How sports drinks actually work

Carb concentration = carbs ÷ fluid

Plain math. 80 g of carbs in 800 ml of water is a 10% w/v drink. 60 g in 800 ml is 7.5%. Your stomach starts to slow gastric emptying when concentration climbs above roughly 8–10%, so this number is your first reality check.

Target zone for sugar-only mixes: 6–8%. Modern race fuels run 14–16% by using polymers (next row) to keep osmolality down. The other way past the ceiling is to split-stream: a concentrated carb flask plus plain water or weak electrolyte in a separate bottle. Split-stream is the default for ultra-distance, hot conditions, and any event where fluid needs are unpredictable.

Carb mix = which sugars to use

Same concentration, very different gut comfort, depending on molecule size. Bigger molecules contribute fewer particles per gram of carb — so they can pack more energy into the bottle without breaking your osmolality budget.

1. Dextrose, fructose, sucrose— small molecules, high osmolality per gram. Cheap. Fine at low concentrations.
2. Maltodextrin— 3–20 glucose units (sports grades typically average 5–10). Same energy, ~5× lower osmolality per gram than monosaccharides. The sports-nutrition default.
3. Cluster Dextrin (HBCD)— branched cyclic polymer, mean MW ~160,000+ Da depending on grade. Lowest osmolality on the shelf. Premium price; earns its keep above ~100 g/hr.

See it side by side on the ingredients page — the “Sugars compared” section makes this trade-off visible at a glance.

Glucose : fructose ratio = which absorption pathways you load

Glucose-yielding carbs (dextrose, sucrose, maltodextrin, HBCD) all share one transporter (SGLT1, ~60 g/hr in a typical gut, higher in trained ones). Fructose has its own (GLUT5, ~30 g/hr alone, ~45 g/hr when paired with glucose — co-ingestion drags fructose along). Use only one and you hit the ceiling fast. Use both and you can ride two parallel pipes.

• Below ~60 g/hr:ratio barely matters — you're under the SGLT1 ceiling on glucose alone, so the second transporter isn't needed. Sucrose works fine and costs nothing.
• 60–120 g/hr: the modern 1:0.8ratio loads both transporters in parallel. Up to ~100 g/hr it sits each near its working capacity; above that, you're in trained-gut territory where both ceilings shift upward. O'Brien & Rowlands (2013) found 0.8:1 fructose:maltodextrin beat both 0.5:1 and 1.25:1 on oxidation efficiency; industry-funded follow-on trials report similar gains over the older 2:1 ratio.
• Pushing past 1:0.8 toward fructose-heavy: performance drops. Glucose is still the primary fuel; fructose has to detour through the liver before muscles can use it.

The full ratio breakdown on /science →

Sodium per bottle = sweat target ÷ bottles per hour

Sodium scales with sweat and hours, not with carbs. If you lose 900 mg/hr and refill ~1.5 containers per hour, you need ~600 mg per container — whether that's a 500 ml soft flask or a 750 ml bike bottle, the math runs the same and runs independently of your carb intake.

When sweat sodium gets high enough that the salt is the limiting factor on palatability, brands cut sodium chloride with sodium citrate — same sodium load, smoother taste. Some go further: Tailwind runs citrate-only, Maurten stays all-chloride, Skratch blends both. At 1500 mg/hr you usually have to decouple sodium from the carb bottle entirely (electrolyte tabs, salt caps, or a separate hydration drink).