It's now well documented that 60 to 90g of carbohydrate an hour works very well for the marathon distance. But does that range still apply over the much longer distances of the ultra world? And what about hydration? Let's see what the evidence suggests...

In this article we explain how the road marathon plan might need to change over longer distances, starting with the part most runners skip: hydration. We look at how your fuel needs shift as races get longer and slower, how much carbohydrate ultra runners are actually taking in today, why your gut can so often be the real limiter, how to mix real food and gels, and how to get in and out of an aid station well. We finish with fuelling templates for 50k, 100k and 100 miles.

What makes ultra fuelling different?

Ultra fuelling can crudely be split into three jobs: managing fluid and sodium over many hours rather than a couple, getting enough energy in, and keeping your gut working so it can keep absorbing that energy. A road marathon plan is built mostly around the second. Over 50k and beyond, all three start to decide your race.

Time is the big difference. In a 100 mile race you are asking your body to keep drinking, eating and digesting for a day and a night, often in the heat, often on very little sleep, while everything else is under enormous stress.

Why is hydration the gateway to carbs?

It's tempting to start a fuelling plan with carbohydrates. But the carbs you take in only reach your working muscles efficiently if your blood volume and electrolyte balance are in decent shape, so hydration comes first.

The tricky part is that while carbohydrate needs are fairly predictable, hydration needs vary enormously between runners. One athlete might sweat 0.5 litres (17 fl oz) an hour and lose around 250mg of sodium in each litre. Another might sweat 1.5 to 2 litres (51 to 68 fl oz) an hour and lose 1,500mg in each litre. Drinking to thirst may serve the first runner well, but the second will very likely come unstuck.

So one of the first steps in your ultra fuelling plan is a general understanding of how much you sweat and how salty that sweat is. We say general, because conditions change and your sweat rate shifts from day to day and even hour to hour.

A simple sweat rate test:

• Weigh yourself before and after a 60 to 90 minute session, ideally in minimal clothing and after towelling off.

• Add the weight of anything you drank or ate during the session to the difference between the two weights.

• Divide by the length of the session in hours. The result is your sweat rate in litres per hour, since 1kg of weight lost is roughly 1 litre of sweat.

• Want a higher-tech option? Wearable sweat sensors such as FLOWBIO measure your fluid and sodium losses as you train, sampling your sweat continuously through the session rather than giving you a single before-and-after number.

Heat, humidity, intensity, clothing and how well you're acclimatised all change your sweat rate, so a single test only tells you about one set of conditions. Test across the conditions you'll race in. And don't aim to replace 100% of what you lose. You can tolerate some dehydration, and chasing like-for-like replacement raises the risk of hyponatraemia, a dangerous drop in blood sodium that is usually caused by drinking too much.

The second step is working out how much sodium you lose, because sweat sodium concentration can vary by as much as ten times between athletes. You can get a sweat test done, or start with a simple self-assessment. Precision Hydration suggest looking for six signals:

• White salt marks on your skin or kit after a hard session.

• Sweat that stings your eyes or tastes noticeably salty.

• Feeling faint when you stand up quickly after exercise.

• Muscle cramps after long, hot sessions.

• Feeling terrible when exercising in the heat.

• Craving salty food during or after exercise.

Putting the two together, Precision Hydration's general guidelines look something like this:

• Lower (1 to 2 signals): a sweat rate of around 450 to 700ml an hour and 200 to 700mg of sodium per litre. Aim for roughly 350 to 600ml (12 to 20 fl oz) of fluid an hour, with 150 to 800mg of sodium per litre.

• Medium (2 to 4 signals): a sweat rate of around 700 to 1,000ml an hour and 700 to 1,100mg of sodium per litre. Aim for roughly 600 to 1,000ml (20 to 34 fl oz) an hour, with 600 to 1,250mg of sodium per litre.

• Higher (5 to 6 signals): a sweat rate above 1,000ml an hour and over 1,100mg of sodium per litre. Aim for roughly 1,000 to 1,200ml (34 to 41 fl oz) an hour, with 1,000 to 2,000mg of sodium per litre.

One caveat worth knowing: sodium isn't a magic fix. In a study of runners covering 161km (100 miles) in hot conditions, Martin Hoffman and Kristin Stuempfle found that cramping, dehydration, hyponatraemia and nausea were all unrelated to how much sodium runners took in overall. The aim is to roughly match what you lose, not to reach for a handful of salt tablets at the first twinge.

Once you have a flexible hydration plan built around your own needs, it's time to look at carbs!

