I read a lot of people saying you shouldn't fuel with sugary/high GI (Glycaemic Index)1 foods, as you'll get a blood sugar spike, an insulin release to cope with it, and then a subsequent blood sugar crash afterwards that will leave you feeling crap and with little energy. I can understand this if you're just sat on a sofa doing nothing, but have there been any studies monitoring blood sugar in cyclists/athletes as they take in something like energy drinks and exercise at medium-high intensity?

The body can apparently absorb 60-90g of carbs an hour - is there a problem with taking on that amount of carbs in one go (e.g. a common combination of maltodextrin/glucose/fructose) and then cycling for an hour, rather than spreading it over an hour?

I've tried riding with low GI foods and they just didn't seem to give me enough oomph, maybe okay for relaxed touring but I tend to like making progress even if I'm not racing. The boost from an energy drink or handful of dates doesn't last too long, granted, but I can definitely feel it when it's there.

Footnote 1: Glycemic index is a measure of how quickly food is digested and how quickly it affects blood sugar. The GI of pure glucose is 100, and foods are measured in relation to that. High GI foods include white rice, white bread, energy gels, and similar foods which have simple carbohydrates. Fiber, fat, and protein lower the GI. More information can be found on Wikipedia.

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    People say many things. My favorite so far is a construction consultant who claimed that water drips upwards.
    – ojs
    Commented Jan 10, 2021 at 12:23
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    Can someone who knows what "GI" is edit the question to add that information so that the question and answer are self-contained and readers don't have to go elsewhere to Google the information and possibly get the wrong meaning? The most common meaning of "GI" is "gastro-intestinal," which doesn't make sense in this usage.
    – shoover
    Commented Jan 11, 2021 at 18:08
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    @shoover I thought GI was Glycemic Index - The lower the GI of a specific food, the less it affects your blood sugar levels. Something like Peanut butter is low GI, but icecream is high GI.
    – Criggie
    Commented Jan 12, 2021 at 9:24
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    Added the full words into the question, but yes thought it was an accepted concept in this context that didn't need explaining.
    – Wilskt
    Commented Jan 12, 2021 at 11:02
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    GI is an established concept, and I knew about it, but I doubt everyone does. I added an explanation and a link.
    – Weiwen Ng
    Commented Dec 5, 2023 at 16:05

3 Answers 3


During a short ride you mostly fuel with pure sugars and as simple as possible - so mainly glucose in gels. For longer events you also want more complex sugars that take longer, but still mostly carbs - rice cakes, croissants, sandwiches, musli (granola) bars.

You are correct that you cannot sustain a long ride (a road race) just from the glucose gels, you will get hungry. But it is possible for short two hour or slightly longer events. Professionals in Ski Classics mostly only use energy gels and drinks for the long-distance skiing. Cyclocross events are one hour or less and you won't see any eating during that, just some drinks or perhaps a gel.

It is impossible to overeat when going full gas. Your concern is to give your body enough fuel in frequent small doses.

Large amounts of proteins and especially fats are too difficult to digest during the race, you have them later for dinner.

You were asking for studies and this one should be of interest for you: International society of sports nutrition position stand: nutrient timing

I select just one paragraph:

Extended (> 60 min) bouts of high intensity (> 70% VO2max) exercise challenge fuel supply and fluid regulation, thus carbohydrate should be consumed at a rate of ~30–60 g of carbohydrate/h in a 6–8% carbohydrate-electrolyte solution (6–12 fluid ounces) every 10–15 min throughout the entire exercise bout, particularly in those exercise bouts that span beyond 70 min. When carbohydrate delivery is inadequate, adding protein may help increase performance, ameliorate muscle damage, promote euglycemia and facilitate glycogen re-synthesis.

They recommend every 10-15 minutes.

It is supported in the article body:

For example, Fielding and colleagues [50] required cyclists to ingest the same dose of carbohydrate every 30 min or every 60 min over the course of a four-hour exercise bout. When carbohydrate was ingested more frequently, performance was improved.

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    It is impossible to overeat when going full gas. Not if you eat so much that the food doesn't stay down... Commented Jan 9, 2021 at 19:39
  • That's perfect thank you, that should give me lots of followup reading. From the study referenced in your last paragraph, "Blood glucose was significantly elevated in the 60-minute-group 20 minutes post-feeding, but had returned to control levels by 50 minutes. In the 30-minute-group, blood glucose was maintained at a constant level throughout the entire 4 hours."
    – Wilskt
    Commented Jan 9, 2021 at 21:33
  • It also states "Despite the differences in blood glucose levels between the two trials, there were no significant differences in the rate of muscle glycogen utilization" but I don't really know how to interpret that, and I can only access the abstract so can't see the detail. Not sure I need to go that deep though.
    – Wilskt
    Commented Jan 9, 2021 at 21:34
  • @Wilskt I think you should be able to access the full text at journals.lww.com/acsm-msse/Abstract/1985/08000/… Commented Jan 9, 2021 at 22:54
  • Got it, thanks. Interestingly they were only giving around 21g of carbs per hour, for four hours, and the riders were doing 20 minutes of 50% VO2MAX along with 10 minutes of 30s 100% VO2MAX/2.5 minutes of recovery, so a decent workout, and as far as I understand it both groups and the control group depleted around the same amount of glycogen. So eating only 21g of carbs an hour does nothing to boost your glycogen stores! Will look through some of the other articles referenced in the 'nutrient timing' link.
    – Wilskt
    Commented Jan 10, 2021 at 0:21

For what it's worth, I participated in an ultraskate 2 years ago. The principle is easy enough: push on a skateboard as far as you can in 24h.

