I had one of my slightly drunken profound thoughts a while ago about why people were encourage to give up rich food etc for Lent.
Came to the conclusion that as this time of year is known as the "hungry gap" in allotmenting circles, ie not much growing really, just a few spring greens, you're also at the end of your stored onions and poatatoes, and if you managed to dry beans etc they'll be running low too. Carrots & stuff will be getting limp if you buried them in a clamp. I don't know about the storage of meat before refridgeration but would guess if you had any salted down that too would be running low, your chickens probably wouldn't be laying as often.
So...if you have a religious reason to eat less, the little food you have left is going to last longer. Would people have been eating richer food during winter and like us could stand to lose a little weight?
Just a thought though, feel free to shoot me down in metaphoric flames!