How to fuel a long endurance session or race? (Carbs/hour, sodium, before/during/after)

Why does fueling matter so much in endurance sport?

Your body has two main fuel sources during exercise: fat and carbohydrate. Fat is effectively unlimited — even a lean runner has enough of it to run for days. Carbohydrate is the problem. Your muscles store a few hundred grams of it as glycogen, your liver stores a bit more, and that's it. A hard one-hour session can burn through 60–80 grams of glycogen. A four-hour ride at tempo can burn 300+ grams. Once the tank drops below a critical level, your brain throttles output to protect itself — and that is what bonking feels like from the inside.

The second thing that matters is *which* fuel you burn at any given intensity. At easy effort (Zone 2), fat provides most of your energy and carbs are a small supplement. At threshold (around 85–100% of FTP), carbs dominate because fat can't be oxidized fast enough to keep up with demand. This is why race-pace efforts chew through glycogen far faster than easy miles — and why fueling matters even more at intensity than at duration.

How many carbs per hour should you eat? (Carbs per hour by intensity and duration)

This is the single most useful number in endurance nutrition. Modern sports science, driven largely by Asker Jeukendrup's work on intestinal carbohydrate transporters, has produced a clear set of targets that scale with how hard and how long you're going. The table below is the quick reference most endurance coaches now use.

Above 60 g/h, glucose alone saturates the SGLT1 intestinal transporter. Fructose rides the separate GLUT5 transporter, so mixing the two is the only way to raise total absorption past the 60 g/h ceiling. That is why every modern 90 g/h product is a glucose-fructose blend.

How long does a session need to be before fueling matters?

Under about 60 minutes, if you started well-fed and well-hydrated, you don't need to eat anything during the session. Your glycogen stores are more than enough. Water is usually sufficient.

Between 60 and 90 minutes, fueling becomes optional depending on intensity — a hard interval session benefits from 20–40 g of carbs, an easy run doesn't really need anything.

Beyond 90 minutes, fueling is no longer optional. Any time you plan to be out the door for an hour and a half or more, you should be eating while you move. This is where most amateur athletes still get it wrong — they treat fueling as something reserved for race day, instead of something built into every long training session.

What should you actually eat during a session?

The goal is to get the target grams of carbohydrate into your system as fast and as comfortably as possible. The classic options, from easiest-on-the-gut to most fiddly:

• Sports drinks with a glucose-fructose mix. The cleanest way to deliver carbs because you're drinking them — fast absorption, built-in hydration, and easy to stack to 60–90 g/h with a concentrated mix.
• Gels. The standard because they're dense, portable, and reliable. A typical gel contains 20–30 g of carbs. Three gels per hour is how most athletes hit 90 g/h.
• Chews. Essentially gels in solid form — slower to eat, easier on the stomach for some, harder to take on the run.
• Real food. Rice cakes, bananas, dates, waffles, flapjacks. Comfortable, cheap, and psychologically satisfying on long rides. Less precise but perfectly workable at 30–60 g/h.
• Purpose-built high-carb drink mixes. These are the Norwegian-style products that enable 90–120 g/h by delivering a precise glucose-to-fructose ratio in one bottle. They're the most efficient option for very high intakes.

Running generally forces you toward drinks and gels because chewing real food at pace is hard. Cycling is permissive — almost anything works on the bike.

What does 'training your gut' actually mean?

If you've ever tried to eat a lot during a session and ended up sprinting to a bush, you already know that the gut is a limiter. Here is the most important thing in modern endurance nutrition: your gut adapts to what you practice eating, in roughly the same way your legs adapt to what you practice running. The transporters that absorb glucose and fructose up-regulate with repeated exposure. Athletes who have never tried to eat 60 g/h will have a terrible first experience. Athletes who have been eating 60 g/h every long ride for three months can often push to 90 g/h without trouble.

The practical protocol is simple and takes a block of training, not a week. Start at whatever you can tolerate without GI distress. Every long session, push the target up by 10–15 g/h. Use the same product or class of products you plan to race on. Track how your stomach feels, and back off if it complains. Six to eight weeks of this progression gets most athletes from 30 g/h to 90 g/h reliably.

