Sports nutrition gels

Endurance exercise is limited in part by carbohydrate availability: muscle and liver glycogen are finite, and once depleted, intensity falls. Feeding carbohydrate during exercise therefore raises performance — but only up to a ceiling that is set not by the muscle and not by the stomach, but by the small intestine.

The transporter ceiling

Glucose crosses the intestinal brush border through SGLT1, a sodium-dependent co-transporter. Like any transporter it has finite capacity, and in humans SGLT1 saturates at an intake of roughly 60 grams of glucose per hour. Beyond that, additional glucose is not absorbed faster; it stays in the lumen, draws in water osmotically, and causes the bloating, cramping and diarrhoea familiar to anyone who has over-fuelled during a race.

For years 60 g/h was treated as the physiological maximum. It is more precisely the maximum for a single transport route.

Two sugars, two doors

Fructose does not use SGLT1. It is absorbed by GLUT5, a facilitative transporter with its own separate capacity.

Ingesting glucose and fructose together therefore recruits two independent absorption pathways, and total carbohydrate oxidation rises well above the single-sugar ceiling — measured rates of around 90 grams per hour, and higher in trained subjects with adapted intakes. This is the entire mechanistic basis for the glucose–fructose ratios printed on sports nutrition products, conventionally 2:1 and in more recent formulations closer to 1:0.8, the ratio at which the two routes are used in balance.

Maltodextrin substitutes for glucose in these formulations for a reason that follows from osmolality, below; sucrose works because it is hydrolysed at the brush border into one glucose and one fructose, delivering both substrates in a single molecule.

Osmolality governs the stomach

Gastric emptying slows as the osmolality of the stomach contents rises, and a strongly hypertonic solution also draws water into the gut lumen — the opposite of what is wanted during exercise.

Osmolality depends on the number of dissolved particles, not their mass. This is why maltodextrin is used: a maltodextrin chain of ten glucose units contributes one particle where ten free glucose molecules would contribute ten, delivering the same carbohydrate at a fraction of the osmotic load. It is hydrolysed by intestinal amylase and absorbed as glucose, so it meets the same SGLT1 ceiling — the advantage is entirely in the stomach and in fluid balance, not in absorption capacity.

Gels are concentrated and therefore hypertonic, which is why they are taken with water; taken without, they slow their own delivery.

Gut training is a real adaptation

Repeated high carbohydrate intake during training increases intestinal absorptive capacity, with evidence of upregulated SGLT1 expression. Tolerance is therefore trainable rather than fixed, which is why elite intakes exceed those tolerable by an untrained gut, and why a new fuelling strategy should not be attempted first on race day.

What is weaker

Alginate-based “hydrogel” fuels are formulated to gel at gastric pH, with the claim that this improves gastric emptying and tolerance. The chemistry is straightforward — alginate cross-links with acid and calcium — but the performance and tolerance evidence is mixed, with several controlled trials finding no advantage over conventional formulations at matched carbohydrate intake. The transporter mechanism is settled; this refinement is not.