Nutritional ketosis achieved through various methods in animals and humans has been shown to augment cardiac efficiency and function. However, this response during exercise has not been well characterized. Recreationally active adults (n = 12) completed a double blind, balanced, placebo-controlled, crossover study to examine the effects of bis-octanonyl (R)-1,3-butanediol (BO-BD) ingestion on cardiopulmonary function at rest and during a maximal oxygen consumption (V̇O) treadmill test (Bruce Protocol). Participants presented to the testing facility fasted. Capillary blood samples were obtained to measure glucose and beta-hydroxybutyrate (R-βHB) prior to consuming the BO-BD or a calorically matched placebo (PL) beverage. Metabolic and cardiovascular measures were collected every 15-30 min following beverage consumption. Participants began the V̇O test 120 min post-beverage ingestion. At rest, capillary R-βHB elevated rapidly after BO-BD ingestion and continued to steadily increase to 2.4 mM prior to the maximal exercise test. During the 120 min rest period, BO-BD increased resting heart rate (HR) (p = 0.001), ventilation (p < 0.001), and V̇O (p = 0.002) relative to PL. Although the total time to exhaustion was similar between conditions, V̇O was lower after BO-BD (p < 0.001). There were no differences in exercise lactate, RER, respiration, or rating of perceived exertion (RPE) between conditions. Compared to PL, BO-BD rapidly achieves nutritional ketosis, increases resting cardio-respiratory parameters, but somewhat paradoxically decreases peak aerobic exercise oxygen consumption despite achieving similar peak workloads.