AbstractElevating blood ketones may enhance exercise capacity and modulate adaptations to exercise training; however, these effects may depend on whether hyperketonemia is induced endogenously through dietary carbohydrate restriction, or exogenously through ketone supplementation. To determine this, we compared the effects of endogenously‐ and exogenously‐induced hyperketonemia on exercise capacity and adaptation. Trained endurance athletes undertook 6 days of laboratory based cycling (“race”) whilst following either: a carbohydrate‐rich control diet (n = 7; CHO); a carbohydrate‐rich diet + ketone drink four‐times daily (n = 7; Ex Ket); or a ketogenic diet (n = 7; End Ket). Exercise capacity was measured daily, and adaptations in exercise metabolism, exercise physiology and postprandial insulin sensitivity (via an oral glucose tolerance test) were measured before and after dietary interventions. Urinary β‐hydroxybutyrate increased by ⁓150‐fold and ⁓650‐fold versus CHO with Ex Ket and End Ket, respectively. Exercise capacity was increased versus pre‐intervention by ~5% on race day 1 with CHO (p < 0.05), by 6%–8% on days 1, 4, and 6 (all p < 0.05) with Ex Ket and decreased by 48%–57% on all race days (all p > 0.05) with End Ket. There was an ⁓3‐fold increase in fat oxidation from pre‐ to post‐intervention (p < 0.05) with End Ket and increased perceived exercise exertion (p < 0.05). No changes in exercise substrate metabolism occurred with Ex Ket, but participants had blunted postprandial insulin sensitivity (p < 0.05). Dietary carbohydrate restriction and ketone supplementation both induce hyperketonemia; however, these are distinct physiological conditions with contrasting effects on exercise capacity and adaptation to exercise training.