Are any of the commenters here actually runners or involved in a high intensity sport/hobby? This is very, very true in my experience, if I lose weight it means I am not fuelling my body enough. Yes runners are very thin but that doesn't mean they should lose weight, if they lose weight it does indicate that they're not eating enough calories. You need to fully fuel your body for this type of thing, performance does suffer if you eat fewer calories than needed and lose weight. When I have been training for ultras, any time I would lose weight, I would add time to my run, like clockwork. I would also feel that my muscles just didn't have the same power etc. Look at any top runner, they don't lose weight, they maintain their weight and for good reason.