Carbs per hour: How much your body can really absorb

120 grams of carbohydrate per hour: the number has been making the rounds in cycling for a few years, ever since WorldTour pros started talking openly about their fueling. At the same time, plenty of amateurs ride long sessions on half an energy bar and wonder about the collapse in hour three. Between those extremes sits a surprisingly well-researched truth that is worth knowing.

Why carbohydrate is the limiting factor

Your body stores carbohydrate as glycogen in muscles and liver, roughly 400 to 600 g in trained athletes. That sounds like a lot but equals only about 1,600 to 2,400 kcal. At high intensity, when fat can't deliver fuel fast enough, those stores are noticeably depleted after 90 to 120 minutes. The classic „bonk" is nothing more than an empty glycogen tank. If you ride longer than two hours with ambition, you have to refuel, otherwise the tank dictates the pace, not your legs.

The bottleneck isn't the stomach, it's the gut

For a long time the rule was: the body can't use more than 60 g of glucose per hour. The reason sits in the small intestine: the glucose transporter SGLT1 saturates at about 1 g per minute. Anything beyond that isn't absorbed, it sits in the gut and causes trouble.

The breakthrough came with the finding that fructose uses its own transporter (GLUT5). Combine glucose and fructose and both channels run in parallel, pushing measurable utilization well above 1.5 g per minute (Jentjens & Jeukendrup 2004). That's exactly why modern gels say „dual source" or „2:1", and why 90 g/h is possible at all, but only with mixed sugar sources.

The rules of thumb by duration

The widely cited framework by Asker Jeukendrup (2014) scales intake with the duration of the effort, and its core still stands today:

• Under 1 hour: no intake needed. Interesting aside: merely rinsing your mouth with a carbohydrate drink measurably improves performance in short, hard efforts, the brain responds to receptors in the mouth (Carter et al. 2004, Chambers et al. 2009).
• 1 to 2 hours: about 30 g per hour.
• 2 to 3 hours: about 60 g per hour, glucose alone still works here.
• Beyond 2.5 to 3 hours: up to 90 g per hour, necessarily as a glucose-fructose mix. That is also the upper limit in the official sports nutrition guidelines (Thomas et al. 2016).

The 120-gram trend: what's actually behind it

Pros now often race on 100 to 120 g/h. The research here is younger and, honestly, not settled. What has been shown: with a fructose-to-glucose ratio of about 0.8:1 instead of the classic 1:2, more of the ingested energy is actually burned at 120 g/h (Podlogar et al. 2022). In mountain marathon runners, 120 g/h reduced markers of muscle damage compared to 60 and 90 g/h (Viribay et al. 2020). What has not been cleanly shown: a clear performance advantage of 120 over 90 g/h for the rest of us. The honest summary: 90 g/h is the well-supported standard for long, hard sessions. 120 g/h is a specialist strategy for very long race days that requires a trained gut.

Gut training: absorption is trainable

Absorption capacity is not fixed. Regularly fueling during training measurably increases carbohydrate utilization and reduces gastrointestinal complaints (Cox et al. 2010, Costa et al. 2017). In practice, that means race nutrition gets rehearsed in training just like intervals. Try 80 g/h for the first time on race day and you'll most likely get a stomach problem instead of a performance benefit. Start at an amount you tolerate well and build up over weeks.

Before and after the session

The hour on the bike is only half the equation. Daily intake scales with volume: the guidelines suggest roughly 5 to 7 g per kg of body weight at about one hour of training per day, 6 to 10 g/kg at one to three hours, and 8 to 12 g/kg on extreme days (Burke et al. 2011). After hard sessions, glycogen refills fastest at about 1 to 1.2 g/kg per hour in the first few hours, which matters most when the next session comes soon. And before a hard day, it pays to top up the stores the evening before rather than at breakfast.

How to put this into practice

Nobody has to do this math in their head. In trace, every planned session comes with a concrete fueling plan: from which minute, at what interval, how many grams, matched to the duration and intensity of the session. On top of that there's a daily target that distinguishes rest days, easy days and race days, and before a hard morning trace reminds you in the evening to top up. For racing, the platform computes a complete fueling strategy with carbohydrate and fluid amounts from your actual data. Your own inputs, say a proven race nutrition setup, are taken literally instead of being overwritten.

The short version: under an hour you need nothing, from two hours on about 60 g/h, on long hard sessions 90 g/h from mixed sugar sources, and everything beyond that is a trainable extra for long race days. The biggest mistake isn't the wrong number, it's having no plan at all.

Sources

• Jentjens & Jeukendrup (2004): Oxidation of combined glucose-fructose ingestion, Journal of Applied Physiology
• Jeukendrup (2014): A step towards personalized sports nutrition, Sports Medicine
• Carter et al. (2004), Chambers et al. (2009): Carbohydrate mouth rinse, MSSE / Journal of Physiology
• Thomas et al. (2016): Position Stand Nutrition and Athletic Performance, ACSM
• Podlogar et al. (2022): 120 g/h at a 0.8:1 ratio, European Journal of Applied Physiology
• Viribay et al. (2020): 120 g/h in a mountain marathon, Nutrients
• Cox et al. (2010), Costa et al. (2017): Gut training and tolerance
• Burke et al. (2011): Carbohydrates for training and competition, Journal of Sports Sciences