Chuckie,

I've been doing a lot of intermittent fasts recently (usually between 16-24 hours, although a few at 42), and the thing that I've noticed is that hunger comes in waves. Often for me, it can be very strong at hour 16, and not even noticeable at hour 20. It somewhat makes sense, as it's there, reminding me to eat, but fades, so that it won't interfere with the "hunt" for food. Some fasts are also easier than others.

At some point, I'm going to try out a longer fast (probably 72 hours) to see what I experience.

Great article, as always.
I only saw it now because I just returned from the mountains (European Alps) myself.
What you describe is very much what I experience every day that I go climbing.

I ALWAYS climb fastet, because any food in my stomach diverts blood and energy away from my muscles. Performance invariably dips. I always feel my best and most energetic with an empty stomach. I dislike eating when I have to work hard and avoid it at all cost.

ALL my climbing partners have a high carb breakfast and consider it essential, and of course they have to continue all day topping up. Alpine climbs more often than not start some time between midnight and 3am, and usually some time after lunch the people around me start to falter and crumble, despite constantly gulping down energy drinks and carbohydrate gels and what not. I don't get tired, ever, I can go on and on and on, for as long as it takes.

I've even had arguments over it. People demand I eat because otherwise I would put them into danger as well when I hit the inevitable wall. It has never happened yet.
Haven't been able to convince people yet that I am not some genetic freak, but that it's normal and a function of what I eat, or rather, what I don't eat, and how I train.

Having said all that, the kind of climbing I do is not so extreme yet that I would have to dip into my glycogen stores often. Like you wrote, the key is to keep the intensity below the basal rate of fat oxidation. If I did empty my glycogen stores, I'd run into trouble because even fat oxidation requires some corbohydrate to be burned in the background. (Can't recall the exact details, just remember reading about all this in "Extreme Alpinism" by Mark Twight.)

Mark Twight also describes an interesting phenomenon that was new to me, hitting the wall when burning fat. Apparently in his experience, and also in that of other extreme climbers, there comes a point where the body seems to be unwilling to release any more fatty acids. And the cure is to have some fatty food. Even tossing back a shot glass of olive oil works. The moment the body knows more fat is coming (there isn't enough time to actually digest the fat), it's happy to release more from the stores.
He has no explanation for this, just recounted the experience and the fact that many climbers share it.

Fuelling 24-48 hours of intense effort in the mountains is an interesting challenge.
Eating for a day or two of sustained moderate effort on the other hand is easy. Don't 🙂

Thanks, JS.
I've actually been around since the early days. Also bought, read and loved your book. I'm just usually very quiet, unless you mention mountains in some form or other :-).

"That's some really interesting info from Twight: thank you! I'll have to look up the book."

The book is from 1999, and there isn't any real science in it. Just an incredible amount of experience, and very useful and detailed advice on how to train, eat and climb to return alive. (Plus lots of awesome photos and stories.) Twight sure knows how to treat the human body to get the utmost out of it.

"Alpinism is one of the few remaining pursuits where survival is completely in the hands of the participant."

Which is exactly why I love it so much.
My biggest pet peeve is that these days hardly anybody is willing to take responsibility for anything in their lives. Not for the bad things that happen (always gotta sue someone), not for their living or financial situation, not for their health or their happiness. It's always the circumstances or the genes or other people or something else.

Our society has developed accordingly, so these days most of the responsibility is taken out of people's hands anyway. Responsibility and freedom go hand in hand. Give up the first, lose the latter.
If we want 100% of the freedom we need to be willing to take on 100% of the responsibility. So I go where I'm allowed to do that.

Btw, I can't tell you how much I appreciate all the work that you and others do that makes it so much easier for us to take responsibility for our health.
Thank you!

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