How do your fuel needs change from a marathon to an ultra?

From an energy point of view, the biggest difference between a road marathon and an ultra is time, and therefore the intensity you can sustain.

A well-conditioned marathoner is walking the line between what's sustainable and what isn't, where 5 to 10 seconds per mile can be the difference between a great day and a blow-up. At that intensity, they're burning as much carbohydrate as they can and using fat to make up the rest.

A useful way to picture this is the crossover concept, first described by George Brooks and Jacques Mercier in the 1990s. Think of fuel use as a continuum. At very low intensities, such as walking, easy jogging or Zone 1 to 2 running, fat is your main fuel. As intensity rises, carbohydrate takes over, until at hard, sustained efforts it's doing almost all the work.

Ultras, aside from at the elite end, are run well over to the left of that picture. The International Society of Sports Nutrition's position on ultramarathon nutrition notes that at the relatively low intensities of a 100 miler, below about 65% of VO2max, fat becomes the primary energy source, which helps preserve your limited muscle glycogen and avoid hitting the wall.

From that you could conclude that the marathoner needs more carbohydrate than the ultra runner. But that's only true to a point...

How much carbohydrate do ultra runners actually need?

Historically, ultra runners often got by on 30 to 60g of carbohydrate an hour, from a mix of sports drink, gels and real food. A 2019 review by Ricardo Costa, Beat Knechtle, Mark Tarnopolsky and Martin Hoffman found that when ultra runners were left to eat freely, intakes typically landed at just 20 to 40g an hour.

That has changed. Sports drinks and gels have become far more sophisticated, making it much easier to absorb larger amounts of carbohydrate for longer. Today the approach we see in ultras isn't too dissimilar to the marathon, with 50 to 90g an hour working well, largely from drinks and gels (and of course the odd hot snack too!). Faster athletes who spend less time on course tend to push towards the top of that range, while runners taking 30 to 40 hours for 100 miles will usually sit nearer the bottom.

Physiologist Trent Stellingwerff saw the same thing in his work with elite athletes. In a 2015 study of the competition nutrition of elite ultra runners, he found that although many in the sport claimed to follow some version of a low-carb approach, their actual race-day intake was high in carbohydrate, at around 70g an hour, and very low in fat. That was in 2015, and the needle has shifted even further towards carbs since, something coach David Roche has advocated for widely.

All of which comes with one big condition. You have to train your gut to cope with it.

Why can your gut be the real limiter?

The numbers here are striking. In a study of the 161km (100 mile) Western States Endurance Run, Kristin Stuempfle and Martin Hoffman found that 96% of runners experienced at least one gastrointestinal symptom during the race. Among those who didn't finish, 35.6% said gut problems were a reason they dropped out, with nausea by far the most common complaint. Among finishers, 43.9% said their gut had held back their performance. Nausea peaked during the hardest and hottest part of the course.

An earlier study of two 161km races by Hoffman and Fogard found nausea and vomiting were the single most common reason non-finishers gave for dropping out, at 23%. As coach Jason Koop has pointed out, that is more than injury, blisters, cramps and exhaustion combined.

The reason is blood flow. During hard or hot running, blood is diverted away from your gut towards your working muscles and your skin, so digestion slows right down. Add heat and you make it worse, which is why so much of the advice in our guide to running in the heat applies to fuelling as well.

The good news is that your gut is trainable. In a 2017 study, Ricardo Costa and colleagues had runners take in 90g of carbohydrate an hour during training runs over just two weeks. When they were tested again, the trained groups reported a 44% reduction in gut discomfort and a 60% reduction in total GI symptoms, and they ran further in a one-hour performance test. Interestingly, both the groups using gels and the group using real food improved.

Just like we train the legs, we also need to train the gut. David Roche's approach is simple and works well:

• Rehearse on your quality long runs. Take in fluid and carbs exactly as you plan to on race day. It also helps your recovery and adaptation, and gives you the chance to adjust your plan based on how you feel.

• Push past your race target on some harder long runs. If you're aiming for 75g an hour on race day, try 90 to 105g an hour during a moderate 32km (20 mile) run.

• Build up your fluid tolerance too. Over time, work towards taking in your full hourly fluid needs when you need them during a run, with proper electrolytes so you don't overhydrate.

Two more habits help. Practise in race conditions, because your gut behaves differently on a hot, hilly long run than on a cool, flat one. And keep a simple log of what you ate and how your stomach felt, because patterns show up quickly.

Real food or gels?

Both, and for different jobs.

