In another topic somebody said loss of weight was due to muscle atrophy for being too much time in bed or immobilised, but not sure, because in general, the less a man exercises, the fatter and heavier he gets. Maybe then muscle mass, being heavier, is lost quicker than fat is acquired??
But maybe that explanation was for example for Precice; doing externals allows some body bearing, efforts to move the crutches, the walker or even a wheelchair, as well as desirably, daily physiotherapy and bike from the beginning, so it would be natural to lose some weight from that "side".
This would be also choerent with some reports also here in the forum that people doing LL also lose appetite, at least during some time.