The IMPs were augmented by an arctic supplement in my day-do they still issue? Basically it was two more hot chocolates and a can of Crosse and Blackwell fruitbread. The latter was delicious, all aimed at increasing the caloric intake in extreme cold weather conditions. I know of few people who gained much weight on winter exercises, REMF or not.
The Somalia experience, where tps were on hard rats for almost the whole time proved that the IMP, though lacking in certain areas, eg roughage, was capable of sustaining a soldier for periods far extended from its original intent, ie I think it was to be a max of a month, without harm, and that included weight gain. Obviously if you find you are gaining weight on them, cut out the sweet stuff for starters.