Ultra Marathon Nutrition: Fuelling for Events Over 50km
The first time I went past 50km I did everything the marathon books told me to do, and I was finished by 38. Not physically — my legs were fine — but I could not put another sweet thing in my mouth without gagging, and from that point on I was running on an empty tank and stubbornness. It took me another two years and several bad afternoons to work out that ultra distance is not simply a longer marathon with more aid stations. It is a completely different fuelling problem.
Marathon fuelling is about topping up a tank that will just about last. Ultra marathon nutrition is about managing a deficit you cannot avoid, over a timeframe long enough that your gut, your palate and your body temperature all change underneath you. Get the numbers roughly right and the day stays enjoyable. Get them wrong and you spend six hours walking while your stomach decides whether to forgive you.
You cannot eat as fast as you burn — and that's fine
Running costs roughly 1 kcal per kilogram of body weight per kilometre. A 70kg runner covering 100km therefore burns somewhere near 7,000 kcal on top of resting metabolism. Now look at what you can actually take in. Even at an aggressive 90g of carbohydrate per hour — 360 kcal — a fourteen-hour finisher absorbs around 5,000 kcal. The gap is real and it is unavoidable.
That gap is covered by two things: your stored glycogen, which is only worth about 2,000 kcal in a well-loaded runner, and fat oxidation, which at genuine ultra pace can supply 0.5 to 1.0g of fat per minute. This is why ultras are run slowly. Drop the intensity and fat contributes a far greater share, sparing the carbohydrate you are drip-feeding in. The goal of ultra marathon nutrition is not to break even. It is to keep the deficit shallow enough that you never have to slow down involuntarily.
The hourly numbers that actually matter
Carbohydrate. For 50km to 100km, aim for 60–90g per hour. Beyond 100 miles, most people settle around 40–60g per hour because pace is lower and the gut simply gets tired. The ceiling on glucose absorption is set by the SGLT1 transporter and sits at roughly 60g per hour. Fructose uses a separate transporter, GLUT5, which is why a glucose–fructose blend at around a 1:0.8 ratio lets trained athletes get to 90g and above without the leftover sugar sitting in the gut pulling water in.
Sodium. Sweat sodium losses vary enormously between individuals — anywhere from about 300mg to over 1,800mg per litre. A practical starting point is 400–800mg of sodium per hour in warm conditions, less when it is cold. Do not chase a number blindly. Hyponatraemia in ultras is almost always caused by drinking too much plain fluid rather than by losing too much salt.
Fluid. Roughly 400–800ml per hour, adjusted for heat and your own sweat rate. Weigh yourself before and after a long training run to find yours; each kilogram lost is about a litre of fluid.
Caffeine. 3–6mg per kilogram is the ergogenic range, but taking it all early in a fourteen-hour event is a waste. Save it. Doses of 1–2mg/kg deployed during the night section or in the last quarter do far more good, and caffeine genuinely helps with the perception of effort when everything hurts.
Protein. Largely irrelevant in a marathon, worth including beyond about six hours. Something like 5–10g per hour helps with satiety and may take the edge off muscle damage. This is where a cup of broth or a handful of salted nuts at an aid station earns its place.
Your gut is the limiting organ
Gastrointestinal distress is reported by somewhere between a third and the large majority of ultra runners depending on the event and the survey, and nausea is consistently one of the top causes of people dropping out — often ahead of injury. That should tell you where to spend your training attention.
Gut training works and is boringly simple. Two or three sessions a week, over six to ten weeks, in which you take in carbohydrate at or slightly above your intended race rate while running. The intestinal transporters upregulate, gastric emptying improves, and the bloated, sloshing feeling fades. Runners who do this properly routinely move from tolerating 40g an hour to comfortably handling 80g.
The second gut problem is flavour fatigue, and it is not psychological weakness — it is a real, well-documented aversion response to repeated sweet stimuli. Plan for it. Carry at least one savoury option and one plain option alongside your sweet carbohydrate, and switch before you feel sick rather than after. Crisps, salted potatoes, broth, plain bread. By hour eight you will want them.
Building your actual race plan
- Work backwards from your finish time. Estimate hours, multiply by your target grams per hour, and that total is what you carry plus what the aid stations provide. Know both numbers before race day.
- Set a timer. Every 20 minutes, take something. Left to your own judgement you will forget on the climbs and remember when it is too late.
- Front-load slightly. The gut works best in the first few hours. Eat well early while it is easy, because intake almost always falls off later.
- Rehearse in the dark and the cold. Cold fingers, cold food and a tired brain change what is realistic. Practise on a night run before you find out mid-race.
- Never trial anything new on race day. Obvious, universally ignored, and the reason for most ultra DNFs that get blamed on fitness.
What we use at FTP Endurance Fuel
Our own long days are fuelled mostly on pastilles, for two practical reasons. First, chewing gives you some control — you can take five, not a whole sachet, which matters when your stomach is marginal and you want carbohydrate without a bolus of syrup. Second, they are easy to portion. We decant a race's worth into a small pouch or a hip belt pocket the night before, so there is nothing to tear open with numb hands at 2am.
The Mixed Fruit Pastilles 1kg are the workhorse for long training runs and races, and buying by the kilo means gut training does not cost a fortune — which is the main reason people skip it. For the days after, the Tart Cherry Pastilles 1kg use Montmorency tart cherry, whose anthocyanins have been studied for lowering oxidative stress and easing delayed onset muscle soreness after long, damaging efforts. An ultra is about as damaging as it gets.
If you take one thing from this, make it the gut training. Most runners arrive at a 50km or 100km start line having trained their legs meticulously and their digestive system not at all, and then wonder why the wheels come off at hour seven. Pick your hourly carbohydrate target, rehearse it weekly on your long run, and build a plan you have already proved works. Stock up on a 1kg pouch and start practising on this weekend's long run — your race-day stomach will thank you.
