Bis-octanoyl-(R)-1,3-butanediol (BO-BD) is a novel ketone ester (KE) ingredient which increases blood beta-hydroxybutyrate (BHB) concentration rapidly after ingestion. KE is hypothesized to improve function in older adults. Whilst many studies have investigated KE in young adults, they have not been studied in healthy older adults (HOA), for whom age-related differences in metabolism may alter the effects. This randomized, observational, open-label study in HOA (n = 30, 50% male, age = 76.5y) aimed to elucidate tolerance, blood BHB and glucose concentrations for 4h following consumption of either 12.5 or 25 g of BO-BD formulated in ready-to-drink beverage (n = 30), and re-constituted powder (n = 21) with a meal. All study interventions were well tolerated, and increased blood BHB, inducing nutritional ketosis (≥0.5 mM) until the end of the study. Peak BHB concentration (C