The benefits of exogenous carbohydrate availability for optimal endurance exercise performance are well-established. Research has identified maximal exogenous glucose oxidation rates of ~1 g/min1, which are thought to be limited by intestinal absorption. However exogenous glucose oxidation rates may be increased if the intrinsic activity of the intestinal glucose transport system (SGLT1 and GLUT2) can be upregulated. As SGLT1 is saturated under conditions of high glucose availability2, GLUT2 translocation is particularly important when large amounts of carbohydrate are consumed. The presence of extracellular calcium in rodent intestine has been found to increase GLUT2 translocation to the apical membrane3. Whether this translates to increased intestinal glucose absorption and oxidation during endurance exercise in humans, is unknown. Therefore, the aim of this study was to investigate the effect of calcium co-ingestion during endurance exercise on exogenous glucose oxidation in healthy men. The relative utilisation of exogenous glucose was hypothesised to be higher with calcium-glucose co-ingestion compared to glucose ingestion alone. Eight healthy male volunteers completed a 120-minute cycling bout (50% peak power output), ingesting either 1.2 g/min dextrose (CON) or an identical solution containing 2000 mg calcium (CAL), distributed across eight 100 ml boluses consumed every 15 minutes. After a minimum 5-day washout period, participants performed the alternate trial in a randomised counterbalanced order. Single-breath expired air samples were collected every 15 minutes and analysed using isotope ratio mass spectrometry to estimate exogenous glucose oxidation (analysis ongoing). Whole-body substrate oxidation was estimated using indirect calorimetry for quantification of total carbohydrate and fat utilisation. Ratings of perceived exertion (RPE) and gut discomfort, blood glucose and lactate concentrations, and heart rate, were also recorded every 15 minutes. Values are means ± S.E.M, compared by paired t-test. No significant differences were observed in total carbohydrate (CON vs. CAL; 265.9 ± 14.1 vs. 274.1 ± 16.1 g) or fat (51.8 ± 6.7 vs. 49.2 ± 5.3 g) oxidation between trials (Figure 1). There were also no significant differences between trials in heart rate, RPE, gut discomfort, blood glucose or blood lactate concentrations (p = .11 – .84). The results thus far suggest no effect of calcium on the characteristics of fuel use during endurance exercise in men. However, mass spectrometry analyses may yet show an increased contribution of exogenous glucose to total carbohydrate oxidation with calcium, suggesting a maintenance of endogenous stores. If demonstrated to have a functional effect, there may be a role for calcium in contemporary nutritional guidelines for endurance exercise performance.