Sports dietitian Aidan Muir, founder of Ideal Nutrition, spends a lot of his time telling runners the same thing: you're probably eating less than you think during your runs, and it's likely costing you performance. Here's what he had to say about fuelling before, during and around your training.
Weight loss versus maintenance during race prep
Aidan's ideal scenario: get your body composition to a good place a couple of months out from your race, then spend the rest of your prep at maintenance calories, or even a small surplus, so your body can actually recover and adapt from training rather than fighting a deficit at the same time.
The risk of staying in a calorie deficit through a heavy training block is what's known in the research as low energy availability, and it comes with real downsides: impaired recovery and adaptation, increased illness risk (particularly upper respiratory infections), loss of menstrual cycle regularity in women (a warning sign for decreased bone density and stress fracture risk, the kind covered in our piece on tibial stress fractures), and a reduced ability to tolerate the carbohydrate intake that would actually help you perform better.
One study Aidan references stuck with us: elite runners had their training volume increased by 50%, with one group given no dietary intervention and another force-fed a high-carbohydrate diet. The no-intervention group got worse, classic overtraining. The high-carb group actually improved, despite the sudden 50% jump in volume. It wasn't even the study's intended finding, just a striking side effect of adequate fuelling.
Why most runners are under-fuelling mid-run
Glucose is your body's best fuel source for running, and "hitting the wall" is essentially glycogen depletion in real time. Aidan's view is that intra-run fuelling is one of the most underappreciated levers available to recreational runners.
The numbers most people don't realise: glucose alone can be absorbed at up to roughly 60 grams per hour. Combine glucose with fructose (multiple carb sources), and that climbs to around 90 grams per hour. A standard energy gel contains 15-21 grams of carbs. Do the maths, and one gel an hour, which is what most recreational runners default to, is a fraction of what's actually achievable.
The good news: gut tolerance for carbohydrate is trainable, for everyone, not just elites. The practical approach is to start low (10-30 grams an hour, lower if you have a sensitive gut) and build a positive experience first, then progressively increase intake over successive long runs. You're aiming for the sweet spot on a U-shaped curve: too little limits the performance benefit, too much causes GI distress.
You don't need to train this every single run, once to three times a week, prioritising your longer or more intense sessions, is enough to build the adaptation without it dominating your overall diet.
Addressing the sugar fear
A lot of runners hold back on intra-run fuelling because of a general wariness around sugar. Aidan's take: in isolation, sugar isn't strongly linked to poor health outcomes once you control for total calorie intake, fibre intake, and overall micronutrient status. The one clear exception is dental health, there's a well-established link between frequent sugar exposure and tooth enamel erosion, which is worth being mindful of if you're fuelling with gels or sports drinks regularly over the years.
The fear of a "sugar crash" mid-run is also largely unsupported by the data. Reactive hypoglycaemia is genuinely rare. For most people, blood glucose rises, insulin brings it back down to a healthy range, and that's the end of the story, which lines up with the simple fact that race times consistently improve when runners fuel adequately during exercise.
Pre-run fuelling
The same underlying logic applies before you run as during it: keep your glycogen stores topped up. Relatively low fibre and low fat are the general rule pre-run, both slow digestion, which isn't ideal right before you head out the door. Start with an amount you know you can tolerate and build from there if you're sensitive.
Caffeine
Caffeine, taken before a run, can improve performance by roughly 1-3% on average, largely by reducing your perceived effort at a given pace. The typical effective dose sits around 3-6mg per kilogram of bodyweight. Coffee itself is surprisingly variable, one study testing espresso shots from cafes found caffeine content ranging from 25mg to 200mg per shot, with most landing around 80mg, which makes coffee alone an unreliable way to hit a precise dose if that matters to you.
Caffeine tolerance is real but relatively minor: going roughly 30% above your normal daily intake still produces a similar benefit to someone who doesn't drink caffeine regularly. For multi-day or ultra events, small ongoing "top ups" throughout the event (rather than one big dose upfront) have been shown to help, which also has an obvious upside for staying alert through sleep-deprived stretches of longer races.
Beetroot juice
A less obvious but genuinely evidence-backed option: beetroot juice, taken 2-3 hours before a run, works by dilating blood vessels via dietary nitrates. Research on recreational runners over 5K distances found a performance improvement of around 1.5%. A concentrated 70ml shot is the standard, practical way to get an effective dose (roughly 300mg of nitrate), since getting the same amount through whole vegetables would mean eating close to a kilogram of beetroot right before running.
Carb loading, properly done
For any event over 90 minutes, carb loading is worth doing. The protocol itself is simpler than the old-school approach of deliberately depleting glycogen first: just 1-2 days of high carbohydrate intake beforehand, no depletion phase required, since research hasn't shown the more complex approach to produce better results.
The standard target is 10-12 grams of carbohydrate per kilogram of bodyweight per day (scaled down to around 6-8g/kg for athletes with higher body fat percentages, since glycogen is stored in muscle tissue). For a 60kg runner, that's roughly 600-720 grams of carbs in a day, a genuinely large amount most people underestimate.
On the fear of feeling sluggish or bloated from a high-carb, high-sugar day: you cannot meaningfully gain body fat in a single day (a kilogram of fat represents roughly 7,000 calories), and given that even a full carb load still gets burned through during a long race, there's little basis for concern about short-term "damage" from the approach.
Fat as fuel, and why distance matters
Research comparing low-carb and high-carb approaches for endurance performance has consistently favoured higher carbohydrate intake, including in studies specifically designed by low-carb advocates to prove otherwise. That said, the picture shifts somewhat at truly long distances.
The logic: glycogen stores are limited, even a full carb load plus aggressive in-race fuelling still tops out around 2,000 calories worth of stored glycogen. Body fat, by contrast, is a virtually unlimited fuel source, even a lean athlete carries tens of thousands of stored calories. As events stretch well beyond marathon distance into ultra territory, the relative importance of fat oxidation increases simply because glycogen alone can't cover the full energy demand.
This also partly explains why the performance gap between men and women narrows significantly at ultra distances, women tend to be somewhat more efficient at fat oxidation, an advantage that compounds the longer an event runs.
Training your gut, specifically
There's a real physiological reason gut training works, and it's twofold: practising intra-workout fuelling improves your body's ability to actually oxidise (burn) the carbohydrate you're taking in, and it improves gastric emptying and absorption, meaning your stomach gets genuinely better at processing food while you're running. Skip the practice and go into race day trying to consume 90+ grams of carbs an hour for the first time, and you're very likely to run into GI distress regardless of how good the fuelling plan looks on paper.