Anne Haug's Whiteboard Lesson: The Real Science of Glucose and Fructose in Running Gels
Updated: 2 days ago
Written by Andy Hood · Ultra runner, cancer survivor and Runna ambassador
Published 14 September 2026
Ironman World Champion Anne Haug picked up a marker pen at a Club La Santa meet and greet and drew the clearest explanation of carb absorption I've ever seen. Here's the science behind her 60g-plus-60g whiteboard sketch, and what it means for how you fuel your next long run.
Spending time with the pros is something I have always taken advantage of when the opportunity presents itself. And that opportunity came again whilst I was at Club La Santa, Lanzarote, on what my family thinks is a family holiday but I secretly know is some warm weather training for my next ultra.
Anne Haug, Ironman World Champion and two-time Olympian, has been associated with La Santa for many years as a sponsored athlete. On Sunday evening she hosted a meet and greet session. Not one of the selfie-hunter sessions that exist to boost the ego of the host, but a genuine, relaxed conversational session. There was no preamble, no introduction slides, no video about how amazing an athlete she is, and she is a remarkable athlete. The hour with Anne kicked off straight away, inviting the audience to ask questions.
Contents
Who is Anne Haug?
For anyone who doesn't follow long-distance triathlon closely, a quick word on who was standing at that whiteboard.
Anne Haug is a German professional triathlete, a sports science graduate from the Technical University of Munich, and one of the most decorated athletes her sport has produced.
She came to triathlon relatively late, turning professional at 27 after years competing at a high level in duathlon and short-course racing, and represented Germany at two Olympic Games, London 2012 and Rio 2016.
Her move to long-distance racing in 2017 changed everything. She won her IRONMAN 70.3 debut in Lanzarote that year, and two years later became the first German woman to win the IRONMAN World Championship in Kona, overhauling Lucy Charles-Barclay on the run to take the title in 2019. She went on to win Challenge Roth three times, including a world-best full-distance time of 8:02:38 in 2024, before retiring from professional racing in 2025.
Lanzarote has been part of that story throughout, which is exactly why she was standing in front of a whiteboard in a Club La Santa meeting room rather than a lecture theatre.
The question I knew was coming
Within two or three questions the topic came round to nutrition. I had fully expected this to come up quickly, it's an area I have written about twice recently and one that occupies my thoughts frequently. For the average sportsperson it has fast become an incredibly confusing place. My article 110 Brands, One Stomach brings home how hard it is for the average person to make an informed choice on what products to use, and we're not just talking about in-race gels and other elixirs, but the dizzying array of influencers and companies telling you that ashwagandha, collagen, creatine, turmeric and a dozen others are like McDonald's secret sauce, but unlike McD's they are going to make you healthier, fitter or faster.
Anne summed this up beautifully. I had to resist a standing ovation here, I have history of solo standing ovations much to the embarrassment of those I have attended with, when she said succinctly, "it's all marketing with little to no science." Hooray, Anne. You had me 100% engaged at this point.
The whiteboard: two separate pathways, not one big tank
Take energy gels. There are too many on the market, and more entrants and new products every day. How does a runner, a cyclist, a triathlete, make the choice on which product will propel them to the finish line in great shape?
Anne, with marker pen and whiteboard, broke it down into the simplest terms. Drawing a circle representing a muscle cell, she wrote "Glucose, 60g" and a clean arrow to the muscle cell.
Anne, with marker pen and whiteboard, broke it down into the simplest terms. Drawing a circle representing a muscle cell, she wrote "Glucose, 60g" and a clean arrow to the muscle cell. "The body has a limit, in this pathway, the glucose pathway. The body can consume a max 60g of this carb per hour." Makes you think back a few years to when the headlines all preached that you should be aiming for a max of around 60g of carbs per hour.
The eagle-eyed among us will note that this figure has crept up in recent years, to 90g, and now north of 100, settling at around 120g per hour. How do we get to this figure if the body can only absorb 60g of glucose? Here Anne drew her second arrow, a distinctly separate pathway to the muscle cell: Fructose. This second pathway is separate to glucose, and the muscle cell can take on around another 60g of fructose. The magic 120g number.
