Running vs. cycling: how carb needs and absorption really differ
Runners often assume their gut is worse at handling carbs than a cyclist's. The research says something more specific: your absorption machinery doesn't actually change between the two sports. What changes is how much mechanical load the runner takes on at the same time — and that's why the same on-paper numbers can play out very differently in practice.
Start with the part that stays the same. Studies comparing exogenous carb oxidation directly between running and cycling at similar effort found no meaningful difference in how much carb the body could actually process per hour. The gut's absorption ceiling — set by the sugar transporters in the intestinal wall, explained in why you can't absorb more than about 90g of carbs per hour — is a property of your gut, not of the sport you're doing it in.
So why does running so often feel worse? Because absorption capacity and comfort aren't the same thing. Running adds mechanical stress cycling doesn't: every footstrike bounces the stomach and raises intra-abdominal pressure in a way pedaling never does. That extra jostling — on top of the blood flow shifted away from the gut that any hard effort causes, covered in whether pace or power affects what you can absorb — is enough to tip a stomach that would have been fine on a bike into real trouble on foot.
The numbers back this up. Studies of ultra-distance events put gastrointestinal complaints at 70-85% of runners over multi-stage or 24-hour races. Comparable studies of cyclists found no link at all between what riders ate or drank and whether they had GI symptoms. Same fuel, same rough intensity, very different outcome — because the sport itself is part of the stress, not just the effort.
The practical takeaway isn't "eat less because your gut is weaker." It's "eat with less margin for error, because the same gram-for-gram plan has less room to go wrong." The carbs-per-hour ranges by intensity and duration in how many carbs per hour you actually need were built around cycling; running the same duration and intensity, it's sensible to aim for the lower end of that range, or a step below it, rather than the top.
This is also why gut training doesn't transfer perfectly between sports. A stomach that handles 80g/h comfortably on the bike hasn't been tested against footstrike impact at all — the mechanical stressor is simply absent from that training. See training your gut. In practice, this means that if you want to raise your carb tolerance for running, you need to do the progression and train your gut while running, not just on the bike.
Sources: Pfeiffer et al., Med Sci Sports Exerc 2011 (carb oxidation, running vs. cycling); Costa et al., Sports Med Open 2016 (GI symptoms in ultramarathon runners); Eur J Appl Physiol 2024 (GI symptoms and nutrition in a non-professional cycling event).