I do a ~24 hour fast weekly, and a LOT of sites go into this "burning off garbage" or "toxin cleaning" rhetoric which always puts me in "bullshit warning" mode. Is there actual science behind these claims?
A 24-hour fast is different; your body will burn fat at sufficient rate to avoid glycogen depletion (it's pretty simple; the harder you exercise the less energy comes from fat and more from 'carbs').
From a weight-less perspective though all that matters is calories out exceed calories in - it's quite possible though that fasts end up making you less hungry than hard exercise does.
Long-distance cyclists don't worry about any of this though - the approach is actually pretty simple:-
Press food to face.
Suck.
Repeat.
Edit: formatting.
The numbers suggest 60 to 70 per cent of max heartrate (which is pretty easy) is the sweet spot for general fitness and fat burning (i.e. weight loss). For improved endurance, spend a lot of time in that zone; for improved performance spend shorter periods (well) above it.
Still seems a little too "high level" to actually believe though. In any case, I still do IF by skipping breakfast, mostly because I'm lazy, and after fasting for so many years hunger doesn't really bother me that much.
I'm not qualified to judge the literature as a whole, and it appears to be an open question whether this can be translated to actionable advice, but we're not in complete bullshit territory here.