Getting fat very qickly

I have a hamster that is about a year and a half old and all of a sudden he has got fat very quickly. He is still quite active but looks very obese. a week ago he was climbing al around his cage but now he is not so agile. i thought maybe he was constipatedand so I fed him fruit and a little veg oil to see if that helps. is there anything else I can do for him?

[Aleda M Cheng, D.V.M., C.V.A]

Hamsters can suffer from the same diseases and problems as humans and other animals.  If your hamster has been exposed to what he perceives as stress, or a stressful situation,  it could have caused your hamster to overeat.  Hamsters overeat due to stress.  A stressful situation could be any change in your hamster's environment, in and outside of his cage, such as: too much noise, a change in activity,  or a change in temperature, and more.   You will have to try to be a veterinary medical detective to figure this out.  

Other causes of unexplained obesity are diabetes or other endocrine disorders, and an inappropriate diet lacking in adequate amounts of fruits and vegetables.  If your hamster has just gained weight in one area, such as your hamster's belly, it could be a tumor.