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I'm a long distance hiker and a beginner runner. I crave spending time in the mountains, hiking, occasionally running on the flat and downhill sections of the trail and have even signed up for a beginner event of 16 km distance in the Polish mountains, a couple of months away.

I am going to hike the Tour Du Mont Blanc on the 14th, August which is a circuit trek, with some gorgeous mountains and picturesque views, amidst the jingle of cows and the nimble chamois around the Mont Blanc massif in the alps. This is about 174 kilometers with some 10,000 m of elevation gain.

I am 35, male of Asian origin. Waist = 94 cms, height = 167 cm, Neck = 38 cm, Weight = 73.5 kgs, so one of the online calculators approximates my body fat percentage = 24%. I have a larger midsection, so I'm probably skinny fat, due to years of inactivity, a sedentary lifestyle combined with eating processed foods here and there. I so want to get rid of the belly fat.

I've started eating clean and I've hiked more this year than all past years put together. I have a small city hill - where I do 12 x 100m sprints.

Generally speaking, I've read, that you always emerge stronger, fitter after a good intense hike. I've also read that a calorific deficit of 500-1000 calories/day is considered healthy. Let me cut to the chase. While on the TourDuMontBlanc hike, I'm considering having oatmeals with chai(tea) followed by a chicken meal at night. I think my total calorie intake would be ~1200-1400 food calories, against a total expenditure of 3500 calories+.

Can fasted hiking such as this cause extreme muscle loss without any fat burn? Does the calorific deficit rule still apply, when you're doing an intense activity such as hiking? Or is hiking with a decent backpack weight and some good ascents more like strength training that exercises one's core, quads, so it helps retain muscle while burning fat? Will cutting back on one meal, accelerate some fat burn?

Cheers, Quasar.

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    This sounds like a bad idea. I think you will be fatigued and miserable. Instead I think you should eat sufficiently during your hike and loose fat when not hiking by strength training + small caloric deficit + high protein intake. At least the caloric deficit during your hike should be much lower. The hike itself sounds great though. Even the more reason to be well fed and enjoying yourself.
    – Andy
    Aug 9 at 11:03
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Does fasted cardio eat muscle mass?

A small study of 20 men tested this theory by comparing two groups, one that trained fasted and one that trained fed, and gave them similar caloric deficits with macronutrient profiles. Meaning, they ate relatively the same foods, but the fasted group ate more food later to make up for the lost meal.

It found that there were no differences in body composition. Though the limitation of the study is how few people there were and it was a relatively short training window.

Another study studied 19 people during Ramadan. 10 people trained in aerobic exercises while fasted and 9 trained while fed. The authors concluded that only the fasted people lost bodyfat percentage while the fed trainers just lost weight. That would mean the fed trainers would have burned more muscle. Although there's nothing in this article that talks about calories or macro-composition so it could be the fed trainers just had a very suboptimal diet.

The point being, that working out fasted does not appear to inherently cause muscle loss.

HOWEVER

I think my total calorie intake would be ~1200-1400 food calories, against a total expenditure of 3500 calories+.

A very severe caloric deficit will absolutely cause muscle loss. I would say anything beyond a 1000 calories is extreme. Keeping it around 500 as you noted would be ideal, but sometimes, as in a long hike, that's not possible.

HOWEVER

I am going to hike the Tour Du Mont Blanc on the 14th, August which is a circuit trek, with some gorgeous mountains and picturesque views, amidst the jingle of cows and the nimble chamois around the Mont Blanc massif in the alps. This is about 174 kilometers with some 10,000 m of elevation gain.

It sounds like it's an amazing hike. Eleven days of hiking (based on the wiki article) of severe caloric deficit is not going to result in a massive amount of muscle loss. So don't let that discourage you. Also, you have muscle memory to your advantage to rebuild when you're back and recovered.

HOWEVER

There's more to hiking than just body composition. Lengthy cardio needs fuel. I suspect that if you don't eat throughout the day then you're going to feel really miserable. Walking 174KM on a flat surface is pretty hard on its own. Hiking 174KM will be really intense.

I HIGHLY recommend you eat something substantial before the hike and bring food with you. Not for body composition reasons but for your own wellbeing. Oat bars, protein bars, trail mix, nuts, and meat jerky are lightweight foods you can take with you. Some long trails have outposts that sell food and it's good to take advantage. Do research about how people hike this trail do for food.

Summary

Can fasted hiking such as this cause extreme muscle loss without any fat burn?

Fasted hiking will have no effect on body composition. An extreme calorie deficit will. Keep calories in check and protein high will ensure less muscle loss. Fat loss will always occur even when muscle is lost.

Does the calorific deficit rule still apply, when you're doing an intense activity such as hiking?

Yes

is hiking with a decent backpack weight and some good ascents more like strength training that exercises one's core, quads, so it helps retain muscle while burning fat?

It is similar to weight training and will help maintain muscle mass like weight training. However, even in weight training you'll lose muscle if there isn't enough energy to maintain it. The body needs to get that energy somewhere and bodyfat and glycogen aren't always enough.

Will cutting back on one meal, accelerate some fat burn?

Technically, yes. It'll accelerate muscle loss too.

My recommendation

Don't worry about losing fat or muscle during this hike. If you're doing the full 174 KM, then your focus should be on getting through the hike. Go back to your focusing on your goals afterwards.

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