OBJECTIVES: Ingested ketogenic agents offer the potential to enhance endurance performance via the provision of an alternative exogenous, metabolically efficient, glycogen-sparing fuel (i.e. ketone bodies). This study aimed to assess the impact of combined carbohydrate and 1,3-butanediol (CHO-BD) supplementation on endurance performance, blood beta-hydroxybutyrate (βHB) concentration and glycolytic activity, in comparison to carbohydrate supplementation alone (CHO). DESIGN: Eleven male runners (age 38±12years, mass 67.3±6.5kg, height 174.5±5.0cm, [Formula: see text] 64.2±5.0ml⋅kg⋅min) performed two experimental trials in a randomised crossover design. METHODS: Each trial consisted of 60min of submaximal running, followed by a 5km running time-trial (TT), and was performed following the ingestion of an energy matched ∼650ml drink (CHO-BD or CHO). RESULTS: There was no difference in TT completion time between the trials (CHO: 1265±93, CHO-BD: 1261±96s; p=0.723). However, blood βHB concentration in the CHO-BD trial was at least double that of the CHO trial at all time points following supplementation (p<0.05). While blood lactate concentration was lower in the CHO-BD versus CHO trial after 30min submaximal exercise (CHO-BD: 1.46±0.67mmol⋅L, CHO: 1.77±0.46mmol⋅L, p=0.040), it was similar at other time points. Blood glucose concentrations were higher post-TT in the CHO-BD trial (CHO-BD: 5.83±1.02mmol⋅L, CHO: 5.26±0.95mmol⋅L, p=0.015). CONCLUSIONS: An energy matched CHO-BD supplementation drink raised βHB concentration and acutely lowered blood lactate concentration, without enhancing 5km TT running performance.