The Science of Endurance Fueling

Carbohydrate absorption is bottlenecked by specific transport proteins in the intestinal wall. Glucose uses SGLT1 (max ~60g/hr). Fructose uses GLUT5 (max ~30-45g/hr). Using both simultaneously is the key to high carb intake.

The ratio of glucose-yielding carbs to fructose determines how efficiently your body oxidizes ingested fuel. The classic 2:1 works well up to 90g/hr. The modern 1:0.8 ratio achieves 74% oxidation efficiency vs. 62% for 2:1.

Your stomach has caloric sensors that regulate how fast it empties. Solutions above 8-10% carbohydrate concentration trigger a slowdown — fuel and fluid sit in your stomach instead of being absorbed. This is the key constraint for drink formulation.

Osmolality measures the concentration of dissolved particles in solution. Blood plasma is 275-295 mOsm/kg (isotonic). Hypertonic solutions can delay absorption and draw water into the gut. Larger molecules like maltodextrin deliver more energy per osmotic particle.

Sodium constitutes 90% of electrolytes lost in sweat and is a biochemical prerequisite for SGLT1 glucose transport. Adding sodium to a glucose solution increases water absorption 5x. Other electrolytes (potassium, magnesium) have much smaller sweat losses and no proven intra-exercise benefit, though they're cheap and harmless to include.

Each carb source has distinct properties affecting absorption speed, osmolality, sweetness, and cost. Maltodextrin is the workhorse (low osmolality, tasteless, cheap). Cluster Dextrin is the premium option (ultra-low osmolality, steady release). Sucrose is the budget choice (natural 1:1 ratio).

Coupled: one drink provides carbs + electrolytes (simple but inflexible). Decoupled: separate carb gels + electrolyte drinks (complex but lets you scale fluid/sodium independently of fuel). The right choice depends on conditions and intensity.