Unless you have low body fat, which is less common than it used to be. I maintain body fat under 7%, and I have to eat regularly or I will become very ill very quickly. If I'm to fast, I have to cut something from my diet, not cut my diet.
I successfully lost 50 lbs a few years ago doing keto and brought myself to exactly the middle of normal BMI.
While I was losing the weight, keto was easy -- I always had energy (more than ever before), felt great, wasn't getting sick.
But once I hit my target weight... all of a sudden keto became miserable. I had brain fog all the time, except for about the window of 2-4 hrs after each meal when my digestion seemed to peak. I started getting colds all the time.
Once I switched back to eating carbs, I felt great.
My theory is that your body burns fat proportionally to the amount of fat you have... and that it's fine to fuel yourself until you just don't have enough fat anymore to keep it going to even things out between meals.
I was pretty bummed actually to discover that keto doesn't work for me anymore, when I'm already at a healthy weight. I just loved the constant energy. But for however my body's built, it's just not an option for me anymore.
Are you talking of this kind of thing, low body fat makes even this difficult, or are you talking of multi-day fasting?
Fasting, to me, begins when all previous meals are digested
Although there are other benefits to IF, the weight loss is principally from calories in, calories out, because it’s much harder to cram an excess of calories into a small time window unless you purposefully eat beyond satiation.
Is it? Study [1] claims "Mice fed an FS [fat and sugar] diet ad libitum (FSA) consumed the same amount of calories as mice fed within a 9 hr window of the dark phase (FST) [fat and sugar, time restricted] (Figure S2A, i), yet the FST [time restricted] mice gained less body weight"
and then "to investigate the therapeutic potential of TRF [Time Restricted Feeding], we tested whether TRF could reverse or arrest body weight gain [..] Within a few days the mice were habituated to the new feeding paradigm and continued to consume equivalent calories (Figure S2A, iv). The 13:12 FAT mice showed a modest drop in body weight"
[1] https://www.cell.com/cell-metabolism/fulltext/S1550-4131(14)...
From related work of https://www.salk.edu/scientist/satchidananda-panda/
I think "it has to be calories in/calories out because it's just common sense" is not nuanced enough for biological systems.
My wife and I are long distance hikers. That means carrying 10-15kg up and down mountains for 10-14 hours per day every day for 3-6 months.
It also means literally counting every calorie--because you have to plan it in week-long intervals and also optimize it for minimal weight and sufficient nutrients.
Sounds like a perfect recipe for weight loss right? Enter 'hiker hunger.'
Appetite is completely elastic. Your ability to perform depends on your brain's ability to produce happy endorphins and satisfaction signals etc.
That's where intermittent fasting is crucial. You exercise your body's regulatory functions. It's not just some belief or willpower or placebo I mean: at a chemical level you transform the function of your neurochemistry to improve your metabolic process.
Would you be able to use IF during a thru hike? I suspect that the answer is no. The weight gain problem for distance hikers is after the hike is finished and we reduce activity significantly. I've solved that by training for distance races when I'm not hiking.
Zach Bitter, the current 100 mile record holder, put it in a way that makes a lot of sense to me[0]. For a mere mortal, fasting for 6 hours puts you in the hole for about 700 kcal. A world class athlete like him is at more like a 2000+ kcal deficit. It's impossible to catch up once you get that far behind, and you end up losing weight and injuring yourself. I only burn about 800 kcal during training on an easy day, but I still find it nearly impossible to catch up if I miss a meal. I'm already eating 5x a day. It's hard to cram 5 full meals into a few hours.
[0]: https://zachbitter.com/blog/2014/04/06/intermittent_fasting_...
And weight gain is not contained to "after the trail"
And weight loss is not guaranteed.
It's great that you've found something that works for you. Like I said, it works for me too.
Bodies are different though.
"I've hacked around this problem" != "This problem is illusory"
Highly active is something you can be when you're at or approaching a healthy weight; the causality is IMO almost backwards from what you've described.
In spite of jamming as much junk food down my throat as I possibly could, I struggled to maintain weight on the Appalachian trail. You can definitely outrun a bad diet. (Not that it's a healthy way to live either.)
Gary Taubes book “Why we get fat,” is a great read on this subject.
I have lost weight like that before, but can't maintain a 2 hour a day exercise routine.
I imagine you can't stay on the Appalachian trail forever.
So eventually a bad diet will lap us.
But, yknow, HYOH
> According to the doctor it's because we're not supposed to keep a full schedule while fasting.
That's a standard answer that likely does not draw on his medical knowledge. People pass out even when not fasting (women more so than man, something to do with vagus nerve IIRC), and thousands of people do long wet fasts while keeping a full schedule with no ill effects.
As mentioned from fasting you still get major benefits from autophay, sirtuin activation, longevity, low insulin variability.
Some of the top fitness and health practitioners who are big on fasting are very low body fat, incredible shape.
Leading figures like Peter Attia, Tim Ferriss, Ben Greenfield, David Sinclair, Kevin Rose to name a few, all pro fasting, all in top shape/low body fat.
> and you become very injury prone
No, you become temporarily weaker, and that's only if you're fasting multiple days. It's recommended you work out during fasts... your body remains anabolic to muscle (barring other variables like elevated stress).