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Going on long bike rides (or otherwise doing any form of endurance workout) can result in "hitting the wall" or "bonking".

The well-known solution is to be mindful of one's carb intake.

But very many folks go on long bike rides (or other endurance workouts) precisely for this reason: to force their bodies to break down fat.

Can someone who is not trying to lose fat (and who really doesn't want to lose weight) take some inspiration from those folks and avoid hitting the wall? Surely they do not practice carb loading (before), gel supplements (during), and glycogen replenishment (after) a ride.

Why is it interesting (and important) to know—even for those who are not trying to lose weight, or who actively do not wish to lose weight? Because it is easy to overdo the starch intake, and so knowing what one should do at a minimum to avoid bonking will also stand as a useful limit. In other words, the cyclist can then "do this" and also take some extra carbs, rather than simply indulge in carbs "to be on the safe side".

It is clear that by erring on this minimum one will be taking the risk of losing fat, but then it appears that even the most athletic still have fat.

I hope an answer is known without using supplements, by using only what grows on trees or is raised in farms, rather than what comes out of a chemistry lab or a factory.

Two options are possible:

  1. Conceivably, the answer may be "it cannot be done," or: taking inspiration from those who succeed in losing fat means that one must also take a risk, and routinely suffer from bonking.
  2. Earlier this summer, with gyms closed, I bonked on two occasions right after picking up road biking, but this was a fluke. My "glycogen stores" were pathetically low. They may have now improved enough that I need not worry about bonking.

Epilogue

To appreciate just how difficult it is to get this right, read the comments under this answer.

In retrospect, the question I wanted to ask (too late now!) is: What is the minimum carb intake that will be certain to neither: 1- make me bonk, nor 2- "cannibalize" my muscles. When initially asking the question, I expected a simple continuum. Imagine a line with zones A, followed by B, followed by C. The diet in zone A ensures no bonking (that's easy: eat lots of carbs. Drawback: it'll also be too easy to put on weight). The diet in zone C makes the rider lose fat (as many questions here show, that's hard). The surprise is that even identifying just zone B—the objective of this question (no bonk, no targetting of fat) is hard. Oh well!

And if all this is not tough enough to understand, I don't actually mind losing fat, but since my BMI is quite healthy, I suspect my body will end up rebuilding these fat reserves again. The next question in this sequel is then: does one's liver get exhausted from so much work (metabolize fat cells, rebuild fat cells, repeat) ? If you understand enough about all this, feel free to formulate that question as a proper question.

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  • One important point is to approach it slowly, with longer and longer exercise sessions, over a period of months. This helps build up the body's ability to tap the energy in stored fat. Oct 9 '20 at 17:35
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    I'll offer a suggestion for your research, mainly because I don't have much experience: Fasted cardio apparently allows the body to more quickly switch to burning fat as an energy source. But, avoiding the bonk... For me, a physically unusual specimen in the biking world, it became a matter or trial and error. Did I bonk at mile X last time? Eat at mile X-3, let's say. That sort of thing. Not a very satisfying answer, for sure.
    – Andrew
    Oct 9 '20 at 17:50
  • Not sure that weight control and traditional bonking prevention methods are as incompatible as the OP presumes. Its important someone exercising does not run out of energy, as it discourages further effort and subsequent gains in fitness and can cause the bodies feast/famine responses to kick in. The time to reduce calorie intake for weight control is post recovery. (i.e. Skip the afternoon tea cream bun, not the pre ride gel)
    – mattnz
    Oct 28 '20 at 9:18
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What is the minimum carb intake to not bonk (but also not put on weight)?

Simple Answer: 30-60g of carb per hour to not bonk (can't comment on the weight bit though).

*Real answer: There is no calculable answer for these reasons:

  1. The type of fuel you burn is dependent on the intensity of the exercise (heart rate zones)
  2. The amount of fuel you burn is dependent on your efficiency
  3. The starting amount of fuel is determined by your diet and build

Fuel Type

At "walking intensity", you burn fat, and because fat is so energy dense even a very lean person could walk for days without running out of fuel and bonking.

