When should I use an energy gel?

Great you are experimenting with these things. Food and energy input, along with hydration, is something any endurance rider needs to be on top of. Everyone is different and you can't know what works and doesn't until you tried it. However in its current form, this question would take an entire (large) book on sports nutrition to get to a full and useful answer, so here is a starting point.....

First thing to keep in mind is energy gels are just food. They are engineered to provide a certain kind of food has some benefits over other kinds of foods. The marketing and packaging are engineered to convince you they are have special powers that you will receive by consuming them. Mostly this is not the case.

Based on this I suggest you study up on sports nutrition and understanding the difference between simple carbs (sugars), complex carbs, protein and fats, and looking at when these various food types are most useful. Once you get a basic understanding, look at where the engineered foods such as Gels, Powders and bars claim to provide benefit over real foods. When doing this consider that no one makes money saying 'A cheese sandwich and banana are as good as a $10 gel"

If you want a straight answer about best time to use Gels, small amounts regularly from the start of the ride is better than waiting till hungry or out of energy.

If you go for a ride in the morning, your body's main source of energy is glycogen stored in your leg muscles, which came from the food you ate for dinner the night before. (There is also liver glycogen, and you can burn fat, but it's a slower process.) For most people, muscle glycogen is sufficient for about 2 hours of continuous, strenuous exercise. Once glycogen is stored in a particular muscle, such as a leg muscle, it can't be taken out of that muscle for use anywhere else. (Liver glycogen can be donated to other parts of the body that need it.) This two hours is if you're really putting out your maximum effort the whole time. Most cycling isn't actually that continuously intense over long periods of time -- the bike is after all a fancy device for improving your body's efficiency. In many cases you may be able to cycle for much longer, like 4 hours or more, without running out of glycogen.

So in terms of energy available to perform muscle contractions, typically it's not really necessary to eat anything unless your ride is very long. However, it's possible to start feeling hungry even when your legs still have plenty of glycogen. Being hungry is going to make your ride unpleasant, and may have the effect of decreasing performance, since fatigue is a complicated phenomenon mediated by non-conscious parts of the central nervous system. So basically it's just what you would think based on common sense: if you're hungry, eat.

At very long distances, where muscle glycogen is in danger of getting depleted, your body will try to switch over to burning fat, but that's slow, so you'll start to feel like you're bonking. In this situation it becomes important not just to avoid hunger but to provide your body with some energy that it has some hope of digesting fast enough that it can be used for pushing the pedals. If you anticipate this kind of long ride, then you can basically eat as many carbs as possible, starting even before the ride, in hopes of being able to use the energy. However, your body doesn't do a very good job of digesting food while you're exercising, and digestion takes time. That's why people often take small amounts of carbs, spread out over time.

Different people also differ in their ability to handle food while exercising strenuously. Some people barf if they try to do it. Do whatever works for you.

The main differences between a GU and some other carb-based food like a granola bar are that the GU doesn't require chewing, and it's also conveniently packaged for use while running or riding. Basically a GU is like cake frosting (or actually maltodextrose) in a convenient package.

There seems to be pretty solid science behind the concept of carbo-loading, i.e., eating lots of carbs in the day or days before the activity. Studies have shown it increases performance by pretty hefty amounts. Probably it just helps to top off your glycogen supplies. Having that energy already digested and available as glycogen is a big win. Eating during the activity is at best a minor benefit in comparison.

And BTW please don't be like a lot of the mountain bikers on my local trail system and leave your GU wrappers on the trail!

I mostly ride road, so my method has been to have a gel 5~10 minutes before a big effort. That might be a flat sprint or a decent grade where I want to beat my PR.

I might also have a gel on a long flat where its just boring.

Packaging says you should have 2-3 gels an hour, which is way too much. I'd have one after an hour, and then 1 an hour but have something else on the half hour, like a square of chocolate, a lolly, or a bliss ball.

There is no time where food is more important than safety.

so for you, I'd suggest having a gel after you've ridden for an hour, whether that be one run down the hill or whatever, just at a point you feel comfortable riding with one hand off the bars for a minute. If you're doing downhill shuttle runs, simply have one while being shuttled up.

Carry it with you on longer rides but not as a meal planned in but just for use in an “emergency“ Gels have the advantage of raising your blood sugar very fast which can be useful if you are in danger run completly out of muscle glycogen and would otherwise not be able to ride back home. For a normal snack as you would have one after like 90-120 mins into cycling a gel is not that well suited as it then will raise your blood sugar too much and drop it afterwards due to insulin. Also gels are expensive and unless you are pushing your limits of endurance a normal cerealbar or banana will be be a better option.

One time I find gels useful is if I'm wearing full-finger gloves -- they're easier to get into than most actual food, and easier to hold while riding (but I'm on drop bars, on road). I'm still experimenting myself with nutrition strategies, given that a long ride can burn more than twice as much energy as a person can store as glycogen.

A couple of years later, I stand by this, I'll also add that if you're trying to keep the carbs in your digestive system topped up, a gel for the first feed after a proper meal can be good. If I go too long without food, I find it hard to start eating again, and end up running at fat-burning power, which is relatively low, not really enough for the hills round here

One minor point may have been missed in the above answers. Long duration, high-intensity exercise burns a lot of energy, and you mainly get that from carbohydrates. The conceptual diagram below is taken from a discussion on Road Bike Rider.com, and it is from an academic article.

The y-axis is calories per hour for a reference rider of unspecified weight. Being an older article, the x-axis is parameterized in terms of percent of max heart rate - low intensity is 65%, moderate is 75%, high is 90%. In modern terms think high is probably around functional threshold or higher; for the OP's reference, functional threshold power is the maximum you can sustain for about an hour, and it's not that comfortable to hold but it is doable. Regardless, at high exercise intensities, you burn carbohydrates rapidly. As discussed, these come from muscle glycogen and blood sugar. If you deplete that, you bonk.

Relatively simple carbohydrates like energy gels are ideal for long exercise durations in these situations. I personally don't find it necessary to consume a gel beforehand for races of an hour to maybe two hours, although some people do this (and personal preferences and metabolisms differ!). I absolutely need gels or energy chews on high-intensity, long-duration rides like a competitive Gran Fondo or century ride. In mountain biking terms, if you're trying to set the fastest known time (FKT) or your personal fastest time around some long course, that would be a similar exercise context.

For less-intense exercise, I'd argue that gels or energy chews are not necessary. You can rely more or exclusively on natural foods or non-specialized prepared foods like cookies, granola bars, bananas, and the like. Generally, the fat, fiber, and protein content will slow the rate that carbohydrates are absorbed. In the context above, you want to get the carbs digested as fast as possible, and with as little strain to your digestive system as possible (although too much of any one item will become psychologically unpleasant, and even if it's 'just' psychosomatic you can feel it in your gut as well). Actually, in my last century ride where I was trying to set a personal best time, I did bring some solid foods like energy bars, so I'm not trying to say that all people will exclusively need energy gels even in the context above. Naturally, eat the gels if you want, even if just to break up the monotony.

One minor note is that professional cyclists may have high rates of oral disease. They ride very fast and very far, and they take in inhuman quantities of sugary energy drinks and gels. For us mortals, I would merely counsel taking a sip of plain water and rinsing your mouth after an energy gel or drink, just to reduce the probability of suffering tooth decay. It probably isn't that big a deal, but you will need to drink anyway and it is an easy thing to do.