What is the minimum carb intake to not bonk (but also not put on weight)? Going on long bike rides (or otherwise doing any form of endurance workout) can result in &quot;hitting the wall&quot; or &quot;bonking&quot;.
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 &quot;do this&quot; and also take some extra carbs, rather than simply indulge in carbs &quot;to be on the safe side&quot;.
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:

Conceivably, the answer may be &quot;it cannot be done,&quot; or: taking inspiration from those who succeed in losing fat means that one must also take a risk, and routinely suffer from bonking.
Earlier this summer, with gyms closed, I bonked on two occasions right after picking up road biking, but this was a fluke. My &quot;glycogen stores&quot; 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- &quot;cannibalize&quot; 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.
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:

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

Fuel Type
At &quot;walking intensity&quot;, 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 &quot;exact intake&quot; but keeping with the spirit of the question, the suggested solution is to eat lots and ride even more.