> Also... I know nothing about wait loss, but surely a lot of the mass is in water?
Not really. Yes, you loose water (and its weight) when you e.g. sweat a lot when exercising, but it gets replaced when you drink. More generally your body keeps the amount of water more or less at the same level through intake (drinking, but also water in food), urinating and sweating. There are short-term variations, but besides exceptional circumstances on a day-to-day basis the amount of water in your body should be pretty constant.
Real weight loss is through loss of muscles (not preferable) or loss of fat.
Not really. Yes, you loose water (and its weight) when you e.g. sweat a lot when exercising, but it gets replaced when you drink. More generally your body keeps the amount of water more or less at the same level through intake (drinking, but also water in food), urinating and sweating. There are short-term variations, but besides exceptional circumstances on a day-to-day basis the amount of water in your body should be pretty constant.
Real weight loss is through loss of muscles (not preferable) or loss of fat.