Most people think of carbohydrates in a drink as calories. However they are doing something in addition to that: they are driving fluid across your intestinal wall.
Water absorption in the small intestine is passive. It follows solute. The main mechanism is SGLT1, a sodium-glucose cotransporter in the brush border membrane that moves two sodium ions across for every glucose molecule.¹ Sodium and glucose move into you body, an osmotic gradient is created, and water follows. This is the physiology that oral rehydration therapy products such as Dioralyte are built on, and it's why the WHO formulation for treating dehydration contains glucose rather than just salt.
The practical side of this is that a drink containing both sodium and glucose is absorbed faster than either alone, and considerably faster than plain water.¹,² If you're trying to restore fluid quickly — after a weight cut, between rounds of a competition, in the heat — that rate matters.
Concentration of the drink is a factor. Gastric emptying slows as carbohydrate concentration rises, and a strongly hypertonic drink can transiently draw water into the gut lumen before it's absorbed.² That's why you can get a bloated feeling from over-concentrated drinks. Sports drinks conventionally sit between 4% and 8% carbohydrate, but that is a compromise weighted toward fuel, which works pretty well for cycling and running. Go lower and both gastric emptying and fluid delivery improve. If hydration is the priority, dilute wins.
For fuelling, the ceiling is transporter saturation. Glucose oxidation from a single-transportable source plateaus at roughly 1 g/min because the SGLT1 receptor saturates — Jentjens and colleagues confirmed this directly by pushing glucose intake up to 1.8 g/min and finding oxidation didn't rise. Add fructose, which crosses via GLUT5 on a separate route, and peak exogenous carbohydrate oxidation rose to 1.26 g/min, around 55% higher than glucose alone.³ Later work combining three carbohydrate sources has pushed that figure higher still, but 1.2–1.3 g/min is what the glucose-fructose pairing itself delivers.
Currell and Jeukendrup showed this translates into performance. Eight trained cyclists rode two hours followed by a roughly one-hour time trial, taking either glucose or a 2:1 glucose-fructose blend, both at 1.8 g/min. The blend produced an 8% faster time trial than glucose alone.⁴ These are very specific conditions: a very high carb intake rate (108 grams/carb an hour), and two hours of prior work before the performance test.
Duration of exercise affects the target carb dose. Jeukendrup's framework is straightforward: for exercise of around an hour, a small amount of carbohydrate is enough. A single carbohydrate source can be oxidised at up to about 60 g/hr, and that's the recommendation for prolonged work of two to three hours; ultra-endurance events go to roughly 90 g/hr, which requires multiple transportable carbohydrates to achieve, and often gut training to get used to the very high carb loads.⁵
A really fascinating study was done based on carbohydrate mouth rinsing. It shows the effect of carbs isn't purely metabolic. Carter and colleagues had nine endurance cyclists rinse a 6.4% maltodextrin solution and spit it out — never swallowing a drop — and time trial performance improved by about 2.9% against a water rinse.⁶ Carbohydrate receptors in the mouth are signalling to the brain, and that alone is enough to change output.
Practical position:
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For a one-hour session, carbohydrate is more about absorption than energy. Keep it dilute.
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Sodium and glucose together move fluid faster than either alone.¹,² Don't strip the carbohydrate out of a rehydration drink.
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Above roughly 8% concentration you're trading fluid delivery for fuel.² Know which one you're after.
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Multiple transportable carbohydrates only matter once you're past about two hours.⁴,⁵ For most combat sports and Hyrox work, they aren't the limiting factor.
What's in DRVN Threshold
30 g of carbohydrate per serving, mixed to a litre — a 3% solution, below the conventional sports drink band. 120 kcal per litre, made up of 15 g maltodextrin and 15 g fructose.
30 g carbs in DRVN was a deliberate choice. The 60–90 g/hr protocols come from cycling, where you're seated and the gut is relatively undisturbed. In sparring, grappling or a Hyrox station, that load is a gastrointestinal liability. 30 g sits at the effective end without the risk, and a second serving in a second litre is there for longer sessions.
Alongside the carbohydrate, each litre provides 1000 mg sodium, 400 mg potassium, 200 mg magnesium and 1062 mg chloride.
References
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Wright EM, Loo DDF, Hirayama BA. Biology of human sodium glucose transporters. Physiol Rev. 2011;91(2):733–94.
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Leiper JB. Fate of ingested fluids: factors affecting gastric emptying and intestinal absorption of beverages in humans. Nutr Rev. 2015;73(Suppl 2):57–72.
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Jentjens RLPG, Moseley L, Waring RH, Harding LK, Jeukendrup AE. Oxidation of combined ingestion of glucose and fructose during exercise. J Appl Physiol. 2004;96(4):1277–84.
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Currell K, Jeukendrup AE. Superior endurance performance with ingestion of multiple transportable carbohydrates. Med Sci Sports Exerc. 2008;40(2):275–81.
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Jeukendrup AE. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med. 2014;44(Suppl 1):S25–33.
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Carter JM, Jeukendrup AE, Jones DA. The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Med Sci Sports Exerc. 2004;36(12):2107–11.