All participants had a VO₂max of 50 mL/kg/min or higher and completed either a half-Ironman triathlon in under 6 hours or a full-distance Ironman triathlon in under 13 hours.

The participants ate two isocaloric diets for 6 weeks: a ketogenic diet (<50 grams of carbohydrate, 60%–80% of calories from fat, 20%–25% from protein) and a high-carbohydrate diet (60%–65% of calories from carbohydrate, <30% from fat, 15%–20% from protein).

At the end of each dietary intervention, the participants performed 2 time to exhaustion tests on a cycle ergometer (pedaling at 70% of VO₂max) after a 15-hour overnight fast. The participants ingested a small amount of carbohydrate immediately before and during the test (approximately 10 grams of carbohydrate per hour) or a noncaloric placebo. There was a 1-week washout period between the carbohydrate supplementation interventions.

During both diets, ingesting carbohydrates before and during exercise improved performance by approximately 22%, on average.

Exercise performance according to diet group and carbohydrate supplementation

	Ketogenic diet + pre/periworkout carbs	Ketogenic diet + placebo	High carbohydrate diet + pre/periworkout carbs	High carbohydrate diet + placebo
Time to exhaustion* (trial 1)	108.9 minutes	83.8 minutes	100.6 minutes	87.8 minutes
Time to exhaustion* (trial 2; 20 minutes after trial 1)	20.2 minutes	18.4 minutes	14.5 minutes	19 minutes

*While cycling at 70% of VO₂max after a 15-hour overnight fast.

Two of the researchers were authors of low-carbohydrate nutrition books. One holds patents related to exogenous ketones and is the owner of Ketone Technologies LLC, and one holds patents related to ketone bodies.

The effects of a ketogenic diet on endurance exercise performance Original paper