Fast food can kill brain cells, but it’s not like every time you eat at McDo you’ll lose a few. It doesn’t work that way.
The terms you’re using are a bit too simple. Your body is a complex machine, and anything you eat has complicated effects on it that never just boil down to taking one day off your life span or lowering your agility with -5 points.
Fast food contains a lot of saturated fat. Saturated fat is not bad in itself, your body needs it just like any nutrient, but the problem is that there’s way too much of it in fast food, much more than your body needs.
Eating too much saturated fat raises your cholesterol levels, which causes your vein walls to become less flexible, which can leads to clogged up arteries and result in a heart attack or stroke. Strokes kill brain cells (the brain cells get no blood, so they starve) and leave you seriously mentally handicapped. Heart attacks can kill the entire person.
So yes, your life span can be shortened by eating too much fast food, in the sense that the fast food itself can kill you.
But I also think that if you change your life style now, if you’ve started eating healthy and working out and everything, then the effects of all the fastfood you’ve eaten until now will be undone.