this sounds really interesting.
from my own experience, i gained massive amounts of weight (pounds per week) with no obvious cause.
my occupation was manual - landscape gardener - regularly shifting tons of soil, digging etc.
i ate my usual foods but i began putting the weight on.
for a period of 9 months i had bad problems digesting food (for anyone squeamish...the toilet was my friend), and still my weight increased.
eventually, aching, exhausted, and with unrelenting severe, stomach pains, i got diagnosed with chronic fatigue syndrome.
the doctor never took the weight-gain symptom seriously, but it was the first obvious sign of my health problem (whatever this illness really is).
all of this has ended up with me being 6 stone heavier than i used to be, virtually housebound, and extremely frustrated at being unable to follow my old interests - motor-cycling, running, soccer and squash.
i was always 'skinny' and very active, i didn't eat rubbish and i didn't have the time or inclination to slob about watching tv... what happened???