Maybe I’ll regret posting in this thread. There are a couple of things that I think people are forgetting when they are thinking about the concept of fueling on long slow rides (ie: z2).
A training ride is not a race. The goals of a training ride and how you do it, are often very different than what you’d do during an event.
Different people have different purposes for their training. The “optimal” training of a pro riding 20-30h a week training for the TdF will likely be different than the optimal training of a MTB cyclist, will likely be different than the optimal training of someone who only races crits, will likely be different than the optimal training of someone who just rides three TR sessions a week over the winter so they stay in shape for group rides in the spring.
If we are talking about whether to continuously take in calories on a z2 ride or not… this is a slightly different question than whether you should be doing fasted rides or not. Someone who eats breakfast, then rides for 4 hours with only a snack in the middle, is neither continuously fueling their ride, nor riding fasted.
That being said, what is the purpose of long slow rides?
We do them for a number of reasons. People talk about things like “improving muscle capillarization,” or “increasing mitochondrial biogenesis/density,” or “improving mitochondrial energy usage.” They all boil down to adaptations that allow your body to be more efficient at extracting mechanical work out of the stored chemical energy in your body.
Why do these z2 rides need to be so long (and low intensity)? Because the duration of time you are doing this training for is felt to the key variable that elicits these target physiological adaptations. It is the time spent doing the activity that overwhelms your bodies existing adaptations, and forces it to continue to adapt to this new stress.
What exactly is going on in your body that you are trying to overwhelm/train? What is the signal for beneficial endurance adaptations (like increased mitochondrial biogenesis?) THE signal which triggers many of the adaptations you are trying to gain from z2 rides is low energy levels in your muscle cells.
Specifically, AMPK is a major regulator of mitochondrial biogenesis in response to exercise. AMPK activity is triggered proportionally to a reduction in intracellular ATP levels. The lower the available energy in a given muscle cell (ie: lower ATP:AMP ratio), the higher the activity of AMPK, the stronger the adaptive signal for increased mitochondrial biogenesis/enhanced mitochondrial density.
You don’t need to reach a state of “glycogen depletion” for this to happen. Which is great, as your glycogen is never fully depleted anyways. Rather, the more of an intracellular energy deficit you’ve put your muscle cells into, the higher the degree of activity of AMPK, and the stronger the signal for beneficial physiological adaptations is.
This is why long slow rides have to be long. They need to be long enough that you overwhelm your body’s existing adaptive mechanisms to continue to provide energy to your myocytes (in the form of lactate, glucose, fat, etc.) during exercise, such that AMPK levels drop, and you get an beneficial adaptive signal. “Long” here depends on you and your training background.
In addition, low intracellular energy levels in your muscle cells are the major trigger for muscle-derived IL-6 release during and post aerobic exercise.. IL-6 is critical in inducing many of the other goal adaptations of long slow rides. Things like improved fatigue resistance.
Low intracellular energy levels in your muscles are also the key trigger for other cell signalling pathways involved in the adaptive response to exercise (ex: PPAR).
Long story short: the whole point of doing long slow rides/z2 rides is to put your muscle cells into a state where they are struggling to adequately get the chemical energy they need. This low energy state is the fundamental cellular signal that triggers most of the beneficial effects of z2 training.
The implication of this is that you will not achieve the full training benefits of your long slow rides, if you never actually reach a state where your myocytes are struggling to access enough energy to continue riding.
If you are constantly eating the entire time you are doing your long slow ride, the primary signal that elicits all the adaptive changes you’re trying to get by doing this ride in the first place would be expected to be gone. Or at least lower.
Does this mean you shouldn’t eat any food during your events? No, that would be stupid. Training is different than an event. You should definitely consume lots of calories during an event as the goal there is performance, not to elicit a training adaptation.
But so and so pro team does their training with consuming 120g of carbs per hour all day every day for all of their training rides. Sure. But many other pros don’t do this. I have no idea why they’re training that way right now, you’d have to ask them. Without knowing what specific training goals these people are going for when doing this, you can’t infer their behavior onto your own training goals.
Fasted rides (or not) are a related but separate issue from the matter of continuously fueling during a z2 ride or not. You can see from the science above how it seems to make sense that fasted rides may be beneficial; however, I don’t want to get into that as it wasn’t the topic of this thread, and there are other variable to consider.
Edit: Also remember: long slow rides are not just long, they are slow. This is because you want to maximize the amount of physical activity you are making your myocytes do while in a low energy state. Doing a set of VO2 max interval will also put your myocytes into an energy depleted state… but for not as long.
Related - if you don’t eat anything at all during your z2 ride… it is unlikely you will be able to continuing exercising as long as if you stopped when you noticed you were getting hungry or slowing down, and ate some food.
My personal take away from all of this is that I will eat some food during z2 rides. I’m not trying to take in an absurd amount of calories per hour. But I’m also not trying to make myself bonk. I’ll eat breakfast, go for a ride. When I notice I’m getting a bit hungry or performace is dropping/RPE is increasing, I’ll eat a bit of something. Just enough that I’ll be able to actually finish the target duration of the ride without being so ravenous when I get home that I eat an entire bag of chips.