Skateboarding is less efficient than cycling, but I think the effort is comparable. You'd just go faster and further on a bicycle.

My goal was to push at least 200 miles (~320km). It's not easy to eat 4000+ kcal while riding a skateboard, so I decided to try out Tailwind. It's basically a powder with 99% glucose/sucrose and salts that you mix with water. There are other alternatives available (e.g. Skratch's Superfuel or Maurten's drink mix), but I didn't try them yet.

I drank half a liter every 30 minutes. Each liter had 75g sugar, for 300 kcal of energy. During training, I tried to drink 1l at the beginning of each hour, but it was too much at once.

I rode 50km every 3h, with 10 minutes of pause at the end of each 50km section.

I never had a too-full belly, I never had cramps and was never hungry. Just for curiosity, I tried to eat a sandwich after 10h but it really didn't feel good at all and I stopped after the first bite.

After 16 hours and 16 liters (and 1.2kg of pure sugar!), I was really fed up and couldn't drink anything anymore. On the other hand, I cannot think of any other liquid (even pure water), I'd happily drink 16l of. I finished my goal by riding 205 miles, and went to sleep.

I never felt a blood sugar spike, I didn't get bellyaches and didn't get diarrhea.

The next day, I was really hungry and ate 3 full menus in one afternoon.

Some friends tried the exact same product and they got diarrhea after 1l. Be sure to thoroughly test any nutritional product before using it during races, and try it while riding.

During Christmas, I was reminded that I feel really bad if I eat too much sugar while doing nothing else. So I definitely can get sugar spikes, but apparently not while exercising.

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    Yeah that sounds like my experience too, although I've never continued drinking that much sugar for that length of time. I think when I try to fuel with normal food I just don't eat anywhere near enough - 60g of carbs is 6 ham sandwiches, so an all-day ride would be almost 40 ham sandwiches! Even with energy drinks I'm probably underfilling.
    – Wilskt
    Commented Jan 10, 2021 at 13:04
  • @Wilskt: Indeed, it's not easy to eat that much without getting digestion problems, especially during continued effort. Tailwind worked really well, but some friends tried homemade alternatives (e.g. runographer.com/2015/12/… or blogbyben.com/2020/10/my-take-on-homemade-tailwind.html) and had good experiences. I just wanted to remove one variable during preparation, so I simply bought the nutrition package. Commented Jan 10, 2021 at 14:13
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    The norm here is not to favor specific products (exceptions made for Kool Stop rim brake pads, because they are that much better than cheap pads), so I'll just add that Tailwind isn't the only maker of these high-carb drinks. Skratch's Superfuel and Maurten's drink mix are two comparable ones I'm aware of. I am not aware of a standard term for this type of drink mix, however.
    – Weiwen Ng
    Commented Jan 10, 2021 at 17:26
  • @WeiwenNg Thanks for the comment. I added the two products you mentioned, so that my post doesn't look like an ad for Tailwind. I don't know how to describe those drinks either. It's surely not "energy drink". We were specifically told not to drink them during the event. Some tried a few years ago, they puked all over the track and had to go to the hospital after very bad stomach pains. Commented Jan 10, 2021 at 17:39
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    Your last comment does raise an interesting issue all on its own that's not directly on the original topic but is a bit related: Gastrointestinal distress. It's a thing in very long endurance events. And everyone's stomach is different. On a solo century last year, towards the end I didn't feel like eating any gels or drinking any energy drinks. My stomach was just a bit off. That could have been partly due to me noticing some crud that I hadn't cleaned out from my water bottle valves (NB: clean them).
    – Weiwen Ng
    Commented Jan 10, 2021 at 18:32

For what it’s worth: Energy gels are mostly glucose (sometimes a mix of glucose and maltodextrin). As far as I’m aware most professional athletes (my sister included) use such gels (or other glucose source) during competitions.

However, even on hard rides you shouldn’t need food if the ride lasts less than an hour. For rides longer than an hour it really depends on the intensity. I think most pros try to max out on the 60–90g carbs you’ve mentioned for longer competitions.

A lot can also be in your brain. I dimly remember a study where even pure glucose had a ~15 minute delay from ingestion until a significant rise in blood glucose levels. Yet you sometimes feel immediately better after a sugary snack.

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    This is probably more for all-day efforts, I can manage maybe 4-5 hours on water and 'energy food', but beyond that I need to stop for lunch. I have had full days on the bike where I have actually taken in less calories than I would have if I were just sat at my desk at work! You can't drink tiramisu from a bidon though.
    – Wilskt
    Commented Jan 9, 2021 at 21:36
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    @Wilskt not with that kind of attitude you can't
    – Paul H
    Commented Jan 11, 2021 at 20:00
  • At 90g carbs per hour I doubt you are actually running out of energy/food. I think it’s more likely that you are simply exhausted and/or that pure glucose is not very filling, so that’s why your stomach feels empty. You could try eating solid foods. On longer, less intense rides it doesn’t have to be pure carbs.
    – Michael
    Commented Jan 11, 2021 at 20:32

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