I would guess that one effect of going very low fat would be from tipping the Randle 'cycle' (seesaw). In the presence of free-fatty acids, cells burn less sugar. Take the fat away and they can burn more sugar. Burning sugar produces more CO2 per energy than burning fat. The higher CO2 level improves O2 delivery and lots of other things, supporting a continued higher metabolism, as long as there is enough sugar available. If much fat is eaten, or if the sugar supply gets low enough that stress hormones come into play to maintain the glucose supply and release fat from tissues, it starts to tip the seesaw back the other way, especially if the liberated fats are PUFAs.