The main challenge for humans visiting high altitude (HA) is the reduced oxygen availability in the air (“thin air”). How well humans tolerate HA is highly variable, but in order to minimize the risk of developing the feared symptoms of acute mountain sickness (AMS), proper acclimatization is needed. The first symptoms of AMS are “self-limiting forms” like severe headache, tiredness and vomiting, but improper acclimatization can lead to the potentially lethal forms like high-altitude cerebral -and pulmonary edema (HACE/HAPE). Vital in a successful acclimatization is that the blood vessels are able to deliver enough oxygen throughout the body. Previous research have shown that blood vessels tend to contract in HA, thereby we wanted to investigate if we could improve blood vessel function in HA simply by drinking beetroot juice at 3700 meters of altitude. Blood vessel function was measured through a standard test of arterial endothelial function by a flow mediated dilatation test (FMD) using ultrasound.