It's not very likely to happen with water either. 6kg of water is 6 liters, which is probably more than the amount of blood in their body. I have to question the measurements on the 6kg fluctuations, going from 60kg to 54kg in a week is serious.
While most people will feel physical effects after losing 5% of their body weight in water, many can lose up to 8-10% without significant adverse physical or mental impact. Loss over 10% is considered serious dehydration.
World champion marathon runner Haile Gebrselassie famously won the Dubai marathon in 2005 with a 10% body weight loss during the race:
A natural 6kg fluctuation isn't inconceivable. I'm fairly sure I could get there starting dehydrated/carb-depleted and pounding a carb-heavy meal with a lot of fluids.
It definitely wouldn't be accidental, though.
Seems crazy, but try it. I went from like 180lbs to low 160s. Of course, it comes back when you start eating and drinking again.
- "Wow, how did you manage that??"
- "I just emptied the gas tank!"