Science
Glucose and fructose take different routes through the gut wall.
Carbohydrate absorption during exercise is limited by intestinal transport capacity. Two primary transporters facilitate glucose and fructose uptake:
Sodium-Glucose Cotransporter 1
Glucose Transporter 5
Using both transporters simultaneously increases total carbohydrate absorption beyond the single-transporter limit.
SGLT1 is a sodium-dependent transporter. For each glucose molecule transported, two sodium ions must be co-transported. This coupling is essential for glucose uptake.
SGLT1 saturates at approximately 60g of glucose per hour. This represents the maximum rate at which glucose alone can be absorbed during exercise.
SGLT1 carries two sodium ions with each glucose molecule. Sodium in the gut is part of how glucose is absorbed.
GLUT5 is a facilitated diffusion transporter specific to fructose. Unlike SGLT1, it does not require sodium co-transport and operates independently.
Because GLUT5 operates independently of SGLT1, fructose can be absorbed at the same time as glucose.
Fructose on its own is absorbed more slowly, and large amounts can cause gastrointestinal discomfort. In research it is given together with glucose.
Glucose–fructose mixtures have been studied at glucose:fructose ratios from 2:1 to about 1:1 (fructose:glucose 0.5 to 1.25). In a direct comparison of fructose:maltodextrin ratios of 0.5, 0.8 and 1.25 at about 108 g/h, O'Brien & Rowlands (2011) found the highest exogenous oxidation rate with 0.8 (1.14 g/min vs 1.04 and 1.05) and the fewest gut symptoms with 0.8, followed by 1.25.
In laboratory studies, glucose alone has been oxidised at up to about 1 g/min (60 g/h). Mixtures of glucose and fructose have been oxidised at up to about 1.5–1.75 g/min (about 90–105 g/h) (Jentjens & Jeukendrup 2005; Podlogar et al. 2022).
Selected studies and reviews.
British Journal of Nutrition
A glucose–fructose mixture ingested at 2.4 g/min reached a peak exogenous carbohydrate oxidation rate of 1.75 g/min, higher than glucose alone.
Medicine & Science in Sports & Exercise
A 2:1 glucose:fructose drink at 1.8 g/min gave about 8% better time-trial performance than glucose alone.
American Journal of Physiology – Gastrointestinal and Liver Physiology
Compared fructose:maltodextrin ratios of 0.5, 0.8 and 1.25 at a high intake rate: the 0.8 ratio gave the highest exogenous oxidation and the fewest gut symptoms.
Sports Medicine
Critical review of fructose–glucose mixtures and endurance performance.
European Journal of Applied Physiology
Fructose–maltodextrin at 120 g/h (0.8:1) gave higher exogenous oxidation than 90 g/h (1:2), with no further sparing of the body's own carbohydrate stores.
Sports Medicine
Review of current carbohydrate research and practice for endurance athletes.