinal distress.

When small amounts 30 grams or less was provided, no effects on metabolism or performance were observed. I suspect that it is exactly the same with ketone esters. Unless you can give significant amounts the effects will be negligible. Ketone supplements are usually contain 10 grams of ketone bodies per serving, with a maximum of 3 capsules (3x 10 grams) per day.

Although some studies have started to study the effects of supplementation of ketone bodies during exercise, there is no evidence at the present time that ketone bodies have significant effects on performance.

Summary

• Ketone bodies are a good fuel for the muscle

• Ketone bodies are now sold as supplements and products often contain pretty wild claims

• Most supplements contain very small amounts, larger amounts are less well tolerated

• Ketone esters have been developed that can be delivered in larger amounts with less problems but these are extremely expensive

• Therefore either tolerance, taste or costs will limit ketone body intake and supplements

• There is not enough evidence to recommend ketone body supplementation to athletes