100-mile ultramarathon nutrition: how to fuel a hundred

Quick answer: A 100-mile race costs roughly 13,000 to 16,000 calories and you can only replace about half of that on the move, so the aim is to keep the deficit steady rather than close it. Eat 60 to 90g of carbohydrate an hour (240 to 360 calories) from the first half hour, in line with the American College of Sports Medicine's guidance of up to 90g an hour for events lasting more than 2.5 hours.

Written by James Frost, founder of Flaming Phoenix and the maker of Phoenix Bars. I make dense, no-fuss food that runners carry on efforts like this, and I have spent years talking to them about what works at hour four and what works at hour twenty-six. I am not a coach or a dietitian, and a plan built with one for your body and your race will always beat a general guide, but this is the practical version. Updated November 2026.

A hundred-mile race is the first distance where you physically cannot eat enough to cover what you burn, which makes it a different event from a marathon or a 50k with more miles bolted on. Researchers who studied runners at the Western States 100 (Stuempfle and colleagues, 2011) put the cost at an estimated 13,000 to 16,000 calories, and finishers took in roughly half to two-thirds of that, crossing the line with a deficit of several thousand calories. You burn roughly 400 to 750 calories an hour, depending on your pace, and most guts handle perhaps 240 to 360 calories an hour of carbohydrate, so the shortfall is built into the event. That changes the goal. You are not trying to replace what you burn, because you can't. You are trying to keep the deficit small and steady enough that you never run the tank dry, and most people fail at that not in the closing miles but in the first quarter, when they feel great and under-fuel without noticing. Fuel from the start, aim for 60 to 90 grams of carbohydrate an hour in small regular amounts, and change what you eat as the race changes you. This guide walks through how, phase by phase.

The maths you cannot beat

It is worth sitting with the numbers, because they dictate everything else. Your body stores only about 90 minutes of hard-running fuel as glycogen, after which you depend on what you eat plus your fat stores. Across a whole hundred you will burn something like 13,000 to 16,000 calories and take in maybe half, which is normal and not a failure, but it means the race is a deficit you manage. The Western States research is blunt about where it goes wrong: finishers ate at close to twice the hourly rate of runners who dropped out (about 4.6 against 2.5 calories per kilogram of body weight an hour, or roughly 320 against 175 calories an hour for a 70kg runner). So the target is to delay depletion, keep your blood sugar and your spirits up, and never let the deficit run away from you. Everything that follows is in service of that.

How many calories does a 100-mile race burn at different finish times?

The total stays roughly the same whatever your time, because most of the cost comes from carrying your body 161km and up the climbs. What changes is the hourly rate, and how much of the total your carbohydrate can cover. These figures are estimates from typical values, not measurements.

Few runners hold 90g an hour for 30 hours or more, so late-race intake usually sits nearer the 60g column, and fat and protein from real food add a little on top.

The first quarter: fuel before you need it

The opening miles are where many hundreds are lost, because you feel fantastic and food is the last thing on your mind. This is a trap. Start eating within the first half hour, before you are hungry, and hold to a steady 60 to 90g of carbohydrate an hour in small amounts every twenty to thirty minutes, the range the American College of Sports Medicine's 2016 position stand supports for events over 2.5 hours. Our guide to the best energy bars for ultra running works through what it takes to hit that target with different bars. Your gut works best now, while you are fresh and cool and the pace is easy, so this is when to bank calories that you will not be able to take on later. Keep the pace easy, too, because going out hard both burns through your glycogen faster and starts shutting down your gut early. Dense, easy food that you barely have to think about works best here, the same low-bulk carbohydrate logic covered on the ultra-high-carb solid foods guide. A calorie-dense bar or a flapjack broken up on the move banks a lot of calories for very little stomach load, and that calories-to-volume ratio is what you want when the aim is calories without bulk, as the calorie-dense foods guide explains.

The long middle and the heat of the day

The middle of a hundred is a metronome: keep the same steady intake going, hour after hour, long after it has stopped being interesting. Two things tend to break the rhythm. The first is flavour fatigue, the point where your chosen fuel becomes unappealing, which is why variety across sweet and savoury matters, and why it pays to carry several flavours of anything you rely on. The second is the heat of the day, which for many runners is when the stomach first turns, often somewhere around the middle of the race. The fatal mistake is to stop fuelling because nothing sounds good, because that is the moment the deficit starts running away and the spiral begins. If your stomach is rebelling, the fix is to ease off, cool down, and switch what you are asking it to do rather than quit, which is covered in full on the mid-race nausea and appetite loss guide. Lean on savoury and real food, take smaller bites more often, and keep something going in even when it is a chore. This is also where having stashed the right food in your drop bags pays off, because the food that appeals at mile sixty is not the food that appealed in your kitchen the night before.

The night

The night section is where hundreds are won and lost. As the temperature drops and the hours stack up, your appetite craters further, sleepiness sets in, and everything feels harder. Two rules carry you through. First, eat on a schedule, not on hunger, because hunger has left the building and will not return until you stop. Second, go warm. A hot drink and warm food lift both your core temperature and your morale. A bar stirred into a porridge with water at an aid station also helps: it is soft, easy and comforting when chewing a solid is the last thing you want. There is a quick guide to both methods on how to use Phoenix Bars. A little caffeine helps with alertness, small bites beat big efforts, and if solids will not go down at all, liquid calories will. If your race climbs high, altitude flattens appetite and gut function further, which is its own discipline on the high-altitude mountaineering guide. The runners still moving well at dawn are almost always the ones who kept feeding themselves through the dark.

