from https://www.outsideonline.com/health/training-performance/be...

  The key ingredient in beet juice, from an endurance perspective, is nitrate. Once you eat it, bacteria in your mouth convert nitrate to nitrite. Then the acidity in your stomach helps convert the nitrite to nitric oxide. Nitric oxide plays a whole bunch of roles in the body. That includes cueing your blood vessels to dilate, or widen, delivering more oxygen to the muscles, faster.

The article summarizes the results of several studies into beetroot juice for physical athletic performance.

  I sent the following summary to Andy Jones, the scientist most associated with beet juice research, to see whether he would agree:

  “It works. It probably works less well in elites, like most things, but there may still be an effect. Higher doses taken for at least a few days in a row probably increase your chances of a positive effect.”

  Jones thought that sounded reasonable. He pointed out that there’s a separate body of evidence emerging that beet juice also enhances muscle strength and power in some circumstances, an effect that Poon’s review confirms.

I see everybody talking about the nitrate, but there is another substance in beets that might be relevant... betaine, also known as trimethylglycine HCL. I take about 800mg of this pure in my orange juice in the mornings and it gives me a very noticeable boost. But it is possible that this effect is specific to me and maybe other people who are "under-methylators" (which I think is a somewhat pseudo-scientific concept, but it led me to try betaine and I got results).

Betaine was first isolated from beets, hence the name, and as the other name, trimethylglycine, hints it has 3 easily donated methyl groups, so if you do need those for some reason it may be useful to you. It's also pretty cheap and unlikely to be harmful.

Edit: I found this... https://www.strongerbyscience.com/betaine/