At higher intensity, you burn a mix of glycogen store from your muscle and sugars being digested from the food in your gut.

At very high intensity, all energy is directed to your muscles and so your digestion stops and you will be relying purely on glycogen stores.

Note that fitter athlete will be putting out more power for the same heartrate, so how your body responds is specific to you.

This conversation is about carbs, so you would need to know the intensity at which you'll be cycling to know what fuel types you'll use in what proportions.

Fuel Amount

Every body works differently, and many variables go into how efficiently you can convert fuel into power output. For cycling, things like how good your bike fit is, your fitness level, your years of experience on the bike, injuries and flexibility can all affect your efficiency.

For this conversation, this variable is basically unknowable. Professional athletes might have a good idea of their efficiency, but it would be pretty rough.

Starting Fuel Reserves

You start your ride with your glycogen stores and the contents of your stomach. It is easy to control the latter by making the meal before riding carb-rich. Glycogen stores can also be optimised by carbo-loading. There is a university study (that I can't find right now) that demonstrated that in the 24hr before an activity, the athlete could more than double their glycogen stores by eating ridiculous amounts of carbs.

For this conversation, your general diet and you diet in the last 24 hours would be very relevant.

Conclusion

While there is a generally accepted minimum carb consumption to avoid bonking (30-60g of carb / hr), the specific question also required not having surplus energy intake an so to avoid putting on weight. The question requires an exact calculation, which is not possible.

Relaxing the constraint on "exact intake" but keeping with the spirit of the question, the suggested solution is to eat lots and ride even more.

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Some people do ride all day without carb intake, or even without calories. They've adopted ketogenic lifestyles (and it does seem like a lifestyle rather than a diet, not something to be done lightly).

Most of the rest of us carb load, eat as much as we can during a ride, and recovery feed, settling in to a fairly constant weight. My first few 200km+ rides lost me weight even with all this intake, but they need an extra day's worth of food or even more to fuel them and you just can't take that in over the course of a ride.

If you're endurance riding with a goal of losing weight, you'll need some carbs during the ride. How much depends on you (weight, bike, effort level) but it's hard or even impossible to overdo it during the ride. You'll then burn a mixture of fat and carbs. The form of the carbs doesn't really matter. I do use gels but also cereal bars, and sandwiches plus fruit if I'm stopping at a shop.

Fasted training is good for increasing your ability to burn fat, but doesn't replace fuelling for the ride.

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Updated answer as focus of question has changed over time.

I think you are mixing up two different things:

  1. Fueling long or intense rides
  2. Maintaining weight over the long term

I would not stress over trying to ingest the exact amount of calories required to fuel a ride, so that your fat reserves are not increased or diminished. Focus on what works for fueling for you to perform on the ride. You may need to take on more carbs or you may be able to use fat reserves.

Maintain weight by paying attention to what you eat for normal meals and on non-riding days. If you find you are losing weight, eat a bit more. If you are gaining weight check that you are not over-fueling during a ride, and cut back a bit on non-riding days.

Original answer:

It's called fasted training. The basic idea is to work out with a low glycogen level (often in the morning) and train the body to convert fat to usable energy. Obviously this is something that requires a progressive training plan to be feasible.

