Introduction: Recently we developed a novel ketone ester (KE) to deliver nutritional sources of ketones without starvation, intravenous infusion or dietary manipulation [1]. Previous investigation of ketosis has been confounded by the metabolic sequalae required to produce ketones endogenously. Exogenous provision of ketones from the infusion of salts demonstrated rapid alterations in glucose and fatty acid metabolism [2] consistent with the evolutionary role of ketone bodies (KB) to conserve glucose stores. Furthermore ketones regulate their own production via negative feedback on lipolysis. It is unclear whether similar effects are pertinent to nutritional ketosis delivered by KE. The aim of this study was to characterize changes in blood metabolites: β-hydroxybutyrate (BHB), acetoacetate (AcAc), glucose, non-esterified fatty acids (NEFA), triacylglycerol (TAG) and insulin following a KE drink. Results: Following favorable ethical review, healthy, non-obese volunteers (n = 16) completed 2 identical study visits. Following an overnight fast subjects consumed 395 mg/kg BW KE mixed with flavoured water. Blood samples were obtained via an intravenous catheter at baseline (BL) and at regular intervals post-drink. Samples were analysed for metabolites using enzymatic methods, and ELISA. Values are mean ± SEM. A 2 factor ANOVA for repeated measures, corrected with post-hoc Dunnett’s tests were used to determine significance. BHB increased following KE, from 0.2 mM (±0.02 mM) at BL to 3.3 mM (±0.2 mM) 1h later. AcAc levels lagged behind BHB, increasing 2-fold from 0.6 mM (±0.1 mM) to 1.2 mM (±0.1 mM) at 1.5h. BHB:AcAc ratio peaked (3:1) at 1h. NEFA and TAG significantly fell following KE, reaching a nadir at 3h. (NEFA = 0.6 mM (± 0.1 mM) to 0.2 mM (± 0.03 mM) and TAG = 1 mM (± 0.1 mM) to 0.7 mM (± 0.1 mM)). Glucose decreased following KE from 5.6 mM (± 0.1 mM) at BL to 3.9 mM (± 0.1 mM) 1h post-KE. Following KE, Glucose, NEFA and TAG fell remained significantly lower than BL (p>0.05). Insulin was 4.54 mU/L at BL; rose significantly 0.5h post KE (12.5 mU/L) (p>0.05 vs BL) and rapidly returned to basal levels. Conclusions: KE consumption rapidly increased circulating BHB to levels equivalent to several days of total fasting [3] with normal glucose and insulin levels and without increasing NEFA. Nutritional ketosis therefore represents a novel metabolic state with which to re-examine the effects of ketone bodies on human health and disease.