Sciences united

This write-up summarises the key takeaways from the Science of series. It's the one page to return to if you want the evidence base for long-format ultra-fueling without reading the entire series.

Carbohydrate as the primary endurance fuel

During prolonged exertion above ~65 per cent maximal aerobic capacity—which covers all competitive ultrarunning—the body relies predominantly on carbohydrate to produce ATP. Fat oxidation alone cannot meet energy demands at these intensities.

Muscle glycogen stores are depleted in 90 minutes to 3 hours at race intensity without exogenous carbohydrate replacement. Liver glycogen, which maintains blood glucose, is depleted in 2–4 hours. Depletion of either produces a measurable performance decrement.

Exogenous carbohydrate ingested during exercise can replace some of this, maintain blood glucose, and significantly prolong exercise duration.

The dual pathway system

The small intestine has two carbohydrate absorption pathways. The first, often named SGLT1, absorbs glucose and maltodextrin and becomes saturated at ~60 grams of glucose per hour. The second, GLUT5, absorbs fructose and operates independently.

The simultaneous use of glucose and fructose saturates both pathways and allows for total absorption rates of 90–120 grams per hour in trained individuals—50–100 per cent higher than with glucose alone.

The most efficient glucose-to-fructose ratio for maximising exogenous oxidation is around 1:0.8, as shown by the work of Rowlands and colleagues.

Osmolality and gastric tolerance

Osmolality governs gastric emptying rates and the risk of gastrointestinal distress. Blood osmolality is 285–295 mOsm/kg.

Hypertonic solutions slow emptying and draw water into the intestinal lumen, causing bloating, cramping, and nausea. Lower osmolality carbohydrate sources produce less gastric stress at the same doses. DE10 maltodextrin at the concentration of a long-format pouch yields around 133 mOsm/kg—well below blood osmolality.

Microdose delivery (25–30 ml every twenty minutes) further reduces the effective osmotic load per ingestion event, even if the source is hypertonic and concentrated.

DE10 Maltodextrin: why this specification

DE10 is the lowest dextrose equivalent that fully dissolves and flows at operational concentration. Lower DE grades suffer from solubility and viscosity issues at high concentrations. Higher DE grades significantly increase osmolality.

200 g maltodextrin in 450 millilitres: DE10 yields ~133 mOsm/kg, well below blood. DE18 would yield ~244 mOsm/kg, approaching blood. DE25 would yield ~333 mOsm/kg, exceeding blood.

DE10 is the optimal specification for minimising gastric stress at maximum carbohydrate concentration.

Gut training

Sugar-absorbing intestinal transporters increase in response to repeated carbohydrate exposure during exercise. Eight weeks of graded gut training increase absorption capacity, reduce GI symptoms at high intake rates, and improve gastric emptying. Those targeting more than 60 grams of carbohydrate per hour should implement a gut training programme before racing at that rate.

The principle of microdose delivery

Carbohydrate delivery in small, frequent doses rather than large infrequent doses reduces the peak osmotic load per ingestion event, keeps blood glucose more stable throughout the race, lowers the risk of GI distress compared to bolus ingestion, and can be automated with a watch alarm, removing the need for race-time decision-making.

Cold weather performance

High sugar content significantly lowers the freezing point of a solution. The product will not freeze solid at Finnish race condition temperatures. Viscosity increases moderately below freezing but remains functional through a silicone bite valve.

Cold exposure increases carbohydrate oxidation by 10–15 per cent compared to temperate conditions. Hourly targets should be increased accordingly in cold races.

Heat adaptations

Heat stress reduces splanchnic blood flow by 60–80 per cent, slowing emptying and increasing gut permeability. In hot conditions: drop initial intake to 40–50 grams per hour for the first 60–90 minutes, increase water intake to 600–800 millilitres per hour, use cooling at aid stations to partially restore gut blood flow.