A good friend of mine at work has gotten noticeably thinner over the last few months - he wasn't fat to begin with. I asked him what was up and he explained that for the past few months (3-4 months) he has been battling stomach issues/diarrhea. He has lost 20 pounds and is now at about 190 (and he's 6'6).
He's an otherwise healthy 24 year old, was a collegiate athlete, and generally doesn't have any other health problems. His mother has IBS - I don't know if that is hereditary.
He's going in to see the doctor next week. I told him he should have gone months ago but he just shrugged.