Ultra Fueling: How Many Carbs Per Hour Do You Actually Need?¶

More ultras fall apart at the stomach than at the legs. You trained the miles, you nailed the taper, and then somewhere around hour seven you can't take another gel and your pace quietly collapses. Fueling is the part of ultrarunning most people wing — and it's the part most worth getting deliberate about.

So let's talk numbers. Not vibes. Actual starting ranges you can take into your next long run and dial in.

Carbs: aim for 60–90 g/hr, train toward more¶

The old "60 grams per hour" ceiling came from studies on glucose alone — that's roughly the rate a single intestinal transporter (SGLT1) can move glucose into your blood. The breakthrough was realizing that fructose uses a different transporter (GLUT5). Combine glucose and fructose in a mix (a 2:1 or even 1:0.8 ratio) and you lift the absorption ceiling to around 90 g/hr, and with a properly trained gut, up to ~120 g/hr (Marathon Handbook).

Here's the honest version for ultras specifically: you probably don't need the top of that range. Ultras are run at low intensity — 50–70% of VO2 max — so your body is also burning fat, and the field data backs off the lab maximums. When researchers analyzed real races, elite finishers averaged around 70 g/hr while non-finishers came in under 45 g/hr (Cadence). The gap between finishing and dropping is often the gap between 45 and 70 grams.

So: start around 60 g/hr. If your race is long and you've done the work, push toward 80–90. Save the 100+ numbers for trained guts and the hard, runnable sections.

The catch: your gut is trainable, and most people skip it¶

You cannot show up on race day and suddenly absorb 90 g/hr if you've trained on water. The gut adapts like any other system. Start your long runs at 30–40 g/hr, build gradually over weeks, favor small frequent intakes over big boluses, and practice with the actual products you'll race on (Cadence). Gut training is the cheapest performance gain in ultrarunning and almost nobody does it on purpose.

Fluid: roughly 16–27 oz/hr, more in heat, never to excess¶

Hydration is where good intentions cause real harm in both directions. The general starting band is about 400–800 ml/hr (16–27 oz), nudged up in heat and down when it's cool (Pyrène Performance).

The thing to internalize: overdrinking is its own danger. Drowning your sodium with plain water causes hyponatremia, which lands runners in medical tents every season. The cleaner signal is body weight — if you finish more than ~2% lighter than you started, you under-drank; if you're heavier, you drank too much. Sip small and often. Don't slosh.

Sodium: 300–800 mg/hr, higher if you're a salty sweater¶

Sodium replacement runs roughly 300–800 mg/hr, climbing to 800–1,200 mg/hr in hot races or for heavy, salty sweaters (Marathon Handbook). Sweat sodium varies enormously between people — that white crust on your hat after a hot run is your hint that you're at the high end. This is the one variable most worth testing individually, because the range is so wide.

Build the plan, then test it to death¶

Here's the structure that works: pick your per-hour carb, fluid, and sodium targets, then break the day into one-hour blocks with cumulative goals so you can check yourself at each aid station instead of guessing. That's exactly what our free Ultra Fueling Calculator builds from your weight, expected time on feet, and conditions — per-hour targets, daily totals, and an aid-station table you can take with you. No signup.

But a calculator is a starting point, not a prescription. None of this is medical advice, and your gut and your sweat are yours alone. The plan only becomes real when you rehearse it on long runs and adjust.

That's the bigger principle: fueling should be built on your training, not a generic chart. When you connect Strava to NavRun, your long runs, weekly volume, and how your body actually responds become the foundation for your plan — and it updates as your fitness changes. Get the numbers from the calculator, prove them on the trail, and let your training do the rest.