How UFC Fighters Cut 20 Pounds—and What It Does to the Body

A fighter steps on the scale at 155 pounds, then enters the cage a day later weighing considerably more. Where did those extra pounds come from?

Mostly, they were there all along. The dramatic weight changes around a UFC weigh-in are not rapid losses and gains of body fat. They are largely changes in body water, stored fuel, and food moving through the digestive system.

That distinction explains both how a large cut is possible—and why it can be dangerous.

What “cutting weight” actually means

A fighter’s preparation usually involves two different processes:

1. Gradual weight loss over weeks, which may include losing body fat.
2. An acute weight cut in the final days, which temporarily lowers scale weight by reducing water, stored carbohydrate, and gut contents.

Glycogen, the carbohydrate stored in muscles and the liver, holds water alongside it. Using up some glycogen therefore lowers both fuel stores and their associated water. Eating less also reduces the amount of material in the digestive tract.

But the biggest rapid changes typically come from dehydration: losing more water than the fighter replaces.

A lower number on the scale does not necessarily mean a leaner body. During a rapid cut, it often means a drier body.

For scale, 20 pounds is approximately 12% of a 170-pound person’s body weight. That is an extreme acute loss—not a standard or safe target.

UFC official weigh-ins are generally the day before the fight. That gap gives fighters time to drink, eat, and regain weight after qualifying for their weight class. It does not guarantee that every effect of the cut has resolved.

How the final pounds come off

The methods you hear about—water loading, sweat suits, saunas, and hot baths—work through two main routes: urine and sweat. None rapidly removes substantial body fat.

The distinction between sweating and cooling matters. Evaporation—sweat turning into vapor—is what removes heat effectively. A sweat suit can leave you sweating heavily while preventing much of that cooling.

Hot baths also attract a persistent myth: adding Epsom salt supposedly “pulls” water out of the body. In a small study of MMA athletes, Epsom salt did not increase weight loss compared with plain water.

Combining heat exposure with fluid restriction makes the situation more hazardous: the fighter is losing cooling capacity and circulating fluid while adding heat.

Why water loading has only a temporary effect

Your kidneys continuously adjust how much water returns to the bloodstream and how much leaves as urine. They are not simply drains that remain wide open once activated.

ADH: the water-conservation signal

Vasopressin, also called antidiuretic hormone (ADH), is a major controller of this process.

When you drink a large amount of water:

• The concentration of dissolved substances in your blood falls slightly.
• ADH release decreases.
• The kidneys reabsorb less water.
• More water leaves as dilute urine.

ADH works partly through aquaporin-2 channels: tiny water passages in the cells lining the kidneys’ collecting ducts, where final adjustments to urine occur. With less ADH, fewer of these channels are positioned to return water to the blood.

After fluid intake stops, continued losses make the blood more concentrated. ADH rises, the kidneys conserve water, and urine becomes more concentrated.

This response can happen relatively quickly. Water loading does not switch off the body’s defenses against dehydration or create a prolonged “flush mode.” There may be a brief mismatch between intake and output, but its size and duration vary.

In one controlled combat-sport study, water loading produced about 0.6% additional body-mass loss after fluid restriction compared with the comparison condition—far short of explaining a 20-pound cut on its own.

Aldosterone: a related but different control

Aldosterone mainly promotes sodium retention rather than directly controlling water permeability as ADH does. Sodium is a major electrolyte, a dissolved mineral involved in fluid balance, nerve signaling, and muscle function.

If circulating blood volume falls, the renin–angiotensin–aldosterone system—a linked set of hormonal signals—helps the kidneys retain sodium and defend blood volume. Extra fluid intake may suppress that system if it expands circulating volume, but the response also depends on sodium intake and other conditions.

The useful distinction is simple: ADH primarily regulates water; aldosterone primarily regulates sodium.

What dehydration does to the body

Circulation, cooling, and performance

Blood plasma is the liquid portion of blood. Losing water can reduce plasma volume, leaving less fluid circulating.

The heart then has to work harder to support blood flow. Heat regulation and endurance can suffer, especially if the fighter is exercising or sitting in a hot environment.

At the same time, carbohydrate restriction reduces available fuel, while sweating removes both water and electrolytes. A fighter may therefore reach the required weight with depleted energy stores and disrupted fluid balance—not merely thirst.

Kidney stress—and the question of lasting damage

The kidneys need adequate blood flow to filter blood. Severe dehydration can reduce that supply and contribute to acute kidney injury (AKI), a sudden decline in kidney function.

Studies of rapid weight cutting have found changes in blood and urine tests consistent with acute kidney stress or injury. Interpreting those results requires care: a post-cut abnormality does not, by itself, prove permanent damage.

