Ketones. A new superfuel?

Previous Cyclist or Scientist articles have addressed the three main macronutrients involved in exercise metabolism: carbohydrates, fats and proteins. But there is a fourth fuel source, ketones, which have become very fashionable in the cycling media and generated some heated comment sections on articles citing their use in the Tour de France peloton.

Acetone. Image Credit: Pixabay

What are Ketones?

Ketone bodies are produced naturally in the liver in conditions of reduced carbohydrate availability and serve as an alternative fuel source for the brain, heart and skeletal muscle. They are primarily generated from free fatty acids in adipose tissue. The smallest and simplest of these ketones is one you may have already heard of and used before, acetone. So how could a key solvent in nail polish remover be used as a performance enhancer for endurance athletes?

There have been many studies investigating the effects of ketone metabolism on exercise performance through prolonged fasting and ketogenic diets. Any benefits of the natural state of ketosis haven’t been all that useful in professional cycling given racing on rashers of bacon or back to back days without eating aren’t very performance enhancing… Recently however, exogenous ketone esters (ones that you can eat) have been investigated in the laboratory and have made their way into the back pockets of riders in the pro peloton.

Jumbo Visma have openly used ketone esters during the 2019 Tour de France. Image Credit:

How are they consumed?

Rather than rely on the body to create its own ketone bodies, two different forms of ketones can be ingested to allow almost immediate intake into the muscle for use as fuel. Ketone salts were investigated first, though any dose of these large enough to provide useful fuel for exercise had athletes going to the toilet faster than they were getting to the finish line.

Ketone esters are far more tolerable for the gut in substantial doses and provide an acute method of elevating blood levels of beta-hydroxybutyrate (βHB), the ketone body used as fuel within the muscles. Researchers have found the ideal dosage to maximise the benefits, while minimising trips to the bathroom, is around 600mg per kg body mass. 40g for a 67kg rider is relatively easy to mix into a sports drink although all reports are that the taste is less than favourable… This dosage, just 10 minutes after ingestion, elevates blood βHB concentration to the equivalent of a 72 hour fast, and after 30 min, increases concentration to that achieved on a ketogenic diet.  Ketosis and carbs, the best of both worlds.

This βHB is then taken into muscle fibres via the MCT1 receptor. This is significant because this is expressed predominantly in type 1 (slow twitch) muscle fibres. It’s therefore likely ketones are used predominantly during endurance exercise or sections of a race where the intensity is lower.

Image credit:

How do they work?

Ketones are suggested to provide a thermodynamic advantage over fats and carbohydrates as a fuel source because the available free energy to perform work is greater. Less oxygen is required per mole of carbon to oxidise. Basically, you get more for less, in theory…

This theoretic benefit has been supported by recent studies on performance when using exogenous ketone supplementation. Cycling performance in a 30 min time trial was increased by 2% when subjects were given ketones and carbohydrates rather than just an optimal level of carbohydrate alone. Another benefit for road cyclists is the carbohydrate-sparing effect of using ketones as a fuel source. Ketones have been shown to provide up to 18% of aerobic energy production during endurance exercise. This would allow those carbs to be saved for later in a race when the high intensity is necessary.

Benefits are suggested to be heightened in trained individuals who have a greater ability to take up and oxidise ketones within the muscle due to the countless hours of aerobic training undertaken. So, if you’re lucky to squeeze in one or two rides a week you might not see the same benefit from chugging down a bottle of ketones as the pros do.

Ketones could provide more for less in comparison to other fuel sources. Image credit: Pixabay


What else can ketones do?

There’s also some promising research on the use of ketones in aiding recovery. Ingestion of ketones after exercise again could allow the body to save up on carbohydrates as well as protein during this period. This would mean faster replenishing of muscle glycogen stores (up to a 50% increase) before the next race or training session as well as increased protein synthesis and decreased muscle breakdown. Athletes could then experience the benefits of fasted training sessions without the associated increased muscle breakdown. There are limited studies on this, but the work done so far is another box ticked for exogenous ketones.

Of course, you couldn’t have an article about ketones or ketosis without mentioning weight loss. Studies have also suggested that the suppression of appetite after the ingestion of ketone esters could also be helpful in decreasing body fat.

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What to watch out for

A lot of the above research may read as an advertisement encouraging you to go out and fill your pantry with ketone esters (an expensive prospect at up to AU$50 per serving…). It’s important, however, to note that many of the experiments are on small sample sizes (10 or less) and few repetition studies have been conducted to date. For example, it is yet to be determined whether the carbohydrate-sparing effect of ketones isn’t simply due to decreasing the rate of aerobic glycolysis. If this were the case, it would mean a higher reliance on anaerobic energy production, quite a painful phenomenon if required too often in a bike race…

So far, only the short-term effects of ketone ester ingestion have been tested; we’re yet to see what long-term physiological changes could occur with chronic ketone ingestion.

If you are keen to give them a go, its important to be aware of your dosage. In this case, more is not merrier… 20% of participants in one study reported flatulence, nausea, diarrhoea and dizziness when taking just the recommended dose (~600mg/kg body mass) – this increased to almost 100% of participants when the dosage was doubled. Definitely give them a trial run in training before smashing them down in a race…

Could the infamous Dumoulin dump during the 2017 Giro be due to an early trial of ketone supplementation??? Image credit:


The take homes

  • Ketones are a fourth fuel source for exercise along with carbohydrates, fats and proteins.
  • They are produced in the body during fasting or a ketogenic diet but can be ingested easily in the form of a ketone ester.
  • Research suggests they may be beneficial in endurance exercise through efficient energy production (low oxygen cost) and a carbohydrate sparing effect.
  • They could aid recovery through increasing glycogen replenishment and protein synthesis.
  • Overdoses can make for a very unhappy gut.
  • The studies published so far are very supportive of their ability to provide a performance benefit to athletes but there’s still further research to be done.


This was a heavily requested topic and good reason to get back into the research after a lengthy lay-off due to a stolen laptop and heavy racing period. As with most expensive supplements, I secretly hoped the hype around ketones was being overdone and that the research would suggest they’re not worth the coin. Unfortunately it looks like those able to fork out the $ for this product may be getting a genuine advantage on the competition. I’m currently 7 stages into the Tour of Qinghai Lake in the middle of China where I don’t see myself finding a supply of ketone esters anytime soon. Hopefully we can pull off some big results in this second week and maybe the prize purse might allow me to give the ketones a go for myself later in the season… Until then, I’ll keep reading the research papers so you can spend more time on the bike.




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