From what I've seen and read about the people who eat those severly calorie restricted diets they're always cold. They have to bundle up even in the summer. It just doesn't seem worth it to me. Like others have said, moderation is the key.

On a personal note, if I don't get enough protein I get terrible cravings and I get snippy and easily aggravated. We eat a lot of chicken but only have red meat about once a month.