Glycogen stores play an important role in fueling endurance exercise, but are prone to depletion in high-intensity or long-duration events. Some research has suggested that ketones may have a sparing effect on glycogen storage, which could theoretically enhance performance by saving more glycogen for the final stages of long races. This crossover study evaluated the effects of exogenous oral ketone esters on glycogen use and performance in 12 highly trained male cyclists/triathletes.
Subjects were given 65g of ketone esters [(R)-3-Hydroxybutyl (R)-3-hydroxybutyrate (ketone monoester); KE] or a placebo (CON) to be taken as 3 separate boluses (20-25g each) throughout testing. At each testing visit, subjects consumed their first two boluses 60min and 20min prior to the onset of cycling. They did a 3hr cycling session with variable intensity; the 3rd bolus was ingested 30min into the session, and plenty of carbohydrate was provided throughout (60g/hour). After the session, they took a 5min rest, then did a 15min time trial. After another 5min rest, they did a final sprint (to exhaustion) at 175% of their lactate threshold.
The researchers took a ton of blood, urine, and muscle biopsy measurements, but the most notable outcomes were muscle glycogen levels and cycling performance. Plenty of muscle glycogen breakdown occurred throughout testing, but the magnitude of glycogen breakdown was not influenced by ketone ester ingestion. Similarly, ketone esters failed to significantly impact time trial power output or sprint time to exhaustion. Perceived exertion was a little higher in the ketone condition around halfway through the 3hr cycling session, but not in the later stages of testing.
The bottom line: These results indicate that exogenous ketones do not spare glycogen stores or improve endurance performance. While these results can’t necessarily be generalized to resistance training, there’s currently insufficient evidence to suggest that the typical lifter could expect meaningful performance benefits from ketone supplementation.