Milk
Full of protein, fat, carbohydrates and even sugar in the form of lactose, milk is an excellent choice for someone looking to put on slabs of muscle. The only trouble is, you have to drink it after exercise - because sculling down half a litre before HIIT training is simply asking for a treadmill vomit explosion.
This is because milk is incredibly nutrient dense, and the bacteria in your gut takes a considerable amount of time to process each individual molecule. Couple that with the "thickness" of milk, and you're asking for a heavy gut that's likely to bounce up and down inside your body instead of moving as a single unit.
Fast food
This one is a no-brainer, but some people still make the mistake of hoeing into a burger and fries shortly before hitting the gym to "work it off".
Because fast food is extremely high in saturated fat, it forces your stomach to allocate extra resources to the gut to help it process all of this fat. The result is that your body is being primed to go into full "digestion mode" when really you want it to be focused on sports performance. This is the reason why many people feel sleepy after eating Christmas lunch, because the body is asking for rest so it can focus on shifting the huge volume of food from your stomach to your bowels.