Every food we consume, no matter how healthy it may appear, can become unhealthy if consumed excessively. For top-level athletes who will do everything to be the best, what you fuel your body with every day (especially tournament day) is crucial.
It is important to find what foods work best with you. It can be helpful to stick to the same meal plan for each tournament once you discover the best recipe for your optimal performance. Having a good nutritional game plan is part of having an intelligent competition IQ.
Food is a direct source of ATP (Adenosine Triphosphate), a molecule that provides energy. The more food we have in our bodies, technically the more energy we should have. With that being said, we need to make sure we have the right type of energy in our body, so we can get through numerous routines without having an energy "crash."
Carbohydrates are used for quick energy; this is what our bodies need both the night before, and day of competition since we will need to have an abundance of energy for routines. Protein is nutrient-dense, it will stay in the body for a longer amount of time, and help feel satisfied longer (not to mention, proteins are literally the building blocks of the body). Fats are the last source of energy to be burned off, so in case the tournament goes later than expected, you're going to want to eat this so you don't run out of fuel.
EAT: eggs, yogurts, avocado (healthy fat), turkey (lean protein), legumes, fruits (blueberries are a good choice because they have antioxidants), a source of carbs other than fruits like potatoes (not fried).
DON'T EAT: sugary cereals, greasy foods (these can make you feel sluggish) or granola bars (most of these are just sugar and don't provide much of a nutritional value).
For anaerobic athletes like sport martial arts competitors, it is recommended to have about 30-35g of carbs pre-competition (within the hour of competing). It's also recommended to consume 1.0-1.6 g/kg BW (body weight) of protein pre-competition. You can eat fruits ringside. Apples are a quick source of natural sugar and can give you a boost for forms. Try to stay away from anything too heavy.