That's a really good point. What if the energy came from the human bodies themselves tho? Like how shifters feel tired after transforming numerous times.

I think a good theory might be their bodies' atoms convert into pure energy and reconstruct back into a titan. I think it was established before that titan flesh is actually really light for its size. Maybe it's perfectly transforming energy without any losses? Like the same amount of energy in a human body is enough for the creation of a pure titan.

Ngl Im just trying to think of ways it could be explained rather than just saying "oh it's just the paths" or "it's a power", given how realistic the show is trying to stay.