Addition of fructose to a carbohydrate-rich breakfast improves cycling endurance capacity in trained cyclists

It was previously demonstrated that post-exercise ingestion of fructose-glucose mixtures can lead to superior liver and equal muscle glycogen synthesis as compared to glucose-based carbohydrates only. After an overnight fast, liver glycogen stores are reduced, and based on this we hypothesised that addition of fructose to a glucose-based breakfast would lead to improved subsequent endurance exercise capacity. In this double-blind cross-over randomised study (8 males, VO2max: 62.2±5.4 mL·kg-1·min-1) participants completed 2 experimental trials consisting of two exercise bouts. In the afternoon of Day 1, they completed a cycling interval training session to normalise glycogen stores after which a standardised high carbohydrate diet was provided for 4 hours. On Day 2 in the morning participants received 2 g·kg-1 of carbohydrates in the form of glucose and rice (GLU+RICE) or fructose and rice (FRU+RICE), both in a carbohydrate ratio of 1:2. Two hours later they commenced cycling exercise session at the intensity of the first ventilatory threshold until task failure. Exercise capacity was higher in FRU+RICE (137.0±22.7 min) as compared to GLU+RICE (130.06±19.87 min; p=0.046). Blood glucose and blood lactate did not differ between the trials (p>0.05) and neither did carbohydrate and fat oxidation rates (p>0.05). However, due to the duration of exercise, total carbohydrate oxidation was higher in FRU+RICE (326±60 g vs. 298±61 g, p=0.009). Present data demonstrate that addition of fructose to a glucose-based carbohydrate source at breakfast improves endurance exercise capacity. Further studies are required to determine the mechanisms and optimal dose and ratio.