Speaking as someone who, over a few years, lost about 50kg, the problem is that when I most needed to lose weight, my body couldn’t put up with enough cycling or swimming to burn useful calories.
I found it very funny that I spent about 2 years eating less so I could get better at cycling, then suddenly had to start eating more, once I was down 50kg, because I was getting faint on very long cycles.
Cyclists typically have enough energy stored up to ride for about 90 minutes without needing to refuel. For me it's 90 minutes on the button, and it doesn't seem to matter what I've eaten beforehand or how hard I push myself.
Its recovery keeping up with expenditure. -1000 cal deficits limit recovering from muscle/ligaments soreness leading to build up of stress that is more damaging than simply not eating
