If there is a large decrease in energy intake then your body will attempt to reduce energy output to match intake as demonstrated by this study.
I guess the difference between us and monkeys is that people can push through their body trying to reduce energy output...
"And for crying out loud. Don't go into the pain cave. I can't stress this enough. Your Totem Animal won't be in there to help you. You'll be on your own. The Pain Cave is for cowards.
Pain is your companion, don't go hide from it."