This article from WebMD about a European study explains why I lost 20 pounds riding to nearly 16,000 feet in Tibet, and why I was unable to regain it for two months after descending to sea level. I was worried about the loss of weight, probably a combination of poor quality and quantity of protein, and why it took so long for me to begin regaining it when better food was available in SE Asia. Claire lost very little weight, and I think having more natural female fat had something to do with that, and she seemed to have a better appetite at elevation than I did.
If you are fat, move to Tibet! Here’s the WebMd link.