The Fuelling Fundamentals: Fluids, Sodium & Carbs for Ultra Distances

Legs turning into concrete. Head going foggy. Stomach shutting down. You are somewhere in the back half of a long ultra. Sounds dramatic, feels worse, and it's rarely your training. It's your fuelling.

Ultra distances demand something different from your body than a marathon does. Whatever gets you through 3 to 4 hours starts falling apart somewhere past hour 6. Most fuelling advice hands you generic numbers and stops there. This one explains why they exist, how I apply them myself over 100km plus, and gives you a quick reference if you just want the numbers.

If this is your first ultra, don't start with my numbers, start with the general range in the table below, and treat everything I say about my own approach as where experience has taken me, not where you should begin. The why behind each number matters more than the number itself, because this is about knowledge and strategy.

Quick reference: the numbers at a glance

	General starting range	My own approach
Fluids	450 to 750mL/hour	Around 500mL/hour, adjusted for heat
Sodium	500 to 700mg per litre of fluid	600mg/hour baseline, up to 1,000mg on hot Perth days
Carbs	30 to 50g/hour traditional starting point, 60 to 90g/hour where tolerance supports it	70 to 80g/hour starting target, tapering as the race goes on
Caffeine	3 to 6mg/kg, though lower doses can work too	None in the first half, brought in through the night and later stages

These are starting points, not prescriptions. Here's the why.

Fluids

"Drink as much as you can" is bad advice, and sports science has moved away from it. Overhydration causes hyponatremia, a dangerous drop in blood sodium, and for a lot of mid to back of pack ultra runners it's a bigger risk than dehydration. Sports Dietitians Australia is blunt about this: hyponatremia is mostly caused by drinking too much, not by losing too much sodium.

"Drink to thirst" gets thrown around as the fix, and it's a reasonable principle, but it's really only well tested for shorter, cooler, lower intensity exercise. In a multi-hour ultra, especially in heat, thirst can lag behind what you're actually losing. That's why the ISSN's ultra-marathon position stand gives an actual number, 450 to 750mL per hour, as a starting point. Use thirst alongside that range, not instead of it.

I don't sweat test. I go by heat and humidity on the day, and that usually lands me around 500mL an hour, with sodium worked into that volume rather than treated separately. That's my number, built from running a lot of ultras (in training and races). If this is new to you, start in the middle of the 450 to 750mL range and adjust from there in your next long training run, warmer conditions and higher sweat rates push you toward the top of that range, cooler conditions and lighter sweaters toward the bottom.

One thing I do take seriously: a sudden whole body chill, almost like a cold flush, during a hot long run. It could be fluid, sodium, fatigue, or not enough fuel on board. I don't try to diagnose which one mid-tun. I just take it as a sign to slow down, check what I've had to eat and drink, and top up on salt. Ignoring it will cost you and can be dangerous.

Sodium

Sodium loss through sweat varies a lot person to person. General guidance, from the AIS and the ISSN alike, sits around 500 to 700mg per litre of fluid. Here's the bit most fuelling content skips: research in ultramarathons hasn't found a strong link between sodium intake and who does and doesn't cramp or feel nauseous. Sodium isn't the anti-cramp fix it's often sold as. It's role is more about conditions and duration than a flat rule.

My own numbers sit around 600mg an hour as a baseline for long runs, up to 1,000mg on really hot days. At 500mL of fluid an hour, that's a higher concentration than the standard guidance. That works for me. It's not a prescription. My base strategy is a carb electrolyte mix, so fluid, carbs and sodium come together, topped up with dissolvable electrolyte if conditions demand it, with plain salt tablets as backup for the rare occasion my gut disagrees with my usual mix.

Carbs

This is where the advice actually gets interesting, because it depends who you ask. The ISSN's ultra-marathon position stand recommends 30 to 50g of carbs per hour. That figure is still valid, it's not outdated. But plenty of runners and coaches have moved toward higher, individualised intakes, often 60 to 90g/h, and in some circumstances much higher than this.. where intensity, duration and gut tolerance support it.

The reason isn't simply "you burn more fat so you need less carb" during longer slower events. That's an oversimplification worth killing off. Fat oxidation does contribute more at lower intensities and carbohydrate more at higher ones, that part holds up. But total fuel needs, carbohydrate needs, and what your gut can actually absorb on the day are three different things, and conflating them leads people astray. You might have a high total energy expenditure across a long race but only be able to tolerate 60g of carb an hour. That doesn't mean your body only needed 60g, it means that's what your gut could process, with fat and glycogen covering the rest. Someone who can tolerate 90g/h doesn't automatically need it just because they can handle it. Pace, intensity and terrain all matter here. There's no clean formula.

Terrain is the part almost nobody mentions. Elevation, technical trail, and soft surfaces like deep sand can dramatically increase the energy cost of moving at a given pace. Two 100km races can demand completely different fuelling depending on the course, even though the distance on paper is identical. A flat, hard-packed 100km and a hilly, sandy one are not the same job.

Gels are great, but for longer events they need to be balanced with other carb sources from the start, not just once things get hard. Leaning on gels too early because they're easy and you feel good, and you will risk not being able to stomach them later, exactly when you need them most. My own starting target is 70 to 80g/h, which is above the ISSN's ultra-specific starting point and into the range some runners are now training toward. It's only realistic because I rotate carb sources from the beginning, which is exactly why I'm still comfortable using gels at the 100km mark.

That number isn't fixed for the whole race, and it shouldn't be. As distance goes on, my gut's capacity to process that much carb tends to drop, and my pace slows at the same time. Slower pace generally shifts the body back toward fat, though it's not a clean trade, fatigue and total energy demand play into it too. What I know from experience is that the number I start with isn't the number I'm still hitting late into a very long race, and forcing it to stay flat has never worked for me.

If you're not used to training your gut for higher carb rates, or this is your first ultra, start at the lower end of the general range, around 30 to 40g/h, and hold there for a few long training runs before pushing higher. Build up gradually rather than jumping to a number like mine, and pay attention to what you can actually tolerate rather than chasing a figure just because it's achievable on paper.

Caffeine

Caffeine is one of the most well-evidenced performance supplements in sport, classified by the AIS Sports Supplement Framework as a Group A supplement, meaning the evidence is strong. Research commonly uses 3 to 6mg/kg, taken about an hour before the effort that matters most, though lower doses work for some people too. The ISSN specifically backs strategic caffeine use in the later stages of ultra races, particularly once sleep deprivation starts to bite.

I don't cut caffeine before a race. Gut tolerance to caffeine is real, so I go in with my normal intake. Where the strategy actually happens is during the race. In my longer events I don't touch caffeine in the first half at all. I bring it in through the night and the later stages, where it does far more for me than it would if I'd burned through it early just because it was there.

This is a starting point, not your plan

The numbers and explanations above give you the physiology behind why fluids, sodium and carbs matter so much more in ultra distances, and how that plays out for one experienced runner. Your own numbers, warning signs and tolerances will be specific to you, and gut training changes what's realistic over time. This is exactly what I work through with runners one-on-one, building a fuelling plan around your actual physiology and race, not a generic chart. If you want help with that, book a consult here.

Next in this series: why your race nutrition stops working late in an ultra, and what to reach for instead when gels and lollies stop going down.

See you out there!