What is the minimum carb intake to not bonk (but also not put on weight)?

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.

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.

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.

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.

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/