Endurance Trial: Ketone and Carbohydrate Supplementation

Participants ingest 210 mL of a \~15% carbohydrate solution, providing 30 g of carbohydrate per dose, every 20 minutes from the start of the 155-minute fixed-intensity phase, with the final dose taken immediately before the start of the 20-minute maximal effort block. No beverage is provided during the maximal effort. The solution combines sucrose and dextrose so that glucose and fructose are absorbed through separate intestinal transporters. All doses are supplied in opaque, coded bottles prepared by a researcher not involved in the assessments, so that treatments cannot be identified visually.

Participants ingest (R)-1,3-butanediol at 1.0 g/kg body mass 45 minutes before the start of the cycling protocol and a further 0.5 g/kg 5 minutes before the start, for a total dose of 1.5 g/kg body mass. The dose was selected to sustain blood beta-hydroxybutyrate at approximately 2 mmol/L throughout the session, a range compatible with nutritional ketosis and without risk to participants. Doses are supplied in opaque, coded bottles prepared by a researcher not involved in the assessments.

Participants ingest a non-caloric placebo beverage at 45 and 5 minutes before the start of the cycling protocol, at the same time points at which the (R)-1,3-butanediol is administered in the CHO-KET arm. The placebo is matched to the ketone beverage for volume, appearance and flavour, and provides no energy. Doses are supplied in opaque, coded bottles prepared by a researcher not involved in the assessments.

Participants ingest 210 mL of a non-caloric placebo beverage every 20 minutes from the start of the 155-minute fixed-intensity phase, with the final dose taken immediately before the start of the 20-minute maximal effort block. No beverage is provided during the maximal effort. The placebo is water with the same flavouring used in the carbohydrate beverage, matched for volume, appearance and ingestion schedule, and provides no energy. Doses are supplied in opaque, coded bottles prepared by a researcher not involved in the assessments.

On the evening before each experimental session, participants consume a standardized meal providing 1.5 g of carbohydrate per kg of body mass. The same meal is provided before all three experimental sessions and is followed by an overnight fast of 8 to 12 hours.

Each experimental session comprises 5 minutes of warm-up at 100 W, 155 minutes at 55% of maximal power output (Wmax) with the cycle ergometer set in hyperbolic mode, and a final 20 minutes of maximal self-paced effort in linear mode, followed by 5 minutes of low-load cool-down. The same protocol is applied in all three conditions, with load individually prescribed from the maximal incremental test performed at the pre-experimental visit.

On the morning of each experimental session, after the baseline venous blood draw and following an overnight fast of 8 to 12 hours, participants consume a standardized breakfast providing 1.5 g of carbohydrate per kg of body mass, 40 g of protein and 7 g of fat. Participants are instructed to consume the entire meal. The same breakfast is provided before all three experimental sessions.