Brain functions less by what you eat and more by what you feed the brain. Always try feeding the brain with positive thoughts of overcoming the impossible.
There’s a simple trick that you can make use of while studying.
The attention span of an average human brain lasts for about 15–20 minutes after which the brain gets distracted and experiences lag phase, so it’s best to take a break after every 15–20 minutes of whatever you’re doing like reading/studying/work.
Everytime you complete a study/work portion, treat yourself by doing what you like e.g. play a mobile game for a shorter duration or, sketch something or, watch a short inspiring video or, answer a question on Quora, like I’m doing right now.
This will help you get back to work with much greater concentration. It’s best to practice praying or meditating, for it makes it easier to focus on work by increasing the attention span of the brain.