How Fighters Shed Water Weight in the Final Week

A fighter can step onto a scale several pounds lighter without having lost much body fat. The reason is simple: the scale measures everything—including water, food in the digestive tract, and stored fuel—not just fat.

In the final days before a weigh-in, much of a dramatic weight drop comes from temporarily removing water. Water loading, sweat suits, saunas, and hot baths take different routes to that same endpoint. Understanding them means looking at two exits: the kidneys, which produce urine, and the sweat glands, which release sweat.

A rapid drop on the scale is not the same as becoming leaner. In a last-minute weight cut, it often means becoming dehydrated.

What Is Actually Leaving the Body?

Fighters generally do not lose 20 pounds of fat immediately before a weigh-in. Fat loss takes place over a longer period, usually through changes in eating and training over preceding weeks.

During the final days, additional weight changes can come from:

• Less material in the digestive tract: food and other gut contents contribute to scale weight.
• Less glycogen and its associated water: glycogen is carbohydrate stored in the muscles and liver. As those stores decline, some of the water stored alongside them is lost.
• Dehydration: the body loses more water than it takes in, reducing water in circulation and other body compartments.

A reported 20-pound cut may combine several of these changes. It is an extreme amount—not a standard or safe target—and should not be understood as 20 pounds of fat disappearing.

Four Methods, Two Routes Out

The techniques used in the final week differ mainly in how they encourage water loss.

None of these methods rapidly removes body fat. And none makes a large water loss harmless simply because the weight can later be regained.

Water Loading: Why the Kidneys May Briefly Keep Excreting Water

Water loading means drinking unusually large amounts of water before subsequently restricting fluid intake. The idea is to increase urine production, then briefly lose more water than is being consumed after intake drops.

To understand why this can work to a limited extent, think of your kidneys as adjustable water-recycling systems. They filter fluid from the blood, then return much of that water to circulation. Hormonal signals help determine how much gets returned and how much leaves as urine.

ADH controls water conservation

The key hormone is vasopressin, also called antidiuretic hormone, or ADH. “Antidiuretic” means that it reduces water loss through urine.

When you drink a large amount of water:

1. The concentration of dissolved substances in your blood falls slightly.
2. The body reduces ADH release.
3. The kidneys return less water to the bloodstream.
4. More water leaves as dilute urine.

ADH acts partly through aquaporin-2 channels—tiny water passages in cells lining the kidneys’ collecting ducts, the final channels through which urine passes. With less ADH, fewer of these passages are available, so less water is reclaimed.

When fluid intake suddenly falls, urine output may remain relatively high for a short time. But that effect has limits.

Water loading does not leave the kidneys stuck in “flush mode.” The body’s water-conserving response can adjust relatively quickly.

As water loss continues, the blood becomes more concentrated and ADH rises. The kidneys then reclaim more water and produce smaller amounts of more concentrated urine.

A controlled study of combat-sport athletes found that water loading produced approximately 0.6% of body mass in additional weight loss after fluid restriction compared with the comparison condition. That is a modest effect—not evidence that water loading alone can reliably remove 20 pounds.

Aldosterone plays a different role

Another hormone, aldosterone, is often mentioned alongside ADH, but the two are not interchangeable.

• ADH mainly regulates water reabsorption.
• Aldosterone mainly promotes sodium reabsorption, which helps support fluid balance and circulating blood volume.

Sodium is one of the body’s major electrolytes—dissolved minerals that help regulate fluid balance and the function of nerves and muscles.

Aldosterone belongs to the renin–angiotensin–aldosterone system, a set of hormonal signals that helps defend blood pressure and circulating volume. If water intake expands circulating volume, this system may become less active. If dehydration reduces volume, it can become more active.

The response depends on factors including sodium intake and blood volume. It is not simply a switch that turns off whenever you drink more water. This distinction matters: the kidneys respond both to how concentrated the blood is and to how much fluid is circulating. Research on these kidney-control systems explains why the response is more complex than “drink more, then keep urinating.”

Too much water creates its own danger

Water loading is not automatically safe because it involves drinking rather than sweating.

If someone drinks water faster than the kidneys can eliminate it, blood sodium can become dangerously diluted. This is hyponatremia, meaning an abnormally low sodium concentration in the blood. Severe cases can affect the brain and become life-threatening.

This risk does not require prior dehydration. Excessive water intake itself can cause it, as described in medical guidance on exercise-associated hyponatremia.

Sweat Suits, Saunas, and Baths: Losing Water by Adding Heat

The other methods rely mainly on sweating. The important distinction is between producing sweat and successfully cooling the body.

Sweat suits interfere with cooling

Sweat cools you mainly when it evaporates—changes from liquid into water vapor—taking heat away from the skin.

A sweat suit traps heat and humidity. You may continue sweating, but the damp environment makes evaporation less effective. Water can leave your body while providing relatively little cooling.

That is why a sweat-soaked suit is not proof that the body is coping well. It may instead indicate substantial fluid loss alongside rising heat strain. Research on restrictive clothing and exercise illustrates this problem.

Saunas create heat stress without exercise

In a sauna, hot surroundings raise the body’s heat load, provoking sweating. If that fluid is not replaced, body mass falls.

The absence of exercise does not make the process physiologically effortless. The body still has to manage heat while losing fluid. Combining sauna exposure with fluid restriction compounds those stresses.

Hot baths heat the body—they do not extract water through the skin

Hot baths transfer heat into the body and can provoke sweating even while much of the skin is submerged.

They do not work by drawing substantial amounts of body water through intact skin by osmosis, the movement of water across a selectively permeable barrier in response to differences in dissolved substances.

That distinction also undercuts a common claim about Epsom salts. In a small study of MMA athletes, adding Epsom salt to the bath did not increase weight loss compared with plain water.

The relevant mechanism was heat exposure, not a salt bath “pulling” water out.

Why Regaining the Weight Does Not Prove Recovery

After weighing in, fighters often drink and eat to restore lost mass. But returning to the same scale reading does not necessarily mean every physiological system has recovered.

Dehydration reduces circulating fluid, potentially impairing heat regulation and blood flow to organs. Combining fluid restriction, exercise, and external heat increases the risk of overheating, fainting, and other serious complications. A review of rapid weight-loss practices discusses these concerns.

Can repeated cuts damage the kidneys?

Repeated severe dehydration could contribute to lasting kidney damage, but the long-term risk specifically among fighters has not been measured well enough to give a precise estimate.

Studies have found changes in blood and urine markers after rapid weight cuts. Those changes indicate acute kidney stress; they do not prove that every cut causes permanent damage.

The concern is that dehydration can reduce blood flow to the kidneys. In severe cases, this can cause acute kidney injury, or AKI—a sudden decline in kidney function. Recovery is possible, but it is not guaranteed. In broader patient populations, repeated AKI episodes are associated with worse long-term kidney outcomes.

For fighters, that is a reason for concern, not a calculation of individual risk. Research on kidney stress in combat sports and long-term outcomes after AKI supports that distinction.

Very little urine, blood in the urine, or persistent illness after a cut warrants medical assessment. Simply regaining the lost pounds is not a kidney-function test.

The Bottom Line

You can understand these methods without mistaking them for healthy weight loss:

• Water loading can briefly increase urine losses, but the effect is limited and the kidneys respond by conserving water.
• Sweat suits, saunas, and hot baths encourage sweating by increasing heat stress.
• Neither route rapidly removes fat.
• Large losses mean substantial physiological strain, not merely a temporary cosmetic change.

The scale records the missing water immediately. It cannot show you the full cost of losing it.