Obesity is the condition of being grossly overweight. Studies show that obesity is linked to heart disease causing death, even moreso in recent decades.
Do they actually say "grossly overweight"?  That seams a little mean.  I know that obesity is very bad for your health though, that is why I became a vegan.  Is diet the best way to lose weight?
Some use the term "grossly overweight". But it is a condition in which excess body fat has accumulated to the extent that it may have a negative effect on health. A combination of diet and exercise are the best way to lose weight/fat mass, but diet alone is a good method.