Ufc Fighters have to go through hell sometimes. Not all athletes fight in their own weight category, so they'll drop body weight in order to fight in a different [lower] category.
In order to achieve a level playing field for all competitors, weight cuts are unfortunately a necessity. A 115-pound strawweight fighter cannot be expected to fight a 265-pound heavyweight.
To compete in their preferred weight class, fighters often have to lose weight, but for some fighters, losing weight is not necessary at all. For an instance, When Israel Adesanya competes at Light Heavyweight, he can gain weight because he is a much lighter competitor than the average Heavyweight athlete.
When fighters do this, it's extremely difficult to go about. Some fighters have to lose up to ~30+ LBS! That's INSANE!
Many, and I mean MANY people cannot do this. A lot of people cannot endure this process. And In fact, many ufc/mma fighters have passed out on stage, during their weight ins due to malnourishment and/or dehydration.
Losing weight is hard. You need to go into a calorie deficit and do tons of exercise. Lots of working out, cardio which burns calories which helps with weight loss. This in return helps the user lose body fat, and water weight. It also drops the user's energy levels, and performance (if you aren't properly hydrated or nourished).
The Main way of reducing body weight fast and effectively is by cutting water weight.
Water weight loss is the most efficient method for temporarily reducing your body weight enough to compete in a lower weight class as your body is primarily composed of water.
Many people who undergo extreme weight cuts have a hard time performing. Many people are malnourished or dehydrated. And thus can affect your performance inside the octagon.
People do this because they don't want to fight in their own weight category for several reasons. For one, they think it'll be easier for them. Like, it'll give them leverage - an advantage over their opponent. Which can be true or not true. Just depends on the individual.
However, does this mean that these people who undergo massive weight cuts to fight in a lesser/lower weight category?
It really depends. Everyone has a different approach to losing weight and maintaining it... Some people do it properly, whereas some people are becoming ill during and before the actual fight. As I mentioned, some people actually pass out during the weigh in's... If you're properly nourished and hydrated, they can easily undergo the weight cut and perform right. Just all depends.
Safe weight loss can be achieved if diet and exercise are done slowly throughout the training camp. Fighters who dehydrate themselves severely to shed the last few pounds can suffer fainting or even organ failure during weight cuts.
But for the majority of the people who do undergo Extreme weight cuts usually have impaired abilities. Hinder performance, mental & physical state, and much more. Some people feel sluggish, slow, or have no drive or energy. Some people take hits much harder due to being in low body weight. It's much harder to recover and perform in general while being low in body weight if you underwent a massive weight cut.
Has anyone died as a result of Cutting weight?
In recent history, weight-cutting has directly resulted in two deaths in MMA. In 2013, Leonardo Souza passed away as he was preparing for his fight against Shooto Brazil 43, and in 2015, Yang Jian Bing passed away before he was to fight Geje Eustaquio at Flyweight for ONE Championship.