How Many Carbs Per Hour During an Ironman?

60-90g per hour. That's the range for most Ironman athletes, and it comes from decades of research that's held up pretty consistently. If you're experienced and using glucose+fructose products, you can push toward 120g/hr.

But a number on a page doesn't get you across the finish line. How much you can actually absorb depends on your gut, the heat, how long you've been racing, and whether you've bothered to practice eating at effort. So let's get into the details.

Why You Bonk

If you're new to endurance sports, "bonking" is when your body runs out of readily available fuel mid-race. Some people call it hitting the wall. Whatever you call it, it's miserable.

Your body stores glycogen in your muscles and liver. How much depends on your size and how well you carb-loaded, but for a trained athlete in the 65-80kg range, you're looking at roughly 1,800-2,400 calories' worth. Lighter athletes store less. A 55kg woman might be closer to 1,500 calories. Bigger guys might crack 2,500.

An Ironman burns somewhere around 8,000-10,000 calories over 9-17 hours. The math isn't complicated. You will run out.

When it happens, your pace falls apart. Brain gets foggy. Legs go heavy. You start walking through aid stations wondering why you signed up for this. Every Ironman finisher has either bonked or watched someone bonk, and it's almost always a fueling problem, not a fitness one.

Now, your body does burn fat for fuel too, and at Ironman pace (which is mostly zone 2 for age groupers, around 60-75% VO2max), fat actually covers a decent chunk of your energy needs. Maybe 40-60% depending on your fitness. So why not just rely on fat?

Because the total demand is enormous. Fat oxidation can handle a portion of it, but over 9-17 hours the cumulative deficit adds up fast. Carbs fill the gap. Without them, even with solid fat-burning capacity, you're going to fade hard in the back half. You have to eat. A lot. While moving.

The Glucose+Fructose Thing

Here's where it gets interesting.

Your gut absorbs glucose through a transporter called SGLT1. That transporter tops out around 60g/hr. For a long time, scientists figured that was the ceiling. Sixty grams, done, don't bother trying to eat more.

Then Asker Jeukendrup's lab showed something that changed endurance nutrition: fructose uses a completely different transporter, GLUT5. Two separate pathways. Two lanes of highway instead of one.

By combining glucose and fructose (roughly a 2:1 ratio, though newer data suggests 1:0.8 might work even better), trained athletes can absorb and use 90-120g/hr. Products like Maurten, SIS Beta Fuel, and Neversecond are all built around this ratio. And the science genuinely backs them up.

Does that mean everyone should slam 120g/hr? No. But it means the old 60g ceiling is gone if you're using the right products and you've trained for it.

How Much You Should Actually Eat

This depends on where you are in your Ironman career. Your gut adapts over time, and someone on their fifth Ironman can handle volumes that would wreck a first-timer.

First or second Ironman: 40-60g/hr

Keep it conservative. GI distress will ruin your day faster than under-fueling will, and your gut hasn't been trained for this yet. One gel plus 500ml of sports drink per hour gets you around 55g. That's enough.

3+ Ironmans, solid training nutrition: 60-80g/hr

You've eaten on long rides. You know what sits well. Two gels per hour with water, or a gel plus concentrated drink mix in the 40-50g range. You've got options here.

Experienced, gut-trained, glucose+fructose products: 80-120g/hr

You've done the work. You use Maurten or similar dual-transport products. You know exactly what your stomach handles at race intensity. Two Maurten gels (50g) plus 500ml of their drink mix (40g) per hour. Or a concentrated bottle with 80-100g that you sip steadily.

The jump from intermediate to advanced isn't just about tolerance. It's about product choice. Glucose-only gels cap you around 60g absorbed no matter how tough your stomach is. You need the fructose pathway to go higher.

One thing worth flagging: those targets assume a 70-80kg athlete. If you're 55kg, scale down. You don't need the same grams as someone who outweighs you by 25 kilos. And for female athletes specifically, carb oxidation shifts across the menstrual cycle. The luteal phase (roughly the two weeks before your period) actually increases carb burning, so you may benefit from slightly higher intake during that window. Worth paying attention to in training.

Racing a 70.3 instead? Cut these numbers roughly in half. The race is shorter and the intensity is higher, so your gut has less time and less blood flow for digestion. Most 70.3 athletes do well at 40-60g/hr, and you can get nearly all of it done on the bike.

Need exact targets by hour for your race? Build your personalized carb, fluid, and sodium plan here: Use the free EnduranceOS calculator.

Don't Go Full Send in Hour One

Adrenaline is high. You feel great. You just swam 2.4 miles and you're finally on the bike. The temptation is to start crushing gels immediately.

Don't.