Gut training is the single highest-leverage nutritional intervention available to an endurance athlete. Ignoring it is why people show up to an Ironman planning 90 g/h on race day when the highest they've ever tolerated is 45.

How much should you drink, and where does sodium fit in?

Hydration guidance has swung around over the past twenty years. The current evidence-based view is straightforward: drink to thirst most of the time, but in hot conditions, during hard efforts over two hours, and for heavy sweaters, planned intake helps. A reasonable baseline is 400–800 ml per hour, adjusted for conditions, body size, and sweat rate.

Sodium matters because it's the dominant electrolyte lost in sweat and because low blood sodium from over-drinking plain water (hyponatremia) is the one hydration mistake that can actually kill you. Aim for 300–700 mg of sodium per hour in hot conditions or for any session over two hours. Heavy and salty sweaters may need considerably more — some athletes benefit from 1,000+ mg/h and can tell you exactly why from dried salt stains on their kit. In hot events, the sodium target ramps up with sweat rate.

The other electrolytes (potassium, magnesium, calcium) matter far less during exercise than sodium, despite marketing to the contrary. They're appropriate to think about in your daily diet, not your bottle.

What should you eat before a long session?

The pre-session meal has one job: top up your liver glycogen and leave your gut ready to race, not digest. The well-validated guidance is 1–4 grams of carbohydrate per kilogram of body weight, eaten two to four hours before the start. A 70 kg athlete targeting the middle of that range would eat 150–200 g of carbohydrate, which is a serious amount of food — a big bowl of oatmeal with banana and honey, a large bagel with peanut butter and jam, or rice with some chicken. (For the day before a race, the rules are slightly different.)

Keep it low in fat, moderate in protein, and low in fibre. All three slow gastric emptying and can come back to haunt you at hour two. Save the big salad for after the session.

For early-morning sessions where a full pre-race meal isn't realistic, the compromise is a smaller, easily digested carb hit 30–60 minutes before — a banana, a piece of toast with honey, a small bottle of sports drink. It's not ideal but it's far better than starting empty. (Fasted training has its own place, but not for hard or long sessions.)

What about recovery after a long session?

Refueling after a long session is where the old advice has been quietly updated. The 30-minute 'anabolic window' — once treated as a hard rule — turned out to be much softer than originally claimed. What the evidence really supports is that if your next hard session is within 24 hours, recovery nutrition matters a lot; if it's 48 hours away, you mostly just need to eat normally across the day.

The practical rule for serious training: in the first four hours after a hard long session, aim for roughly 1–1.2 grams of carbohydrate per kilogram of body weight per hour — in practice, that looks like a full meal within an hour of finishing plus a proper second meal a couple of hours later. Include 20–40 g of protein in the first meal to support muscle protein synthesis. Rehydrate with fluid and sodium, not just water.

On a regular endurance training block, the biggest post-session mistake is not the timing but the total. Athletes who under-eat across the day find that their next hard session feels terrible, their sleep drifts, and their resting heart rate climbs. Under-fueling over weeks is how relative energy deficiency in sport (RED-S) sneaks up, and that's a far bigger problem than whether you drank your recovery shake within 30 minutes.

What are the most common fueling mistakes?

Five mistakes catch almost every amateur endurance athlete eventually.

• Under-fueling during the session. Taking one gel per hour and wondering why the last hour fell apart. The target is almost always higher than you think — most amateurs need to double their usual intake to reach 60 g/h.
• Fueling only on race day. Gut training doesn't happen on its own. If your race plan is 90 g/h, every long training session should be practicing that intake, not saving your stomach for the big day.
• Fibre in the pre-session meal. That fresh salad, bran flakes, or high-fibre protein bar before a long session is a recipe for stops in the bushes at hour two.
• New products on race day. Never eat anything on race morning or during a race that you haven't tested in training. This is the most common source of endurance GI disasters.
• Ignoring sodium. Plain water plus hot weather plus four hours of hard effort is a classic recipe for hyponatremia or bonking. Sodium is cheap, works, and matters.