What is the healthiest thing to eat/drink before exercise/competition?

Hydrate: Water. You should drink at least a 8 oz glass of water before exercise. You should also be drinking water during exercise especially during endurance events. Lastly, you should rehydrate after exercise. In addition, after high intensity or long endurance events, you should also replace electrolytes and salts.

H2O: Agree with earlier input. Just prior to an endurance event (like a marathon) drinking water makes sense as you are hydrating and will have still have glycogen in your liver to break down for energy. Many people switch to an electrolyte / carbohydrate drink later in the race to replace electrolytes lost to sweating and to provide carbs after other stores are now dwindling.