It is, genuinely, one of the clearest explanations of exogenous carbohydrate metabolism I've ever seen drawn by hand on a whiteboard in about ninety seconds, and it happens to match exactly what the sports science literature has been saying for the past two decades.
Your gut absorbs glucose using a transporter protein called SGLT1. That transporter has a ceiling: pump more glucose in than it can move, and the rest just sits in your intestine causing exactly the bloating and cramping every runner dreads. Fructose doesn't use SGLT1 at all. It travels through a completely separate transporter called GLUT5, largely independent of the glucose system. Because they're two different doors into the body, you can run them at the same time.
Combining glucose and fructose increases total carbohydrate availability and allows for meaningfully higher oxidation rates than pushing glucose alone, which is precisely Anne's two-arrows sketch in a lab coat.
The research backs the scale of the effect up. Trials testing a 2:1 glucose-to-fructose gel found peak oxidation of around 1.4g of carbohydrate per minute, against roughly 0.8g/min for glucose alone and just 0.4g/min for fructose alone. Cyclists given a much higher combined dose, around 154g per hour of glucose plus fructose, pushed peak oxidation as high as 1.75g per minute, well beyond what a single sugar source can ever achieve, however much of it you swallow.
Why the "60g per hour" advice you grew up with is out of date
Fifteen years ago the standard advice genuinely was 30 to 60 grams of carbohydrate per hour, full stop, because the guidance was built entirely around what a single carbohydrate source could achieve. A single sugar oxidises at up to around 60g per hour even when you pour far more than that down your throat. The excess just isn't absorbed and oxidised, it sits there being a problem.
Once the two-pathway science was properly understood, the ceiling moved. Asker Jeukendrup's foundational 2014 review set out guidance that's still the backbone of most fuelling advice today: a single carbohydrate source tops out around 60g/h for exercise of two to three hours, rising to around 90g/h for ultra-endurance events, but only if that 90g comes from multiple transportable carbohydrates rather than one.
And the number keeps moving. A 2025 study on elite male marathoners, run jointly by Liverpool John Moores University and the University of Exeter, compared 60g/h (maltodextrin alone), 90g/h (2:1 glucose to fructose) and 120g/h (1:1 ratio) during race-pace running.
Both whole-body and exogenous carbohydrate oxidation rose at every step up, and running economy actually improved at the 120g/h dose, with a measurably lower oxygen cost than at 60g/h. The researchers were careful to flag that GI symptoms turned up across the board, and that proper gut training has to come before any of this is attempted in a race. But the direction of travel is clear: the ceiling that everyone assumed was fixed at 90g quietly moved again.
Glucose, maltodextrin, sucrose and fructose: what's actually in your gel
This is the bit most runners skip past on the label, and it's the bit that actually matters.
Glucose and maltodextrin both use the SGLT1 door. Maltodextrin is simply a chain of glucose molecules stuck together, produced by partially breaking down starch. Your gut enzymes chop it back into individual glucose units before it's absorbed, and those units then queue up at exactly the same SGLT1 transporter as plain glucose. As far as your gut is concerned, a maltodextrin-only gel and a dextrose-only gel are the same pathway, the same 60g/h ceiling, the same problem if you try to push past it.
Fructose uses the GLUT5 door, on its own. This is the pathway with genuine spare capacity, which is why almost every modern high-carb gel leans on it.
Sucrose is table sugar, and it's a clever shortcut. It's a glucose molecule and a fructose molecule bonded together, and your gut splits it apart before absorption, sending one half down each pathway. Research comparing glucose-plus-fructose against glucose-plus-sucrose found no meaningful difference in peak oxidation between the two, both around 1.4g/min. So a gel listing "glucose syrup and sucrose" on the ingredients panel is doing functionally the same job as one listing "maltodextrin and fructose". Don't be fooled into thinking sucrose is somehow the cheap option.
