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I’m asking about cycling, but do answer if you have experience with any endurance workout (running, cross-country skiing, …).

I’m eating a small snack (one banana, one nutribar, ..) every hour during endurance bike rides. The 100 calories provided by each snack replenish only a small fraction of what an hour of cycling uses.

When I started endurance rides (one good thing that came out of the pandemic), I bonked after 90 minutes. With some training, I could now last with water but no food for up to 3 hours, with not much difficulty, but I’m not sure that:

  1. that’s healthy,
  2. it brings out the maximum benefit of training,
  3. it enables remaining competitive enough to stay at the front on group rides.

How do I determine the minimal bounds of reasonable caloric intake during endurance (cycling) workouts?

Possibly related: Occasionally even elite cyclists will collapse during a race (https://www.theguardian.com/sport/2022/mar/22/italian-cyclist-sonny-colbrelli-recovering-after-collapse-at-volta-a-catalunya), though it's unclear whether that incident is nutrition-related.

Clarification

We can talk about two minimum fueling concerns:

  1. minimum fueling to avoid bonking,
  2. a considerably higher minimum fueling to remain at the front of a group.

In one example on July 13, 2022, Tadej Pogačar famously lost a stage to Jonas Vingegaard on the Col du Granon. One possibility discussed in chatters is that Pogačar was insufficiently fueled. Even if we mortals operate on a different level, the worry is the same. How do cyclists make sure they remain competitive on longer rides by not running out of fuel?

2 Answers 2

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If you consume adequate carbohydrate (8-12 grams per kilogram of body mass per day is currently recommended, but studies have found that professional athletes often consume as little as 4.5-6 grams without trouble), your muscles and liver hold approximately 500 grams of glycogen, of which around 80% (400 grams) of that is stored in the muscles. In highly-trained and carbohydrate-loaded athletes, total glycogen stores can be as high as 700 grams.

Whether or not you have been carbohydrate loading, a carbohydrate-rich pre-exercise meal 3-4 hours can further elevate muscle glycogen content, as well as restoring liver glycogen depleted after an overnight fast. Current recommendations are 1.4 grams per kilogram of body mass. In addition to that, evidence suggests that the ingestion of a minimum of around 0.5 grams of carbohydrate per kilogram per hour is required to improve performance.

Glycogen is the main substrate for exercise intensities over around 70% VO₂max, while maximal fat metabolism occurs at intensities of around 65% and 50% for athletes and the normal population, respectively. And from 65% to around 85% of VO₂max, muscle glycogenolysis, liver glycogenolysis and glucose uptake increase such that carbohydrate metabolism is predominant. In practical terms, this correlates with approximately 80-90 minutes of continuous high intensity (80-85% VO₂max) effort, or between two and four hours' at a lower intensity.

In conclusion, therefore, you can certainly ride for three hours without eating. It is entirely safe and healthful. However, carbohydrate consumption during the ride will improve your performance.

Personally, I have regularly ridden for over 12 hours without ingesting anything other than water. Muscle and liver glycogen is entirely depleted by that time, of course, and performance is greatly reduced. However, it has seldom presented any problems, except in the high mountains during which time higher power and torque output is unavoidable. In such cases, I have found myself prone to cramping. For my regular three-hour training rides, I do not ingest carbohydrate.

I hope that helps.

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  • Excellent answer and great details, but I'm not sure you're hitting exactly the point I'm after. Could you read the "clarification" in the question?
    – Sam7919
    Jul 17, 2022 at 12:51
  • The difficulty is determining requirements without knowing an approximate output profile in terms of VO₂max, as well as the duration of your ride. Were you saying that your rides are three hours, or sometimes longer? It is not normal to bonk on a three-hour ride, but if you are pushing the intensity throughout the ride, it is possible to do it in less. Needless to say, Pogačar and Vingegaard are riding at their limits for a sizeable portion of the race. Do you have some sense of your training zones during your rides?
    – POD
    Jul 17, 2022 at 14:12
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From the British Cycling website:

Carbohydrates need to be consumed early, in small amounts and frequently. Thirty minutes into a ride might seem too early but you are not eating for that moment, but for 15-30 kilometres down the road. You will need 0.5-1g of carbohydrates per kg of bodyweight each hour depending on intensity, and you should aim to spread that over 2-3 micro feeds every 20-30 minutes.

This is a fairly standard recommendation. A couple of things to note:

  • The amount of carbohydrates that you can process will depend on the exact form and composition of your energy drink/gel/snack. For example some modern gels use a maltodextrin/fructose mix which is supposed to aid absorption and enable you to take in more carbohydrates per hour
  • The amount of energy that you can take in is limited by your body's ability to process carbohydrates, regardless of your energy output, hence the .5-1g/kg/hour recommendation

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