I can’t say I’ve heard that one, but it is true at least that a healthy diet in general can help improve focus. Your diet plays a big role in your mental health! I notice that when I’m not eating processed garbage, it’s like I have a brand new brain.
I have heard, though, that supplementing omega-3 fatty acids can help support brain health!
There’s new studies coming out about the MIND diet/lifestyle. MIND stands for Mediterranean-DASH Intervention for Neurodegenerative Delay diet. The idea is that it is composed of foods that promote brain health, since Mediterranean and DASH diets are repeatedly shown to be some of the healthiest. You could also look into nootropics - supplements/substances meant to enhance cognitive performance.