Rice, honey, dates and fruit purée are simply less processed carriers of glucose and fructose, aimed at runners whose stomachs rebel against syrup-based gels rather than at unlocking some third pathway. The transporter limits don't care what the sugar arrived wrapped in.
Why you can't just double up on one sugar
This is the trap I see beginners fall into constantly, and it's exactly what Anne's whiteboard was built to prevent. If you're taking 90g an hour of a gel that's entirely maltodextrin, you are not getting 90g of usable fuel. You're pushing 50% more through a door that can only open 60g wide, and the rest is going to sit in your gut fermenting, which is a fairly accurate description of how a stomach ends up feeling at mile 20.
Ratio matters, but it doesn't need to be exact. Most serious gels sit somewhere around 2:1 or 1:0.8 glucose to fructose, and the newer high-carb products are edging towards something closer to 1:1 to chase that 120g figure. What matters far more than the precise ratio is that both pathways are actually present on the ingredients label, in meaningful quantities, once you're trying to take on more than about 60g an hour.
So how many carbs per hour should you actually take on?
Duration is what should be driving this decision, not what the marketing on the sachet suggests.
Under 75 minutes: you need nothing. Your muscle glycogen covers it, provided you ate properly beforehand.
75 to 90 minutes: optional, and it won't make much difference either way.
90 minutes to around two and a half hours: 30 to 60g per hour, a single product type is fine at the lower end of that range.
Marathon distance and beyond: 60 to 90g per hour, from a genuine glucose-fructose blend, built up gradually in training.
Chasing a serious time, or racing an ultra, with the gut work already done: up to 120g per hour is now supported by the research, but this is the ceiling, not the starting point, and it's not for someone trying it cold on race day.
Gut training: how to handle more carbs per hour
Here's the part of Anne's talk that stuck with me most, and it's the part most amateur runners skip entirely. The SGLT1 and GLUT5 doors aren't fixed in size. A sustained high-carbohydrate diet genuinely increases both the density and the activity of your intestinal glucose transporters, meaning your gut gets measurably better at absorbing carbohydrate the more consistently you practise doing it, and the research also suggests this lowers your risk of the GI distress that stops so many runners hitting their fuelling targets in the first place.
Practically, that means building up your hourly carb intake in training the same way you'd build up mileage. Plan for four to six weeks of progressive intake during your hard sessions, nudging your hourly dose up by around 10g every week or two until you reach a comfortable new ceiling.
Nobody's gut adapts to 90g an hour by trying it for the first time three miles into a marathon.
Anne, as the athlete she is, has spent countless hours, months, years, perfectly fuelling, with access to the best advice, professionals and data that most of us never will. It's why spending time and listening to the pros is so valuable, and I'd encourage you to do it whenever the opportunity arises.
The bit Anne didn't need to say
She never once mentioned a brand name. Not hers, not a sponsor's, not anyone's. An hour on fuelling, delivered by a World Champion with a marker pen, and the entire thing was built on transporter biology rather than a single product claim. Given what I wrote in 110 Brands, One Stomach about a market of over 110 manufacturers competing to convince you their formulation is somehow different, there's a lesson in that alone. The science doesn't care whose logo is on the wrapper. It cares whether both doors are open.
Carbs per hour for running: frequently asked questions
How many carbs per hour can you absorb when running?
Runners can absorb around 60g of carbs per hour from a single carbohydrate source, such as glucose or maltodextrin, and up to 90 to 120g per hour when glucose is combined with fructose. Glucose enters the body through a gut transporter called SGLT1, which tops out at around 60g per hour. Fructose uses a separate transporter, GLUT5, so combining the two sugars uses both pathways at once. How much you can absorb at the top end depends on training status and gut adaptation.
How many carbs per hour should I take during a marathon?
