How Much Carbs Can You Use Per Hour?

William Horkoff
Oct 7
4 min read

Personalizing race fueling without a lab (and without wrecking your gut)

The gold standard to find your peak exogenous glucose oxidation rate (how much of what you drink you can actually burn per hour) is a ^13C-tracer drink + metabolic cart. That’s lab-only and pricey. In the real world, the best practical predictors are body size (especially height) and absolute power output during the effort. Together, those explain a big chunk of why some athletes comfortably use more carbs per hour than others. You still need gut training to claim the rest.

Why this matters

If you under-fuel, you lean harder on stored glycogen and pace falls apart late. If you over-shoot your gut, you’ll meet GI roulette at mile 10. A personalized carbs-per-hour target lets you hit high power/pace and keep RPE and stomach calm.

The lab reality (and why most of us won’t do it)

The only way (as of Sept 2025) to directly measure your peak exogenous glucose oxidation is:

Drink a ^13C-labelled glucose solution,
Ride at a steady workload,
Analyze your expired CO₂ with a metabolic cart.

It’s accurate, but not practical for most athletes.

New data that actually helps

A 2025 study (Podlogar et al.) had trained cyclists ride 2.5 h at around 95% of VT1 while ingesting 90 g/h of glucose. They then measured each rider’s peak exogenous glucose oxidation with the tracer method.

Key takeaways (clean numbers, no fluff):

Height + body mass + power (Watts) together explained ~68% of who could burn more of what they drank.
Looked at individually: height ~35%, power ~35%, mass ~21% of the variance.
Translation: taller athletes and/or those pushing higher absolute Watts tend to tolerate and use more carbohydrate per hour.
Individuals still varied widely (roughly ~42 to ~66 g/h exogenous on glucose-only, despite all drinking 90 g/h).

Why this tracks: Taller = bigger GI tract on average; higher absolute power = higher delivery/uptake demand (and often higher splanchnic blood flow), so more of what’s ingested gets used.

Important: This was glucose only. In real racing we use glucose + fructose mixes to raise the ceiling (often 90–120 g/h if gut-trained). Your oxidation ceiling with a dual-transporter mix will be higher than with glucose alone.

A simple, field-usable way to set starting targets

You don’t need a formula tattoo. Use this Height × Power heuristic to pick a starting range, then refine with gut training.

Step 1 — Classify your typical race-effort power (absolute, not W/kg):

< 180 W → Low
180–240 W → Moderate
240–300 W → High
> 300 W → Very High

Step 2 — Classify height:

< 170 cm → Shorter
170–183 cm → Average
> 183 cm → Taller

Step 3 — Start here (bike intake, dual-source mix like malto+fructose ~1:0.8):

Shorter + Low: 50–70 g/h
Shorter + Moderate: 60–80 g/h
Average + Moderate: 70–90 g/h
Average + High: 80–100 g/h
Taller + High: 90–110 g/h
Taller + Very High (>300 W): 100–120 g/h (only if gut-trained)

These are starting bands, not ceilings. About 30% of the individual variability is not explained by height/power/mass things like transporter density, prior fueling habits, anxiety, heat, osmolality, and simple genetic differences still matter.

How to “earn” a higher carbs/hour: the gut-training ladder (4–8 weeks)

Rules: use a dual-source carb drink (e.g., 1:0.8 glucose:fructose), keep osmolality sane, and don’t spike concentration on minimal fluid.

Weeks 1–2: Sit in your starting band (e.g., 80 g/h). Ride 90–150 min at your race-specific intensity 1×/week.
Weeks 3–4: Add +10–15 g/h if no GI flags (no sloshing, burping, cramps, or urgent port -potty meetings).
Weeks 5–8: Nudge up again if clean. Cap the build if you see HR drift or RPE jump at the same power fuel should lower stress, not raise it.

Green flags: stable HR at given power, steady RPE, no bloating, you can run off the bike smoothly.

Red flags: stitch, gut slosh, burps, bathroom stops, RPE spike at the same Watts → drop 10–15 g/h and increase fluid by 200–300 mL/h.

Fluid & sodium sanity:

Aim ~500–900 mL/h (heat dictates the top end).
Keep drink strength ~6–8% (≈60-80 g per liter) if you’re a big sipper; concentrate more only if your fluid needs are low and stomach says OK.
Sodium ~700–1000 mg/L as a starting point; adjust to sweat rate/losses.

Quick field test to confirm you’re in the right ballpark

Do two identical race-specific rides a week apart (same route, kit, temp if possible).

Test A: lower end of your band (e.g., 80 g/h)
Test B: +15 g/h (e.g., 95 g/h)

Hold the same power. Watch: HR drift, RPE, how the run off the bike feels, and any GI feedback. If B feels easier at the same power and your HR is flatter, you likely unlocked useful exogenous oxidation. If GI or RPE worsens, back off.

What about the run?

Your hourly ceiling usually drops once you’re upright and bouncing. If you’re 90–110 g/h on the bike, expect 60–90 g/h on the run, split across gels/chews + bottle sips. Train that, too.

Where people go wrong (so don’t)

Trying to be a 120 g/h athlete with a 60 g/h gut
without doing the ladder first.
Glucose-only at high intakes. Use a dual-transporter mix to raise oxidation and comfort.
Concentrated syrup + low fluid in heat. That’s a GI hand-grenade.
Basing intake on bodyweight alone. Height and absolute Watts are better guides than kilos.

My coach conclusion (and how I would go about it)

You can personalize carbs/hour without a lab by anchoring to height + absolute power, then progressively gut-training inside that band.
Expect most trained age-groupers to land around 70–100 g/h on the bike, 60–90 g/h on the run, with 100–120 g/hreserved for tall/high-power athletes who gradually trained the gut.
The point isn’t to max the number; it’s to maximize power/pace with low RPE and a quiet stomach.