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  • By roughly weekly fasted training rides I improved my ability to burn fat on long rides, noticeable by not needing carbs so frequently, and having a bit of warning of bonking. The first was my usual half hour commute (familiarity for a test) then a couple at one hour, after that 90 minutes, all at a decent effort. You lose it fairly quickly though - I never got back in to fasted rides after an injury and can really tell
    – Chris H
    Oct 10 '20 at 5:49
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    @ChrisH: You shouldn’t need any food on a 30 minute ride and even 90 minutes should be perfectly doable without. Glycogen storage is about ~2000kcal.
    – Michael
    Oct 10 '20 at 7:13
  • @Michael indeed. 90 minutes is a common fasted training ride; the shorter ones were tests as it was part of my commute so timing was critical. Even after the short fasted rides I had to be careful to stand up slowly after locking up my bike, or go very light headed.
    – Chris H
    Oct 10 '20 at 21:37
  • ... Without an overnight fast, I've also been known to do 4 hours/80km on just water. That's a related form of training that might appeal to the OP
    – Chris H
    Oct 10 '20 at 21:39
  • Understood. But I'm not really sure what you're saying. The first part of your answer suggests that—even someone who is not aiming to lose weight or fat—to be certain to avoid bonking, the cyclist is better off going on an completely empty stomach than after a small (starch-rich) meal. The second part suggests that one could still bonk going on an empty stomach unless one drops down the distance and/or the speed and ramps back up gradually. Is there a way to eat, but eat minimally, and be sure that bonking stays at bay?
    – Sam
    Oct 26 '20 at 18:26
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Obviously it depends a lot on the individual and the intensity. At walking intensity almost nobody is going to ”hit the wall”, no matter how long the exercise lasts. I don’t think there are any good estimates on how many carbs you have to supplement to sustain a certain intensity for a certain duration. From what I can tell, athletes in competitions which last more than about an hour supplement as much as they safely can, often ending up around 100g carbs per hour.

I’d just like to point out that even at relatively low intensity you’ll still burn more calories than you can replenish during a ride. 50g carbs per hour (that’s like two bananas) is already quite a lot and still only 200kcal while you are probably burning >500kcal.

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As others have pointed out, this question can't be answered exactly because there are too many variables. However, building off my previous answer for the amount of hydration required during a long ride, perceived hunger is often a decent guide to how much you need to eat.

For any given ride, you can start with a ballpark figure. For example, Clif Bar recommends 1-2 packets of its Clif Bloks energy chews per hour of activity; 1 packet is 25g carbs. As you ride, you can let your perceived hunger guide your consumption. You should take internal note of how much you are consuming, keeping in mind that you should adjust this for ride intensity and your own body weight. I'm not clear from the original question if you want to lose weight, but if you do, you can further adjust this amount down slightly - keeping in mind that you do want to avoid bonking.

For example, I am a relatively small athlete at 133 lbs. In my experience, I need about 1 packet (or the equivalent in terms of other food) every hour on long, high-intensity rides, and I am fine with about 1 packet every 1.5 to 2 hours on easier rides.

One minor caution is that in competitive and semi-competitive rides, you may want to try to eat ahead of your hunger. By the time you start to feel hungry on such a ride, it can be a bit too late and you would bonking unexpectedly and, in some contexts, getting dropped from a group. You would need to remember that products based on simple sugars still take some time for the sugar to become available to your muscles, and any solid foods (e.g. sandwiches, cookies, jerky) probably contain fat as well, which will slow digestion.

An unrelated observation: if you are consuming a lot of simple carbohydrates, do note that many professional athletes have poor dental health due to their very high consumption of sugary beverages. I would recommend rinsing your mouth with plain water after eating a sugary snack; this recommendation is on the Clif Bloks site and the product packaging. Most amateur athletes probably won't consume enough simple carbs for this to become a significant danger, but I think the issue is worth a quick statement.

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    But going reasonably hard suppresses hunger (unlike thirst). I'm far from alone in needing to eat independently of hunger sensation on long days. Similarly on fasted training rides, I'm only hungry when I stop - again like many people
    – Chris H
    Oct 27 '20 at 19:25
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Although the question is about quantity of carbs. I think it is relevant to mention the following also.

If you are going to consume a sugary drink...find one that is glucose based rather than fructose based or sucrose. Dextrose is also a trade name for glucose.

Explained most simply, sucrose is table sugar, which contains one molecule of glucose and one molecule of fructose. Our bodies (I believe the liver) need to break this sugar down into the glucose that we need. Glucose on the other hand is ready to be used by your brain and muscles, and will get you fuel much faster.

In addition, there is research suggesting that Fructose is the part about sugar that is bad for us.

This is oversimplified, but it is how it was explained to me by a good friend who was very educated on the subject.

Citations: https://academic.oup.com/ajcn/article/86/4/895/4649668 https://www.livestrong.com/article/13727382-fitness-benefits-medicare-uhc/

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