Exogenous ketone esters are a hot topic in cycling, but the best way to use this expensive substance to get faster is still up for debate. In our previous discussion with Dr. Chiel Poffe, we learned that ingesting ketone esters with bicarbonate may enhance racing performance under certain circumstances. In the second part of our conversation with Dr. Poffe, we learn about his research into using ketones as a recovery aid.

For more information on ketone esters and bicarbonate, check out Science of Getting Faster Ep 4.


What are Ketone Esters?

Ketones are molecules naturally produced by your liver in times of low carbohydrate availability. Since ketones can be used by the body as an energy source, there may be potential for supplemental ketones to improve performance in endurance sports, and this is where ketone esters come in. Ingesting this expensive and foul-tasting substance can rapidly raise your blood ketone levels, and as we discussed in our last conversation with Dr. Poffe this may be beneficial to an athlete’s performance in some circumstances.

Read more about ketone esters and Dr. Poffe’s other study here

Ketones and Recovery

One of the most important ingredients in an athlete’s ability to perform is the capacity to recover, and poor recovery can be a limiting factor in training if it restricts volume or intensity. In racing, recovery is especially significant in multi-day events such as stage races. The racer whose performance declines the least during a long event is often the one who comes out on top, so any improvement in recovery could be decisive.

Most of the research on ketone esters has studied their acute impact on exercise. However, preliminary data suggested ketone supplementation might positively impact muscle regeneration and glycogen resynthesis, two key components in recovery. Working with Dr. Peter Hespel at KU Leuven, Belgium, Chiel Poffe and his team decided to take a closer look at this potential use for ketone esters.

Preventing and Measuring Overtraining

The researchers wanted to design a study in which participants were intentionally trained beyond their ability to recover to see if athletes receiving ketone esters experienced fewer negative effects. However, this is harder than it sounds, since there aren’t many objective and universally applicable ways to measure overtraining. There are a few common markers, and the team knew these would be important to measure.

One obvious parameter is a decline in performance. As you overreach and under-recover, your abilities measurably decrease. Likewise, maximal heart rate tends to decrease in conjunction with heavy training and poor recovery, and maximal blood lactate values also decline. None of these markers on its own offers a specific measurement that reliably signifies overreaching, but when taken in combination they can paint a clear picture of fatigue.

Additionally, a newly-discovered indicator that may be significant is the hormone GDF15, a protein found at increasing concentrations in the blood during periods of heavy exercise. While more research into it is needed, the team planned to monitor GDF15 through the course of their study, along with the other common metrics associated with overtraining.

The Study

The study put subjects through an increasingly-difficult 3-week period of high training intensity and volume, to induce a state of non-functional overreaching and limited recovery. Daily workouts varied but generally included two high-intensity sessions per day with one full rest day per week. Before the study began as well as at the end of each week, subjects performed time trial and sprint tests to evaluate their performance. They were again tested 3 and 7 days after the study ended, and standardized carbohydrate-rich meals were provided before each test.

The researchers knew serious cyclists wouldn’t want to participate in a study that intentionally caused overtraining, so they worked with 18 healthy male subjects who weren’t well-trained riders. The sheer physical toll of the study also meant that a crossover design (in which every athlete participated twice and received both placebo and ketones) was not possible. Instead, the researchers used matched pairs, grouping each subject with someone who shared a similar initial time trial performance, VO2 max, and body weight. 

All of the subjects received a 500 ml high-dose protein-carbohydrate drink immediately after each workout, but one subject from each pair also received 25g ketone esters in this drink. This subject also received a drink containing 25g ketones before bed each night, while the other subject received a placebo.  By comparing the changing performance of the subjects in each matched pair, researchers hoped to demonstrate whether or not the ketones would improve performance compared to placebo.

The results

By the end of the brutal 3-week study, almost none of the test subjects could successfully complete all the prescribed workouts. But during the final week, the ketone-supplemented athletes were able to sustain a 15% higher overall training load (measured in kilojoules of work performed). During this third week, the ketone subjects were also were able to maintain a 15% greater power output during the second half of a 120-minute endurance test. Sprinting performance was similar between ketone and placebo athletes throughout the training block.

Other metrics were also favorable for the ketone-supplemented riders. All the subjects experienced a gradual decline in resting and exercise heart rate, but the ketone subjects’ heart rates declined only about half as much as did the placebo subjects’. Training-induced nocturnal adrenaline and noradrenaline levels were lower in the ketone group, potentially facilitating better sleep. Ketone subjects also showed a significantly smaller increase in the levels of the overtraining-related protein GDF15 in their blood.

Finally, the subjects who received ketone supplementation were observed to increase their overall food intake in proportion to their training, voluntarily ingesting more carbohydrates over the course of the study. The placebo athletes, on the other hand, did not voluntarily increase their food intake as training got more intense despite the potential benefit of doing so.

Researchers’ Conclusions

Researchers concluded that Ketone Esters ingested after exercise and before bed improved test subjects’ ability to recover during heavy training. 

“KE markedly inhibited the appearance of (overtraining) symptoms, whilst enhancing tolerable training load, increasing energy intake, and stimulating endurance exercise performance,” they concluded. They also noted new evidence for a correlation between increasing GDF15 levels and overtraining.

One possible interpretation is that the Ketone Esters primarily functioned as an appetite stimulant, and the resulting increase in food intake was the actual source of improved recovery in the ketone-supplemented riders. However, acute ketone ingestion tends to suppress appetite under most circumstances. Additionally, numerous other physiological markers of overtraining (such as GDF15) were detected in placebo athletes too early in the study for a caloric deficit to be the cause, suggesting more was at play than a mere difference in nutrition. 

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Takeaways For Athletes

Is post-workout ketone ester supplementation a way to improve recovery? It would seem it can play a role, but it is important to remember this study’s conditions were not representative of most athletes’ normal training loads. Additionally, since the study only used male subjects who were not serious cyclists, it isn’t clear whether its conclusions can be generalized for all athletes or for more elite riders. 

For those of us training 3 or 4 days a week, good rest and nutrition are probably still our best option. But for athletes engaged in high-volume training camps or long stage races, the expense of ketone esters may be at least worth considering, especially if more supporting data becomes available.

About the Researcher

Chiel Poffe is a post-doctoral researcher in the Exercise Physiology Research Group at KU Leuven in Flanders, Belgium. This study was completed as part of his Ph.D. research at KU Leuven under Dr. Peter Hespel.



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Sean Hurley

Sean Hurley is a bike racer, baker of sourdough bread, and former art professor. He is a connoisseur of cycling socks, and a deep believer in the power of periodized, science-based training. Rumor has it he also runs a famous cycling instagram account, but don't tell anyone about that.