Some physiological principles can inform the discussion:
- Riding burns a lot of carbs. The more intensely you ride, the more your energy comes from carbs.
- Simple sugars get to your muscles about 15-20 minutes after ingestion.
- Your muscle glycogen stores (this means readily available carbs stored in the muscles) are sufficient for most 1 hour rides, or maybe even 2 hours of endurance riding.
If you Google, I believe you'll find that at least 30-60g per hour is a typical recommendation. This is something like 1-2 energy gels, or one small granola bar, or similar per hour. How much exactly? I'd suggest experimenting. However, if it's a long ride, you are better off eating early on, even if not hungry. This will help keep your carb availability elevated through the ride. I have found that later in rides, I feel like I might benefit from a bite, but it's "only 10 miles to home" or something - this is a fallacy like the sunk cost fallacy; I should either have eaten more later, or eaten at the same time as I had that thought.
Some individuals and organizations who I think are well-informed (e.g. TrainerRoad, FastCat Coaching) are starting to argue that for intense rides over 2-2.5 hours, athletes can maximize performance if they take in 60-90 grams of carbs per hour, possibly even more (e.g. some products are geared for 100-120g per hour), discussed more below.
Some modern nutrition products are aimed at making large amounts of carbs digestible. Even if you are targeting 30-60g per hour, those products can still be useful on long rides, you just take less of them. Some people even dissolve sugar or maltodextrin powder in their water, or just bring maple syrup, discussed later. It does seem like drinking carbs is easier than eating even simple carbs, so this is something to consider if it's a very long ride and you physically don't feel like eating.
Note the change in emphasis in the answer. I may previously have overemphasized the 90+g carb recommendation. I'm not sure it's necessary for all riders. People looking to maximize performance on long hard rides can certainly think about it and experiment with it, but I'm not sure about the scientific consensus.
One proposal for high intake was made by Dr. Alex Harrison who posted this over at the TrainerRoad forum. I'm unable to find his original post, but this one references it. Some people argue the evidence for very high carb intake isn't clear (Andrew Coggan is one). Regardless, I think the high carb proponents aren't suggesting you do this all the time, definitely not for recovery rides or shorter endurance rides.
The idea is to make sure your muscles have a lot of glycogen available through the entire ride. And that does include taking in carbs at the beginning of the ride, even though you're not feeling hungry. Remember that this is to maximize performance on long and intense rides.
90g of carbs sounds like a lot. But keep in mind that even 90g of carbs is 360 calories. Over a 4 hours, that's 1,440 calories. Looking at my previous group rides (with a power meter), I've typically burned about that many calories in a 4-5 ride with 3.5-5 hours total moving time.note So, a lot of us may be near to or at calorie balance even with a super-high carb in take.
Of the people on the group rides I do, I think most of us don't even hit the lower end of this intake range. I don't, but I have fellow riders who aim for over 60g. 30-60g is likely sufficient to allow good performance for most riders in many group rides. In any case, inadequate carb intake can definitely explain a drop off in performance at the end of rides. If you are on a long and intense ride, you should probably think about increasing your intake.
Footnote: In these group rides, I was usually riding at 50-70% of my threshold power (FTP) in the draft, was in the draft most of the time, and took occasional pulls at around 90-110% of threshold. These felt well within my limit last year. Hillier rides will have higher average power, as will rides where you spend a lot of time on the front or you do a lot of town line sprints. I'm also a light rider at about 144 lbs, so most adult males will burn more calories. Last, the group I ride with is fairly fast, but not the fastest out there; really hardcore groups with a lot of cat 1/2 cyclists probably burn more fuel.
If you want to shoot for over 60g, some things to note are:
- Most carbs are going to need to be liquid or gels. You probably can't do gels alone, so you will need sports drinks formulated for this intake. This amount of carbs with solid food alone (e.g. cookies, sandwiches) is probably going to be a disaster.
- You probably need to train your gut to digest that much, even if it's liquid carbs. This may mean ramping your intake up over time. Don't let your first time on a high carb intake be at a race or important event.
- Specialty products are really helpful if you want to shoot for really high intakes, but there are people who brew their own drinks from sugar, including table sugar.
- Over 60g of carbs/hour, you may need to pay attention to the ratio of glucose to fructose if mixing your own, explained below.
Regarding the last point, fructose and glucose are two forms of sugar. They are metabolized by two different enzymes, which work independently. You basically want some glucose and some fructose if you are shooting for >60g of carbs per hour. Table sugar or sucrose is 1:1 glucose and fructose. (Based on this thread on the TrainerRoad forum, I have a bottle of maple syrup, which is mainly sucrose, by my indoor trainer setup.) Maltodextrin powder, which you can buy at sports nutrition stores or Amazon, is (I believe) all glucose. In the past, many products shot for 2 glucose to 1 fructose. I haven't tracked this in detail, but some newer products may use different ratios. The bottom line is that if you are home brewing and targeting >60g carbs/hr, I believe that neither pure maltodextrin nor pure sugar syrum is optimal.
For further education, one place to start is this YouTube video by Road Cycling Academy, which has an interview with a sports dietitian. The TrainerRoad and FastCat Coaching pages linked above are other sources, and they may link to some academic sources. If you want more detailed advice on this, the TrainerRoad forum is probably a good place to start; the link about home brewing goes to a thread there.