So true, I have an HRM and I've occasionally worn it on my typical athletic pursuits just so I can approximate what I can treat myself to on a given workout day. I have a pretty fast metabolism for a female, I maintain on 2500-3000 calories. I went on an overnight hike in the mountains with a friend, carrying a 35 lbs backpack. We hiked 12 miles the first day. I thought I would've burnt like 5000 calories doing that, but based on my HRM, I only burnt like 2000 on that entire hike. It's amazing how efficient your body can be at conserving energy. It takes MULTIPLE DAYS to undo the damage of a buffet binge or a full-sized fast food meal.