Most of your carbohydrate will probably come from drinks and gels. They're fast, compact and easy to measure, and they're at their best on runnable sections and whenever the effort is high. Their weakness is flavour fatigue. After ten or twelve hours, plenty of runners simply can't face another one.

Real food gives you variety, savoury flavours and a little fat and protein, which matter more the longer you go. In a study of runners in a 161km race, Stuempfle and colleagues found that those who didn't suffer gut distress had eaten a race diet with a higher percentage of fat, and took in fat and fluid at a higher rate, than those who did. That doesn't mean you should start eating cheese at mile five, but it's a good argument for a mix.

A simple way to think about it:

• Drinks and gels for the bulk of your carbohydrate, especially on runnable sections and when the effort is high.

• Real food on the climbs and at aid stations. Power hiking is the perfect time to eat, because your effort and your bounce are both lower.

• Savoury and warm food later in the race and overnight, when sweet things start to turn your stomach.

What's the best aid station strategy?

Aid stations are where good races are quietly lost. Ten minutes at each of eight stations is well over an hour, and a lot of it is spent wandering around a table deciding what to eat.

• Know what's there before race day. Most races publish what each aid station stocks. Plan around it.

• Arrive with a plan. Decide on the previous climb what you need: refill, food, layer, sock change. Say it out loud if it helps.

• Get in and get out. Fill your flasks first, grab food, then walk out eating rather than standing still.

• Use drop bags and crew for the things you can't get. Your favourite savoury snack, your own drink mix at the right sodium strength, a fresh pair of socks or a warm layer for the night.

• Carry a cup. Many trail races no longer provide them, and it makes soup and flat cola a lot easier.

Sleep plays a part too. As we covered in our guide to sleep and running performance, short sleep disrupts the hormones that control appetite. In a 100 miler, the dip in appetite overnight is very real, so plan small, frequent, easy-to-eat food for those hours rather than waiting until you feel hungry.

What should your fuelling plan look like for 50k, 100k and 100 miles?

Everyone is an individual, and these are starting points for a runner who has trained their gut and knows their sweat profile, not prescriptions. Adjust them in training, not on race day. For shorter road races, our guide to race day nutrition covers the fundamentals.

50k (typically 5 to 8 hours)

• Carbohydrate: 60 to 90g an hour if your gut is trained, 40 to 60g if you're still building up.

• What: mostly drinks and gels, with a small snack at one or two aid stations.

• Fluid and sodium: based on your sweat profile, towards the top of your range in the heat.

• Aid stations: quick refills. Aim to be in and out in under two minutes.

100k (typically 12 to 20 hours)

• Carbohydrate: 50 to 80g an hour, little and often, every 20 to 30 minutes.

• What: drinks and gels as the base, with more real food from halfway. Eat on the climbs.

• Fluid and sodium: based on your sweat profile, topped up with salty snacks at aid stations.

• Aid stations: one solid snack per station, plus a warm layer and head torch planned for the evening.

100 miles (typically 24 to 36 hours)

• Carbohydrate: 40 to 70g an hour depending on how long you'll be out there, and keep it going even when appetite drops.

• What: drinks and gels still do much of the work, with savoury and warm food overnight and a couple of "rescue foods" you know you'll always be able to eat.

• Fluid and sodium: based on your sweat profile, but bring intake down as temperatures drop overnight and be wary of overdrinking on long, slow sections.

• Aid stations and crew: a written plan for each station and a crew who knows it.

Key Takeaways

• Hydration is the gateway to carbs. Sweat rates and sodium losses vary enormously between runners, so start your plan by understanding your own sweat profile first.

• Estimate your sweat rate by weighing yourself before and after a 60 to 90 minute session, test across different conditions, and don't aim to replace 100% of what you lose.

• Sweat sodium can vary up to ten times between athletes. Use a sweat test or a simple self-assessment, but don't treat salt tablets as a cure for cramps.

• At the lower intensities of most ultras, fat provides more of your energy, but carbohydrate still matters a great deal.

• Modern ultra fuelling looks much closer to the marathon, with 50 to 90g of carbohydrate an hour, largely from drinks and gels. Elite ultra runners were already averaging around 70g an hour back in 2015.

• Your gut can be the real limiter. At Western States, 96% of runners had at least one GI symptom, and over a third of non-finishers said gut problems were a reason they dropped out.

• Your gut is trainable. Rehearse your race fuelling on long runs and practise above your race target on some harder ones.

• Plan every aid station in advance, get in and out quickly, and walk out eating.

Your legs get you to the start line of an ultra. Your stomach decides whether you reach the finish.

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