I've been through the Marine Corps Arctic survival & operations training in the Sierra Nevadas, and then helped teach some reservists in Norway ... I went in fit & buff, and still lost 20lbs in a couple months. Granted we were in tents, cooking and heating with whisper lights, and either snowshoeing or skiing everywhere carrying gear. We were issued 4000 calorie arctic MREs (white wrapper - my favorites) plus two hot meals per day, and everyone lost weight. Then we went to Norway, above the arctic circle, and slowed down a bit but ate norwegian food. Couple things I learned about eating in a cold environment:
- eat constantly. like a stove, you have to keep feeding the flame if you can. Keeps your metabolism going all the time, which keeps you warm. I kept packets of raw (unsalted and unflavored) mixed nuts in my pockets.
- eat fat. norwegian food was fatty; game meats were stewed with the fat, and the fish were fatty.
- eat all the animal you can ... meat not necessarily first. Organs are best, richest in nutrients.