I haven't passed out but I've lost vision and come close my fair share of times. If I get up too fast or have been restricting low it happens a lot more. I usually just either sit down or stand still/kneel down for a few seconds and then get up slowly and then drink water. Another issue is that I don't drink enough water so that's a bad thing. I have also come very very close in the shower a few times. It is the most common place for me and it happens even if the water isn't too hot and the fan is on.