Repeated severe cuts could cause lasting harm, but the long-term risk specifically in fighters has not been well measured. In broader patient populations, repeated AKI episodes are associated with worse long-term kidney outcomes. That is reason for concern, not a precise prediction for every athlete.

Regaining weight is not proof that kidney function has recovered. Persistent illness, very little urine, or blood in the urine warrants medical assessment. Blood and urine tests after normal hydration and recovery can help clinicians assess whether problems remain.

Water loading has a separate immediate danger: hyponatremia, meaning abnormally low blood sodium. Drinking more water than the kidneys can clear can cause it even before any dehydration occurs.

The brain: established effects versus uncertain risks

Severe dehydration, reduced circulation, and overheating can impair attention, judgment, coordination, and alertness. Extreme cases can involve confusion, collapse, or seizures.

Whether dehydration makes a given punch more likely to cause a concussion, a brain injury caused by forces transmitted to the head, is less certain.

Cerebrospinal fluid (CSF) surrounds the brain and fills spaces within it. It provides buoyancy—allowing the brain to “float”—and helps buffer ordinary movement. It is not a fixed layer of impact-proof padding.

Small imaging studies have found changes in brain tissue or fluid-space volumes after dehydration, but results vary. A scan showing a volume change also cannot establish how the brain will respond to a punch.

Dehydration can impair brain function. The claim that it definitively increases concussion susceptibility has not been demonstrated.

An American College of Sports Medicine consensus review found direct evidence confirming increased brain-injury risk was lacking. The idea that a cut simply “drains the brain’s cushioning” is too simplistic.

Regardless of hydration, suspected concussion means stopping sparring or fighting until medically assessed.

What happens after the weigh-in?

Fighters typically recover by mouth, drinking electrolyte-containing fluids and eating carbohydrate-rich meals with salt and some protein.

Each component has a role:

• Fluid replaces lost water.
• Sodium helps retain that fluid.
• Carbohydrate replenishes glycogen and its associated water.
• Food restores energy and digestive-tract contents.

Intake is generally spread over the available recovery period rather than consumed all at once.

Why were IVs restricted?

The USADA-era UFC restriction on large intravenous (IV) infusions, which deliver fluid directly into a vein, was an anti-doping rule, not a declaration that saline was itself a banned drug.

Large infusions can expand plasma volume and potentially mask prohibited substances or alter anti-doping measurements. The rule also discouraged unnecessary IV use when oral drinking was sufficient.

For historical context, the applicable limit in 2017 was more than 50 milliliters in six hours, unless an exception or appropriate medical exemption applied. It was not a ban on emergency treatment.

That is historical information: USADA’s administration of the UFC program ended at the close of 2023. The old threshold should not be assumed to describe current UFC rules.

How much weight returns—and does it help?

Ten to twenty pounds is a useful ballpark for many fighters, though the amount varies substantially with body size and the individual.

One study involving UFC and Bellator fighters found an average gain from official weigh-in to fight night of about 10% of weigh-in weight. For someone weighing in at 155 pounds, that is roughly 15–16 pounds.

Those are fight-night measurements—not necessarily what had returned by the next morning.

A size advantage could plausibly help in grappling or clinches. But a larger rebound can also reflect a harsher cut and its lingering costs.

The evidence on winning is mixed:

• One study of 700 professional MMA fights linked greater percentage weight regain with a higher chance of winning.
• Other studies, including research on UFC fighters, found no significant relationship.

These are observational studies: they record what happens rather than randomly assigning athletes to different cuts. They cannot establish that extra regain causes victory.

Can different weigh-in rules solve the problem?

Some organizations are trying to change the incentive rather than merely manage the aftermath.

Same-day weigh-ins

With less time to regain weight, a large dehydration cut may become less attractive. Ideally, fighters choose classes closer to their usual weight.

But timing alone is not protection. Athletes may still cut aggressively—and then compete with even less recovery time. Research across combat sports has found substantial competition-day dehydration under both morning and previous-evening weigh-in systems.

Hydration testing

Hydration testing asks whether a fighter appears adequately hydrated while making weight.

ONE Championship, for example, uses urine specific gravity, a measure of urine concentration, alongside weigh-ins held 24–48 hours before events. It is not a same-day system.

The approach makes simple dehydration-based qualification harder, but urine concentration is only one snapshot—not a complete measure of whole-body hydration.

Both approaches aim for safer competition and fairer matchups. Neither is a complete fix by itself.

The number on the scale is only part of the story

A dramatic cut is possible because body water can change much faster than body fat. The day-before weigh-in creates an opportunity to temporarily reach a lower weight class, then regain substantial mass.

But weight restored is not necessarily health or performance restored. Circulation, kidney function, temperature control, fuel stores, and brain function all matter more than whether the scale has climbed back up.