Most marathon runners should aim for 60 to 90g of carbs per hour from a glucose-fructose blend. Well-trained runners who have practised high-carb fuelling can take up to 120g per hour. A 2025 study of elite male marathoners by Liverpool John Moores University and the University of Exeter found that taking 120g per hour improved running economy compared with 60g. However, gut symptoms appeared at every dose tested, so any target should be built up in training first.
How many carbs per hour for a half marathon?
For a half marathon lasting 90 minutes to two and a half hours, 30 to 60g of carbs per hour is enough, and a single type of gel works fine at this level because you are still within the 60g per hour glucose limit. Between 75 and 90 minutes, fuelling is optional and makes little difference either way. Runners finishing in under 75 minutes don’t need to fuel during the race at all, provided they ate properly beforehand, because stored muscle glycogen covers the effort.
How many carbs per hour for an ultramarathon?
Ultramarathon runners should aim for 60 to 90g of carbs per hour from a glucose-fructose blend, rising to up to 120g per hour once their gut is trained. Asker Jeukendrup’s widely cited 2014 review recommended around 90g per hour for ultra-endurance events, provided it comes from multiple types of carbohydrate. More recent research supports going higher. Runners whose stomachs struggle with syrup-based gels can use less processed options such as rice, honey, dates or fruit purée. These are absorbed through the same glucose and fructose pathways, so they don’t get around the absorption limits.
How many gels per hour should I take?
Most runners need two to three energy gels per hour to reach 60 to 90g of carbs per hour, because a typical gel contains between 20 and 40g of carbohydrate. Gels at the higher end of that range mean fewer sachets. Always check the grams of carbohydrate on the label rather than counting gels, and spread your intake evenly through each hour.
When should you take your first gel in a marathon?
Take your first energy gel around 30 to 45 minutes into a marathon, then one every 20 to 30 minutes after that. Starting early keeps blood glucose topped up before your glycogen stores run low. Runners who wait until they feel tired are already behind, and catching up on fuel late in a marathon is difficult because the gut can only absorb carbohydrate at a limited rate.
What is the best glucose to fructose ratio in energy gels?
Most energy gels designed for higher carb intakes use a glucose to fructose ratio of around 2:1 or 1:0.8. Newer high-carb products are moving closer to 1:1 to support intakes of up to 120g per hour. In research, a 2:1 glucose-fructose mix reached peak carbohydrate oxidation of around 1.4g per minute, compared with roughly 0.8g per minute for glucose alone and 0.4g per minute for fructose alone. The exact ratio matters less than having both sugars present in meaningful amounts.
Maltodextrin vs glucose: is there a difference in energy gels?
There is no meaningful difference between maltodextrin and glucose for absorption in energy gels. Maltodextrin is a chain of glucose molecules that the gut breaks back down into glucose, which is then absorbed through the same SGLT1 transporter. A gel made only from maltodextrin therefore has the same 60g per hour ceiling as a gel made only from glucose. Sucrose (table sugar) is different, because it splits into one glucose and one fructose molecule and so uses both absorption pathways.
How do you train your gut for running?
Gut training for running means gradually increasing how many carbs you take per hour during training. Add around 10g per hour every week or two over four to six weeks, practising during your hard sessions. A sustained high-carbohydrate diet increases both the number and the activity of the gut’s glucose transporters, so you absorb more carbohydrate and are less likely to get stomach problems on race day.
Is 120g of carbs per hour too much for runners?
For most recreational runners, 120g of carbs per hour is more than they need and more than an untrained gut can comfortably handle. The 120g figure comes from research on elite athletes who built up to that intake specifically. For most club runners and everyday marathoners, 60 to 90g per hour from a glucose-fructose blend, practised in training, comfortably covers most races.
Disclosure: Anne Haug has no commercial relationship with this blog. I attended her meet and greet session as a guest of Club La Santa.
About the author: Andy Hood is an ultra and endurance runner, cancer survivor, and unapologetic advocate for men's health and Runna Ambassador. Follow his runs, read his race reports, and grab a pair of Check Ya Balls at runningwestwardho.co.uk.
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