Glucose-Fructose 2:1: More Carbohydrates Per Hour

Most endurance athletes do not fail on race day because of their legs — they fail because of their gut. Sixty grams of carbohydrate per hour was the accepted ceiling for years, simply because one single transporter in the small intestine saturates at exactly that point. The 2:1 ratio of glucose to fructose sidesteps this limit by using two absorption pathways in parallel. Sixty grams become 90, and with an adapted gut up to 120 grams per hour. That is the single biggest lever in fuelling — bigger than any gel, any drink brand and any carb-loading protocol.

Why the gut is the limiting factor

Carbohydrates are not a homogeneous substance. Glucose, maltodextrin and most starch components are absorbed in the small intestine through the sodium-dependent transporter SGLT1. That uptake is saturable: in healthy, untrained people the limit sits at about 1 gram per minute, roughly 60 grams per hour. Anything beyond that overshoots the transporter.

What happens next is not a theoretical problem. Unabsorbed carbohydrate stays in the gut lumen, holds water osmotically and gets fermented by gut bacteria. The result is bloating, cramps, wind and in the worst case diarrhoea — the classic gastrointestinal blow-ups from kilometre 25 onwards that you can observe in practically every marathon field. The problem is not a lack of willpower, it is a transport bottleneck.

Fructose takes a different route. It is absorbed through the transporter GLUT5, independent of sodium and independent of SGLT1. That gives you two parallel channels that do not block each other — provided you feed both of them.

The 2:1 trick: two transporters instead of one

The foundational work on this comes from Asker Jeukendrup and his group in Birmingham. Across a series of studies they showed that combined glucose and fructose intake pushes oxidation rates clearly above the 60-gram mark — to about 90 grams per hour at a 2:1 ratio, and in later work with trained athletes to 105 and 120 grams per hour. A 1:1 ratio also works but shifts more sugar onto the fructose pathway and raises the risk of gastrointestinal distress, because fructose is absorbed more slowly and less completely in larger amounts.

What matters in practice: the ratio decides, not the total amount alone. A product with 90 grams of carbohydrate per serving made entirely of maltodextrin gains you exactly nothing above 60 grams per hour — it ends up in the colon. On labels this is easy to check: maltodextrin, glucose, glucose syrup and isomaltulose count on the glucose side, fructose and fructose syrup on the other. If the ingredient list only says "maltodextrin", the second channel is missing entirely.

To be honest about it: below roughly two and a half hours, the difference is small. One hour more or less in the glycogen tank does not decide a 10k race. The lever becomes big exactly where the stores inevitably run empty — half marathon from about 90 minutes upwards, marathon, gran fondo, Ironman, ultra.

How much per hour — the four bands

Intake recommendations are not a matter of taste, they hang on race duration. Four bands have proven themselves in practice:

• Under 75 minutes: no fuelling needed. Water is enough; glycogen stores cover the effort. A gel does no harm, but gains nothing.
• 75 to 150 minutes: 30 to 60 grams per hour; carbohydrate mouth rinses already act on the central nervous system. A 2:1 ratio is not mandatory here, but it is unproblematic.
• 150 minutes to 3 hours: 60 to 75 grams per hour, and from here the ratio should be right. A half marathon at 75 grams per hour from pure maltodextrin often still works — with 2:1 it sits calmer in most people's stomachs.
• Over 3 hours: 90 grams per hour as the target, 100 to 120 if well tolerated. This is the marathon, gran fondo, half Ironman and ultra band — and the band where the 2:1 ratio actually translates into performance.

Two side conditions that are often forgotten: first, the stomach needs fluid to let the carbohydrate solution leave — drink osmolality should sit below roughly 300 mOsm/L, otherwise emptying slows down. Second, electrolytes belong in there, above all sodium: 300 to 700 milligrams per litre prevent the high carbohydrate load from tipping into dilutional hyponatraemia. Details in the article on electrolytes for running.

