When it comes to running, the part I find most confusing isn’t pace or route planning – it’s fuelling. I can stick to my splits and memorise a loop. Getting my fuel right, however, has always felt like a question mark. I’ve never loved that it’s essentially a trial-and-error experiment.

Yes, there are guidelines. 30-60 grams of carbohydrates per hour for runs lasting longer than 60 to 90 minutes, rising to 60-90 grams once you cross the two-hour mark. But those are wide ranges. And within that large bracket, where are you supposed to sit?

Then there’s the growing narrative that more fuel is better. That runners should ‘train their gut’, pushing carb intake higher and higher to maximise performance. Scroll social media for five minutes or search fuelling strategies online, and you’ll fall into a rabbit hole of gels, glucose-fructose,and stories of athletes pushing higher numbers.

But can a blanket recommendation of 30-60 grams really work for everyone? Do I, a 5ft 3in woman, need the same hourly intake as my 6ft 5in boyfriend? Off the course, I could happily out-eat him at a post-race meal. Mid-run, though, logic suggests his body might need far more fuel than mine.

Until recently, the only real way to figure out what works was guesswork – unless you were an elite athlete with access to laboratory testing. Now, personalised carbohydrate testing is beginning to shed light on how much fuel you, as an individual runner, need, absorb and actually use, rather than relying on broad guidelines. I tried it out, and got some expert advice on finding your own magic carb number…

Why are carbs important for runners?

“Carbohydrates are, simply put, the body’s preferred energy source,” says nutritionist Hannah Norris (@hanutrition). “Unless sprinting, runners will be using the aerobic energy system. Carbohydrates provide quicker, easier energy, meaning the runner can maintain a higher output and therefore get fitter and perform better. She also notes that they’re also important for endurance athletes in maintaining hormone and bone health.

Once eaten, carbohydrates are broken down into glucose, which enters the bloodstream and is either used immediately or stored as glycogen in the muscles and liver for later use. But glycogen stores are limited. Even when well-fuelled, the average person only has approximately 2,000 calories of carbohydrates stored as glycogen. Just like, as the old saying goes, you can’t drive a car with no fuel. The same applies with running.

When glycogen stores drop too low, your body is forced to rely more heavily on fat as a fuel source. Fat stores are abundant, but fat can’t be broken down as quickly to sustain high-intensity running. The result is that familiar heavy-legged feeling, a drop in pace, and the sense that you’ve suddenly hit a wall.

Carbs & running FAQs

How many carbs do runners need?

“The goal is 30-60g of carbs per hour. Most people are able to tolerate that amount over one to two-and-a-half hours of exercise,” Hannah says. That’s why performance nutrition products such as gels and bars have largely been designed around these guidelines. Most gels on the market contain between 20-30 grams of carbohydrates, aligning with the recommendation to fuel regularly during longer runs. Some newer options now push higher, offering 40 grams or more per serving to help runners reach higher hourly targets with fewer products.

Can I just take on more carbs for more energy?

Sadly, no. The intestine has a limit for how quickly it can transport glucose into the bloodstream, and exceeding that limit leaves carbohydrates sitting in the gut, causing bloating, nausea or an upset stomach.

Hannah highlights that evidence for over 90 grams is limited in runners and not “worth considering unless at very elite levels, and often come at a price of GI symptoms like nausea”.

If you’re running a marathon or an ultra, she says that higher levels can be beneficial, but stresses that “more is only better when you can tolerate it”.

“For most runners, hitting 60g of carb per hour is the performance sweet spot.”

Personalised carb testing FAQs

What is personalised carb testing?

Anything personalised in the world of sports performance conjures up images of white lab coats, oversized masks and wires tethering athletes to machines. You’d think that it would only accessible to elite athletes. However, thanks to technological advancements, sports testing is getting more available.

ExoAnalytics is one company offering at-home carbohydrate oxidation testing. The test finds out how much carbohydrate your body absorbs and uses during exercise, or in technical terms, your exogenous oxidation rate. I tried it out.

What happens during a personalised carb test?

The protocol takes around 90 minutes to complete and requires an indoor treadmill. Beyond that, the test itself is fairly straightforward.

Following a day during which you avoid certain foods and drinks, immediately before starting the test, you supply a breath sample by blowing into a vial. During the steady effort (I did a six-minute kilometre pace), you consume a measured carbohydrate drink every 15 minutes. In the final 10 minutes , you wear a metabolic mask connected to an app that records respiratory rate, calorie burn rate and VO2. One final breath sample, and the test is complete.

How do you get your results?

After testing, you receive a report showing how much of the carbohydrate you consumed during the test was processed through your digestive tract, absorbed into your bloodstream, transported to your working muscles and used for energy.

What do the results of personalised carb testing tell you?

During my test, I opted for the 90g drink, as that was the amount I had consumed during my 50K ultramarathon. My oxidative efficiency – the amount my body actually absorbed and used – came back at 77%. In practical terms, of the 90 grams I drank per hour, my body was able to oxidise around 70 grams. Using those results, ExoAnalytics suggested I aim for 87g an hour for the first two hours of exercise and then 99g an hour beyond that.

At first, that felt counterintuitive. If I’m only oxidising around 70 grams per hour, why not aim for 70? The answer lies in efficiency. Research suggests that peak carbohydrate oxidation occurs when efficiency sits between 80-90%. This means you don’t necessarily want to match intake exactly to what you’re currently oxidising. Instead, you want to fuel at a level that allows your body to operate within its peak efficiency range. By slightly increasing intake, the goal is to shift efficiency closer to the optimal zone, improving available carbohydrates for energy without overwhelming the gut.

Do I need to get a personalised carb test?

The experts I spoke to all agreed that the 30-60g per hour guidelines are a good enough starting point for many runners. But, as my results showed, what’s right for one runner may not be right for another.

If you suffer discomfort from fuelling during sessions, or just want to remove the trial-and-error factor from your race-day strategy, personalised testing can bring confidence to your running.

There’s no guesswork in my fuelling now. I know to the gram how much carbohydrate I need to get me over any finish line, feeling strong. The only problem now, of course, is that I can’t use fuel as an excuse for a bad run…