Blood flow is all over the place early in the race. Your stomach isn't ready for full volume. A simple ramp-up that works:

• Hour 1: 60-70% of target. Aiming for 80g/hr? Take in 50-55g.
• Hour 2: 80-90% of target. Push to 65-70g.
• Hour 3 onward: Full target. Your gut's awake. Your effort has settled. Now you eat for real.

This matters most on the bike, where you'll do the bulk of your fueling. Starting too aggressive is one of the most common mistakes in Ironman. Feeling good at mile 20 of the bike doesn't mean your stomach agrees.

Swim, Bike, Run: Three Different Games

Swim (2.4 miles)

You're not eating in the water. Your pre-race meal does all the work here. Eat 2-3 hours before the start: 100-150g of easy carbs. Toast and jam. Oatmeal with banana. Whatever you've eaten before long training sessions. Some people take a gel right before the water start. Fine. Don't overthink this part.

Bike (112 miles)

This is where the race is won or lost nutritionally. You've got 5-7 hours in a stable position, blood flow to your gut is way better than it'll be on the run, and your stomach can handle more volume. Aim to get 80-90% of your total race calories in during the bike leg.

Set a timer for every 15-20 minutes. Eat something. Don't wait until you're hungry because by then you're already 30 minutes behind.

Mix your formats. Gels for the first couple hours, maybe a bar when you want something different, chews when the gel flavor starts making you gag (and it will, eventually). Flavor fatigue at hour 5 is real. Plan for it.

Run (26.2 miles)

Back off. Running bounces your gut around more than riding. Blood shifts to your legs. GI issues spike on the run for good reason.

Drop to 70-80% of your bike intake. Stick to gels, sports drink, maybe cola at aid stations (it's sugar and caffeine, and honestly it works great when nothing else sounds appealing). If your race-day nutrition plan is dialed in from the bike, you've got a cushion here.

Stomach going sideways? Slow your intake but don't stop completely. Small sips. A few chews. Something is better than zero.

Train Your Gut (Or Pay for It on Race Day)

You can memorize every number in this article. None of it matters if you haven't practiced.

And I don't just mean during your long sessions. Practice fueling on your regular Tuesday ride. Your Thursday run. Even a 60-minute workout. Take a gel at minute 30. Drink your race-day sports drink. Make it automatic, like clipping into your pedals or hitting your watch at the start.

Why? Two reasons.

First, your gut literally adapts. The enzymes and transporters that process carbs during exercise get upregulated when you ask them to work regularly. Practice 80g/hr in training, and your body gets better at handling 80g/hr when it counts. Skip it, and race day becomes the first time your stomach has ever dealt with that volume at effort. That's how you end up in a porta-potty at mile 70.

Second, it gives you way more chances to experiment. Long rides happen, what, once a week? Maybe? But if you're testing products on shorter sessions too, you can try five different gels in a single training week without any of them ruining a key workout. You learn fast what sits well, what doesn't, what flavor you'll still want after six hours, and what makes your stomach turn.

Start 8+ weeks out from your race:

• Weeks 8-6: 40-50g/hr during long sessions, and take something on shorter workouts too
• Weeks 6-4: Push to 60-70g/hr on long days
• Weeks 4-2: Hit your full race target
• Final two weeks: Lock it in. Same products, same brands, same flavors you'll use on race day. Nothing new.

Stuff That Goes Wrong

Under-fueling the bike. The number one mistake. You feel fine through 80 miles, then mile 15 of the run hits and you completely fall apart. By the time you feel the bonk, you're 30-45 minutes past the point where eating could've helped.

New products on race day. The aid station has something you've never tried? Skip it. That gel your friend swears by? Save it for a training ride.

All liquid, no variety. Twelve hours of nothing but sweet drink mix will break you mentally. Your brain wants texture. Mix in something solid or semi-solid on the bike.

Ignoring the heat. Hot and humid conditions ramp up your sweat rate, which means more fluid in your stomach, which means less room for food. In heat, drop your carb concentration slightly and bump up total fluid. A 6-8% carb solution (60-80g per liter) is the sweet spot for getting both fuel and hydration through at the same time. The ACSM's position on fluid replacement is worth reading if you want the full picture on this.

"I'll eat when I'm hungry." Worst case Ontario, you bonk at mile 80 and death-march the entire run. Set timers. Count gels. Know exactly what you're eating and when.

Get Your Actual Numbers

Everything above is general guidance. Your specific numbers depend on your weight, pace, what the weather's doing on race day, and how long you've been training your gut. The EnduranceOS planner builds a personalized, hour-by-hour fueling plan that accounts for all of that. Takes about 60 seconds, and it's free.