How dangerous is rapid weight cutting in combat sports?

Short answer

It is dangerous. 3 people died of overheating while sweating off weight right before weigh-ins, and 12 people who cut 5% of body weight in 3 days had kidney markers above the normal range. After weigh-in, 39% of 40 fighters were still severely dehydrated right before their bouts.

How dangerous is rapid weight cutting in combat sports?
'10% of body weight in a few days before weigh-in' — can the body handle it?
Combat athletes often drain water to drop weight fast in the days before weigh-in, then eat and drink after weigh-in to fight at their normal size. We checked what losing this much in such a short time does to the body.
How much do they cut?
About 7% in the 3 days before weigh-in, up to 10%
Records for 616 UFC fighters showed they cut about 7% of body weight on average from 72 hours before weigh-in, then regained about 10% before the fight. For a 70 kg (154 lb) person, that is about 4.9 kg in 3 days. In an analysis pooling 4,432 people, the biggest cut was 7.4 kg (about 10%).
Real deaths
3 people died sweating off weight in the hours before weigh-in
Within one month in late 1997, 3 previously healthy wrestling competitors died in 3 different US states. All of them stopped eating and drinking in the hours before weigh-in and exercised hard in hot rooms wearing sweat suits. The cause was overheating as their bodies lost water and their temperature soared.
The kidneys take the hit first
A 5% cut in 3 days pushed kidney markers above the normal range
When 12 people trained while cutting 5% of body weight over 3 days, blood urea nitrogen, uric acid and creatinine rose sharply. When they did the same training 1 week later without cutting, the markers stayed within the normal range. What pushed the kidney markers out of range was the rapid weight loss, not the training.
Taken to the extreme
1 fighter who cut 18.1% over 8 weeks had acute kidney injury
1 mixed martial arts fighter went from 80.2 kg to 65.7 kg over 8 weeks, an 18.1% drop. He lost 9.3% of body weight as water in the last 24 hours before weigh-in alone, and his blood became concentrated and he developed acute kidney injury. His testosterone fell very low, his cholesterol rose, and he could not finish fitness tests.
Does drinking after weigh-in fix it?
39% of 40 fighters were still severely dehydrated before the fight
40 mixed martial arts fighters regained 3.4 kg (4.4%) on average in about 22 hours after weigh-in. Yet urine tests about 2 hours before the fight showed 39% were still seriously or severely dehydrated. Their weight came back, but the water inside their bodies did not fully return before they fought.
Performance and injury
Strength comes back, but men who cut more reported more injuries
An analysis pooling several studies found maximal strength and repeated high-intensity ability dropped right after the cut, and after eating and drinking there was no difference from the start. A 14-month survey of 155 athletes found that for men, every extra 1% cut in the 24 hours before weigh-in raised the risk of reporting an injury 7 days after the event 1.2 times.
Bottom line: the body pays for rapid weight cuts
You gain a weight class, and risk your kidneys and heat control
Cutting 7 to 10% of body weight in the days before weigh-in pushed kidney markers out of the normal range, and people have died sweating off weight. Even after drinking, 39% fought dehydrated. Performance came back, but the risk stayed in the body.