Sorry but this is complete bullshit. I don't do Atkins any more. I can lose weight other ways more easily now. But when I did, twice many years ago in my twenties and my thirties, there's no way I could lose 20 kg of just water. It was very obviously fat. Anyone could see and touch.
I don't know where this absurdity of water losing came from, but It's utterly ridiculous.
Taking laxatives in large quantities (as some misguided young people do) does indeed reduce water mass significantly, besides screwing up one's digestive system.
It's about 5kg of water loss that will mess up your statistics if you just measure weight. So if you are looking at weight, a low carb diet loses you more weight. If you look at fat, it performs the same as a medium or high carb diet.
That doesn't mean you can't lose fat on a low carb diet, of course.
Except I would have been unable to do low calories at the time because I felt it like starving, while low carbs was pleasant, sustainable (last time for two years) and worked like a charm.
So if I lost 5 kg of water and "only" 15 kg of fat, so be it... as opposed to 0 kg of anything with low calories.
In 30 years I've read a lot against low-carbs, but nothing that can reconcile the fact that it works.
Low carbs did make you eat lower calories, so if it works for you, that's great. A low calorie diet is not opposed to a low carb diet, a low carb diet is usually a low calorie diet.
I don't know where this absurdity of water losing came from, but It's utterly ridiculous.