Gaining

Eat low fat versions of everything like low fat milk and sherbet, cake instead of cookies, chicken instead of hamberger.

If you dont eat healthy or exercise you will be more inclined to develop high blood pressure, high cholesterol, heart disease, bowel cancer etc.
that has absolutely nothing with the amount of fat tissue your body has.

So if you ate a healthy diet, exercised and then used a low sugar weight gain shake every day to bring you over your caloric requirements or something like that you would be gaining weight without making you susceptible to those diseases.

However, and this really seems to be left out of alot of discussion on fat health, is that there are disorders out there that are caused directly by the fat tissue, things like diabetes, meralgia parathesia, hernias, etc. Obviously not all fat people will develop these but I do think they need to be mentioned.