Slow digesting, healthy carbs, source of protein. Those are making buckwheat groats great for boosting your stamina.

There is not much information online, therefore I tested it on myself. More than a year ago I started having buckwheat kasha every day for lunch.  How it boosted my stamina?

I'm not an athlete and don't swim fast, however, since eating buckwheat every day I started swimming* 80 pools. I could never do that before! I was always tired after 60 pools and now, after 80 I am not tired at all! Sometimes it is a hundred 25m pools if I'm in the mood and have more time. It must be buckwheat that boosted my stamina! Moreover, I got rid of my belly fat! It's just slowly disappeared over the last year!

Also, a great feature of roasted buckwheat is being lightweight for your stomach! One day, very hungry I had two portions of kasha before going to bed! After 15 minutes I was asleep without any feeling of late dinner in my stomach! 
Its also great food before doing exercises or other activities, or just work.

*Nonstop, Freestyle, with tumble turn.