The final push

By the closing miles you are running on fumes and your gut may be wrecked. Sweet gels are usually finished as an option, and the strategy narrows to whatever still goes down: flat cola, broth, a few salty bites, a few spoonfuls of warm porridge, small amounts as often as you can manage. Do not aim for your earlier targets here, just keep a trickle of calories and fluid going to hold off the worst of the bonk and protect your decision-making to the line. Calories are courage at this stage, and a small win, a hundred calories you did not think you could take, can be the difference between shuffling and stopping. The case for real food over gels late in a race is on the energy gel alternatives guide, and the broader picture is on the ultra-endurance and expedition nutrition guide. If you fuel plant-based, the fibre and calorie-density angles deserve a read on the vegan endurance nutrition guide, and for staged races run over several days with sleep between them, which are a different problem, see the multi-day ultra running guide.

After the line

Finishing leaves you with a large deficit to repay and a body that has taken a real beating, especially from the descending. You usually have a short window before exhaustion drops you into sleep, so use it: get easy carbohydrate and a little protein in, plus fluids and electrolytes, even if your appetite has not woken up yet. Then keep refuelling steadily over the following days, because a deficit of several thousand calories is not cleared in one meal, and this is recovery rather than weight management. A high calorie bar, eaten as it is or mixed with water into porridge, is an easy first thing to get down when nothing else appeals, and a steady run of small, calorie-dense meals over the next few days does more for how you feel than one big plate ever will.

Common questions

How many calories do you burn in a 100-mile race? Roughly 13,000 to 16,000 in a mountain hundred, the range researchers estimated for the Western States 100, depending on your size, the climbing and your time on feet. It is one of the highest single-event energy costs in endurance sport.

How much should I eat during a 100-miler? You cannot match what you burn, so aim to minimise the deficit rather than close it: around 60 to 90g of carbohydrate an hour, about 240 to 360 calories, taken in small amounts every twenty to thirty minutes from the start, in line with the American College of Sports Medicine's guidance of up to 90g an hour for long events. In the Western States study, finishers ate at close to twice the hourly rate of runners who dropped out.

Why do so many people drop out of 100-milers? Stomach problems and under-eating are among the most common reasons, alongside injury and missed cut-offs. The calorie deficit builds in the first half while runners feel fine and under-fuel, then a turned stomach or flavour fatigue mid-race makes them stop eating, and the deficit runs away. Runners who keep feeding the deficit are the ones who finish.

What should I eat at night during a 100? Eat on a schedule rather than waiting for hunger, lean on warm food and a hot drink for morale and core temperature, use a little caffeine for alertness, keep bites small, and switch to liquid calories if solids will not go down. Porridge made by mixing a bar with water at an aid station is soft and easy when chewing feels like work.

How much should I drink in a 100-miler, and how much salt? The International Society of Sports Nutrition's 2019 ultra-marathon position stand recommends 450 to 750ml of fluid an hour, about 150 to 250ml every 20 minutes, and says sodium may need to be taken at a higher concentration than most sports drinks provide (above 575mg per litre) to reduce the risk of low blood sodium. Drink to thirst within that range, drink more in heat, and do not force fluid down when you are not sweating much at night.

How do I train my stomach for a 100-miler? Practise race-day eating on your long runs for six to eight weeks or more before the race, using the same foods, drinks and timings you plan to use. Start where you are comfortable, often 40 to 50g of carbohydrate an hour, and build up by about 10g an hour every week or two. The gut training for ultras guide sets out a full plan.

Why do gels stop working late in a race? A mix of flavour fatigue and a gut tired of concentrated sugar after many hours. The fix is variety planned in advance and a shift toward real, savoury, and liquid options as the race goes on. Our list of the best foods for ultra running sets out that shift hour by hour.

Are Phoenix Bars good for a 100-miler? I make them, so weigh my view accordingly, but they suit the job. Each one carries up to 557 calories in a compact, 120g bar, so it banks calories without loading your stomach, it comes in six flavours to help with flavour fatigue, it can be eaten as it is early on or mixed with water into porridge at night, and because it keeps for up to two years, it stocks drop bags well. Paul Blinstone, who finished the 2025 Marathon des Sables, found the bars "kept their shape even in desert temperatures". To build a rotation, the 12-bar Starter bundle lets you test flavours in training, while the 18-bar Essential and 36-bar Complete bundles cover a full training block, the race and its drop bags.

The one idea to carry to the start line is that you cannot out-eat a hundred, so the race is really a long exercise in managing a deficit you can never close. Fuel early while it is easy, hold the rhythm through the heat, feed yourself through the night when every instinct says not to, and keep a trickle going to the line. The runners who finish are rarely the most talented. They are the ones who kept eating when it stopped being fun. If you want to go further, the drop bag and aid station guide and the mid-race nausea guide are the natural next reads, and a Starter bundle with two of each flavour is a simple way to find out, in training, which flavours your stomach will still accept at hour twenty.

Sources: Stuempfle and others, Race diet of finishers and non-finishers in a 100 mile (161 km) mountain footrace, Journal of the American College of Nutrition (2011); American College of Sports Medicine, Academy of Nutrition and Dietetics and Dietitians of Canada, joint position statement on nutrition and athletic performance (2016); Tiller and others, ISSN position stand: nutritional considerations for ultra-marathon training and racing (2019); Phoenix Bar values from our own pack labels.