One sign of successful acclimatization to altitude is that the blood vessels are able to deliver enough oxygen throughout the body. But normal blood vessel function depends on the body's ability to naturally produce a compound called nitric oxide. Drinking nitrate-rich beet juice helps improve blood vessel function at altitude by giving the body alternative building blocks to make nitric oxide.
http://www.sciencedaily.com/releases/2015/10/151012083808.htm