Ketones are produced by the body during periods of fasting or very low carbohydrate intake as an alternative energy source for the brain and other tissues, including skeletal muscle. As an alternative energy source to glucose and fatty acids, ketones can be helpful in decreasing protein breakdown that otherwise occurs to produce glucose from amino acids during times of starvation or inadequate carbohydrate intake. Because following a ketogenic or very low carbohydrate diet is not advantageous for improving cycling performance, exogenous ketones have become more popular as a supplement to elevate ketone body concentration in the bloodstream without having to sacrifice carbohydrate intake. Ketone esters are typically consumed in beverage form and, at adequate doses, can temporarily elevate the concentration of circulating ketones in the body.
With all the media hype, it's not surprising that there are so many questions about the use of exogenous ketone supplements and whether they can benefit us. With regard to the ketone ester supplements specifically, we can categorize usage into two main areas:
Pre or during training/racing for some type of performance benefit
Post exercise for recovery benefit
Ketone Esters for Performance
To maximize performance, your body should ideally be metabolically flexible enough to maximize its use of all energy sources to get us to the finish line as quickly as possible. Simply put, we want to be able to maximize fat oxidation at submaximal exercise intensities, but not by compromising the ability to utilize carbohydrates, including both muscle glycogen at higher intensities and exogenous carbohydrates taken in before and during exercise. Recent studies have examined how ingesting ketone esters during exercise might affect the metabolic response during exercise and performance capability. While the relative contribution of ketones as an alternative energy source would be much smaller compared to carbohydrate and fat, it has been suggested that perhaps ketones could spare muscle glycogen and also reduce reliance on blood glucose, which could be argued might be useful in very long and steady sub-threshold-intensity events. However, a recent study shows no difference in glucose oxidation between 110 g carbohydrate alone and 110 g carbohydrate + 573 mg ketone ester per kg body weight. 6.4 km TT performance and TTE were also reduced with the ketone + carbohydrate compared to carbohydrate alone.
As far as performance results, conclusions from studies are mixed. For example, ketone ingestion prior to a 20-minute TT resulted in 2% worse performance compared to a placebo. Another study compared ketone + carbohydrate ingestion compared to carbohydrate alone on glycogen utilization and a 15-minute TT and sprint following 3 hours of exercise. Interestingly, no differences were observed in glycogen breakdown or in TT or sprint performance. Studies have attributed possible negative outcomes of ketone ingestion before and during exercise to inhibition of lipolysis from adipose tissue, GI disturbances, and/or metabolic acidosis (increased blood pH).
Finally, carbohydrates require less oxygen to produce ATP, and carbs are important for supporting higher exercise intensities. Therefore, higher rates of carbohydrate intake and utilization are better for cycling economy (faster speed for lower oxygen cost) and performance. If you are looking for a nutrition supplement to improve cycling performance, evidence continues to suggest prioritizing carbohydrate availability and intake.
Ketone Esters for Recovery
Though the evidence supporting the benefits of ketones on performance has been lacking, other research has focused on possible benefits of ketone ingestion for post-exercise recovery, particularly glycogen resynthesis and muscle protein synthesis. One study examining the effects of ketones + carbohydrate vs carbohydrate alone following previous glycogen depleting exercise on subsequent TT performance showed no difference in performance. However, metabolic differences were observed. Ketone+carbohydrate resulted in lower blood glucose in the first hour after ingestion (possibly due to lower glucose output from the liver) but higher insulin two hours after ingestion, compared to carbohydrate alone. The higher insulin response could, in theory, improve glucose uptake into the muscle leading to more glycogen resynthesis, but glycogen was not measured in this study. Insulin also plays a role in protein synthesis, and ketones can also reduce protein breakdown. When taken with adequate carbohydrates and leucine-containing protein following exercise, it's possible that ketones could offer some additional benefit to recovery. Others have also suggested that post-exercise ketone ingestion could help with decreasing feelings of fatigue, improved mental strength and well being, and improved cognitive function.
Based on these findings, some researchers have suggested that taking ketones during periods of high training load might help blunt the negative effects of overreach and possibly help with recovery and adaptations during this time. One study demonstrated that 25 g ketone ester taken immediately after training with a 60 g CHO + 30 g PRO recovery drink plus 25 g ketone ester before sleep blunted symptoms of overreaching and appeared to improve tolerance to training load, increase energy intake, and improve training quality compared to the CHO+PRO recovery drink alone. Post-exercise and pre-sleep ketone ingestion might help, in part, due to the purported benefits on sleep efficiency, REM sleep, and wakefulness after sleep onset.
It will be interesting to see more research on dosing protocols and effects in different training populations. For the time being, if you want to experiment with ketone esters, I recommend sticking to taking them post-exercise and pre-sleep, but don't forget to continue to include your recovery carbohydrates and protein. Keep in mind that the relative lack of research to date also means that we don't yet know possible negative downstream effects of taking exogenous ketones; therefore, always use some caution when deciding whether and/or how to take supplements such as these.
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Super interesting, Namrita! Definitely looks like the hype machine has been hard at work. After reading your blog it seems like Keytone supplements are not a great idea for during higher intensity events/training, but I'd be curious about the efficacy during much longer lower-intensity ultra-distance events where we are pulling much more of our energy from a fat stores. Clearly there is a huge difference between a 15 min TT and a 200 mile cycling event. Thoughts here?