It's not like "OMG I just ate a Milky Way so now ill never run fast again.." Any kind of junk food is ok to eat in moderation.. But if you eat it excessively without exercising it will eventually cause you to gain weight which will then cause to be physically unfit... So, yeah, it can. However, you can be the healthiest eater in the world but if you don't exercise and work out properly you'll still be physically unfit. Proper diet and exercise go hand in hand... You need them together, they compliment each other!