Also take daylight savings into account, you could get an extra hour one day a year. Thats a lot more breaths.

Supercharge your weight loss by running west.

Supercharge the whole planet's weight loss by spinning counterclockwise... Robbing the planet of it's angular momentum and extending night and day.

But optimize this by not running too close to c (or is it the contrary ? My restrained relativity courses are rusty)

In your reference frame nothing will be different, so it really depends on whether you want to loose weight for yourself or for others.

Except he had to accelerate to speed and back, so only our reference frame matters.

Not sure about you but I keep breathing 24 hours a day.

you're clearly not trying hard enough.

It's pretty difficult to double that number. Can you really double your metabolism without ill effect? Or without massively changing your lifestyle?

The normal expectation is maybe 10-25% change. Any more than than would require things likes 8 hours of exercise per day.

5-6000 Calories is normal for an 8 work hour shift at -10c.

https://academic.oup.com/jcem/article/99/12/E2772/2833739

This is very interesting paper, but is it a right one? I do not see any mentioning of working in such conditions nor 5-6000 calories per day.

That paper isn't the right one. I can't find it now. Here is another interesting one though...

http://www.dtic.mil/dtic/tr/fulltext/u2/a240839.pdf

I will update this if I find it again.

A colleague who was into serious mountain climbing said that at 5km it was not enough to eat like 5000 calories per day to maintain weight. He even on purpose gained weight before his trips, and still returned leaner despite eating a lot.

You mean 5k elevation?

Yes, 5 km elevation

Do you have any sources for those numbers or did it just feel right to type them?

Losing eight pounds during 3-4 hours run (marathon event) is not unheard of: http://www.letsrun.com/forum/flat_read.php?thread=3319287

Like the previous commenter mentioned, that's mostly water. I've seen various numbers between 100 and 150 calories burned per mile for runners, which at the high end is just a bit over one pound's worth of calories for a marathon.

I’ve read about people who do things like swim the channel, and bulk up (10’s of lbs) before doing so. Given that they lose this weight in the process, how does that happen?

I think that's mostly independent. A swimmer bulking up for a channel swim is probably putting on a mix of water, fat, and muscle in the slow lead up to the swim (and maybe extra water in the days before) and then losing water during the swim. Here's [0] a discussion of swimmers strategizing about weight gain, and you can see that it's largely about issues like insulation from cold, buoyancy, and strength, and not because they're worried about losing the weight during the swim itself.

[0] http://marathonswimmers.org/forum/discussion/90/when-to-gain...