Because this is a training & nutrition forum, I wanna share a bit about my experience so far with doing intermittent fasting for weight loss plus cycling for fitness.
First, like anything related to nutrition in general or weight loss in particular, there is a
ton of "bro science" surrounding IF. There are legions of true believers who will assert that autophagy is the panacea for all of life's ills. Only by going extended periods of time without consuming any calories will you trigger your body to enter this magical zone of accelerated weight loss, increased energy and focus, body repair, etc. etc. Put some milk or sugar in your morning coffee?
Sorry bro - no autophagy for you, hope you like being fat and having low energy and probably cancer. What is autophagy? It's when your body starts effectively digesting itself by finding tissues or cells that are ready for expiration, are damaged, or otherwise candidates for deletion, and breaks them down for calories.
More info here.
There's no good evidence for how long one would have to fast to trigger autophagy, or what (if any) health benefits would come with frequent, short periods of autophagy.
Personally, I think it's hype. I think that setting strict eating windows is a valuable tool for managing caloric intake for some people, but it's not a cure-all. I also think that it's not for everybody - I can go a day or even two without food and feel basically OK, but if my wife tried it I'd be dead or divorced by 3:00 PM on the first day.
At this point, my approach is to aim for at least 20 and preferably about 22 hours of fasting each day. Most days I'm having my one meal of the day at about 6:00 PM, and then a snack or two afterwards, and no more eating after 8:00 PM. If I just decided to have anything and everything I wanted during that 2-4 hour window, I could definitely stay obese (although probably not 250+ like I was before starting this cut). For me, this is all about limiting intake in a way that is easy to manage. I've done multiple large (~40 lbs) cuts over the last 25+ years, most of them long before I had ever heard of IF, but I had always followed a similar approach of strictness and flexibility, and that combination is what I like. My target rate for losing the weight is 2-3 lbs per week, and so far I'm right on track. I think that as long as I'm in that territory I'm losing at a sustainable and healthy rate.
It's strict and simple in terms of having no eating outside of the allowed window, so I'm not counting or tracking or even planning anything. But if I get a craving for something, the response isn't "oh, no, you don't eat that anymore". It's "Well, see if I still want it later when it's eating time." For me, this is generally a pretty effective way of dismissing the craving instead of feeling overwhelmed like if certain foods were completely off limits. It's very flexible in that I let myself have anything (but not everything) I want during my window. I do still have to be conscious of eating only to satisfy hunger instead of eating for its own sake.
About 35 years ago when I was first starting to learn about lifting and body building, I read what was already then an old-school book (so probably written in the early 60's) that said something about weight and nutrition that has really stayed with me: "
To gain weight, eat when you're not hungry. To maintain weight, eat only when you're hungry. To lose weight, stop eating while you're still hungry." Nothing in my experience as a fat slob, a lean runner/cyclist, or a relatively muscular powerlifter has contradicted that simple guidance. With that said, I've never been a competitive athlete fueling for top performance or extreme leanness, so I'm sure it breaks down when trying to push into any kind of elite territory - or when using PEDs.
I will likely do almost all of my training rides of 100 km and under while fasting. My reasons for doing this are:
- I've done it in the past and was able to do it without experiencing dizziness or other problems that would make it dangerous.
- It means greater calorie deficit and therefore burning fat.
- As I've noted elsewhere, I think that any suffering serves the end goal of HTFU. Riding fasted or in crap weather, or tired all contribute to the ability to just keep going - which will be an important ability when facing the Haleakala-specific difficulties that I can't directly train for.
Training rides above 100 km will include some fuel past the 5-6 hour mark. Fuel at that point is to allow me to keep the intensity from just completely collapsing. I have done 160 km with no fuel in the past as an experiment (at a bodyweight of ~185 lbs), and while it felt OK at the time, the recovery was so long and difficult that I think the last hour or even two was probably of very little training benefit.