Wow. I feel bad for Sakurai, especially since I've seen it happen several times before. It's easy to correct, but it's still embarrassing.
Anyway, low calorie, high protein diets are good and all, but most meats tend to have a decent amount of sodium in them, even lighter stuff like grilled chicken, turkey breast, and tuna. Having a higher level of sodium in the body can force out fluid from cells and dehydrate a person.
So yeah, hydration is key with these diets, because you gotta replace all that fluid you're losing. Sodium and exercise can be a dangerous combination without fluid replacement.