How does a professional fighter, who is in the gym supposedly every day, all muscle, in far better shape than the average person walking around, put on 10 or 20 pounds after a weigh-in? This is after a hard training camp. How does a person within 24 hours of a weigh-in (or whatever it is), put on such a ridiculous amount of weight in such a short time? This has always baffled me.
When the fighter weighs in they are "Dry" they then rehydrate. it's not like that the weight gain is muscle it's just rehydrating fluid. things like IV's can help counter some issues but usage is banned in most sports. In modern boxing it's really just boilng down to a weight division well below your natural weight so that you can come in 15-20lbs heavier so you can bully the opponent. The way around this would be to weigh in before the ring walk so there's no time to gain any additional weight.
As the other commenter stated, it’s rehydration. Your muscles can carry or lose tens of pounds of water “easily”, so a lot of people sweat out the water, don’t drink etc until the weigh in. And then they gulp down tons of water. (Or use illegal methods).
It's always pizzed me off when a fighter fights for the middleweight title and one look at him and you know he's way closer to 180 than 160.
Yeah we need to go back to same day weigh ins but with several weigh in moments before that so its not possible for fighters to lose weight to fast and be drained nor be a light heavy on the night.
Because they dehydrate their muscles which are entirely made of water. Then they rehydrate, 15-20lbs not uncommon. Guys that are super lean like Canelo can rehydrate more weight than guys who have higher fat %'s (like Donaire) because you can dehydrate muscle, but you cannot dehydrate fat. There's also a science that goes into a bit too. Guys in their early professional careers are not gaining 20lbs after weigh-ins. That comes with time and experience knowing their bodies. With saunas, hot baths, etc they know exactly how much weight they can take off leading into a weigh in. The target I've heard in boxing circles is to add 10% of your weigh-in weight, more than that is dangerous (welterweight target adding 15lbs, super middleweights 17lbs , so on and so forth). Any given person has about 5-10lbs of food digesting in the intestines at any given time. Laxatives are banned, but if they are desperately trying to lose the pounds then natural laxatives are used. This however depletes the fighter of vitamins and minerals. After the weigh-in the race is on to fill up again. It's an intense process you never see on 24/7 or the other documentaries. If you're really interested here is a great article that always blows my mind - it really is a science and art: https://tim.blog/2008/01/18/how-to-cut-weight/
If I fast and do hard cardio, I can easily weigh 10 lbs less than I would after a big but not excessive dinner… keto diet can knock off 2-3 lbs on top of that.