What this looks like on race day

Theory only helps if it translates into a fuelling plan. A worked example for a marathon at a 3:30 target — 210 minutes:

• Total need: 90 grams per hour × 3.5 hours = around 315 grams of carbohydrate. That is roughly 1,260 kilocalories, more than a normal meal.
• Split: 210 grams of glucose and 105 grams of fructose. In practice that means six to seven gels with a 2:1 ratio (30 grams each) plus a sports drink, or a mix of drink, bars and gels.
• Timing: first intake 15 to 20 minutes after the start, then half a gel or a full gel every 20 minutes. Do not wait for hunger — at high intensity it never arrives, and by then it is too late.
• Fluid: 400 to 600 millilitres per hour, more in heat. If you carry a concentrate, you must dilute it at the aid stations — otherwise osmolality climbs into the intolerable range.

A five-hour gran fondo looks different because you can carry everything yourself: 500 to 600 grams of carbohydrate split across bottles, bars and gels. The bigger challenge there is not the amount but the logistics — anyone who notices at kilometre 150 that the second bottle is still untouched has already lost the plan. More on that in the article on gran fondo nutrition.

Training the gut: two to four weeks

The ability to process 90 grams of carbohydrate per hour is not a genetic trait, it is an adaptation. The gut responds to repeated high carbohydrate intake with more SGLT1 and GLUT5 transporters in the brush border membrane, faster gastric emptying and a changed microbial composition. That adaptation takes time — realistically two to four weeks with two or three sessions per week.

The build-up is gradual, never all at once:

• Week 1: one long session at 60 grams per hour with a 2:1 ratio. Observe how the stomach reacts — note bloating, do not interpret it.
• Week 2: two sessions at 75 grams per hour, at least one of them longer than 90 minutes. Test new gel flavours here, not later.
• Week 3: one session at 90 grams per hour over 2.5 hours. That is the dress rehearsal for the marathon.
• Week 4: 90 to 100 grams during the race-pace block of the long run. If that works, it will work on race day.

The important distinction is between training and simulation. An easy base session with high carbohydrate intake trains the gut, not the metabolism — fat oxidation is blunted in the process, which is suboptimal for adaptation to long efforts. So: high intake deliberately in a few key sessions, not on every easy loop. The systematic build-up is covered in the article on gut training.

Five mistakes that keep repeating

• Maltodextrin only. The most common mistake. A drink with 80 grams of maltodextrin per serving delivers the same 60 grams per hour as a simple one, costs more and tastes sweeter. Without a fructose fraction the second transporter stays unused.
• More on race day than in training. The classic: 60 grams in training, suddenly 100 in the race because "more must help more". The untrained gut answers with nausea, and the performance loss outweighs the gain.
• Fructose as the main source. A 1:1 ratio, or outright fructose-dominant products, sound progressive but overload the GLUT5 pathway. The result is precisely the discomfort the 2:1 ratio was meant to avoid.
• Carbohydrates without water. A gel without fluid draws water out of the intestinal wall and slows its own absorption. Every gel needs 150 to 200 millilitres of water, ideally at the aid stations.
• Fat and protein in race nutrition. Bars with a high fat or protein content slow gastric emptying considerably. Over three hours that can block the whole plan — the carbohydrates simply do not arrive in time.

What this means for training plans

Carbohydrate intake is not a constant in the plan but a function of duration and intensity — and it has to match the training phase. In the base phase with long easy runs, high intake is counterproductive for fat oxidation adaptation; in the race-specific phase it is mandatory. The article on carbohydrate periodization describes how that shifts across a training block, and the piece on energy gels puts the product formats in context.

The second point concerns race-day planning: a fuelling plan that only says "bring gels" is not a plan. It needs amount per hour, ratio, timing and fluid allocation — and it has to match the course profile, because aid stations are not always where the 20-minute rhythm would need them.

How Peakora calculates this

The fuelling calculator in Peakora does exactly this arithmetic: from your estimated race time and body weight, the engine derives carbohydrates per hour, the total amount, the 2:1 split in grams of glucose and fructose, plus sodium and fluid per hour. A 3:30 marathon produces different numbers than a 1:45 half marathon — and in heat the fluid component shifts upwards. The values are a starting point, not dogma: what your gut actually tolerates shows up in training, and adaptive planning takes into account how the sessions actually went. Anyone who wants to go deeper on the physiology will find the system in the marathon training guide and the definitions in the Peakora glossary.

Frequently asked questions about the 2:1 ratio