Athletes in weight-class sports like MMA, wrestling, boxing, and judo typically cut weight in two distinct phases: a slow fat-loss phase over several weeks, followed by an aggressive water cut in the final days before weigh-in. The slow phase sheds actual body fat, while the rapid phase manipulates water, glycogen, sodium, and gut contents to temporarily drop several more pounds. After weighing in, athletes rehydrate and refuel before competing. Here’s how each piece works.
The Slow Phase: Losing Fat Over Weeks
The real foundation of a weight cut starts well before fight week. During what combat sport athletes call “fight camp,” a period typically lasting 8 to 10 weeks, fighters gradually reduce calorie intake to lose actual body fat. The recommended target is 0.5 to 1 kilogram (roughly 1 to 2 pounds) of body mass per week. This pace is slow enough to preserve muscle and training quality while steadily bringing body weight down toward the competition weight class.
To create the necessary calorie deficit, fighters primarily reduce dietary fat intake, often to between 0.7 and 1.3 grams per kilogram of body weight per day, while keeping protein high to protect lean mass. The key guideline is that total calorie intake shouldn’t drop below the athlete’s resting metabolic rate, the baseline energy the body needs just to function. Going below that threshold risks excessive muscle loss, hormonal disruption, and poor training performance. An athlete who needs to lose 10 pounds of fat over an 8-week camp, for example, would need a daily deficit of roughly 600 calories.
Water Loading and Sodium Manipulation
The acute phase of a weight cut, usually the final 5 to 7 days before weigh-in, is where things get more extreme. The goal is to trick the body into flushing out water faster than it takes it in.
Most protocols start with “water loading.” Athletes drink unusually high volumes of water, sometimes 6 to 8 liters per day, for several days. This trains the kidneys to ramp up urine production. Then, in the final 24 to 36 hours before weigh-in, water intake drops dramatically or stops entirely. The kidneys don’t adjust immediately, so they keep producing urine at the elevated rate even though fluid intake has plummeted. The result is rapid water loss.
Sodium manipulation amplifies this effect. Athletes eat high-sodium foods during the water-loading days, then sharply restrict sodium in the final days. When plasma sodium drops, the body reduces its drive to retain fluid. Lower sodium concentrations in the blood suppress the thirst stimulus and reduce the hormonal signals that tell the kidneys to hold onto water. Together, the water loading and sodium restriction can shed several pounds of fluid in a short window.
Glycogen Depletion and Carb Restriction
Your muscles store carbohydrates as glycogen, and each gram of glycogen holds onto at least 3 grams of water. This ratio is well established in exercise physiology. That means an athlete carrying 400 to 500 grams of glycogen (a normal amount for a trained individual) is also carrying 1,200 to 1,500 grams of water just bound to that fuel source.
By cutting carbohydrate intake in the final days before weigh-in while continuing to train, athletes burn through their glycogen stores and release the water bound to them. This alone can account for 3 to 5 pounds of scale weight without any actual dehydration. It’s one of the less physically punishing parts of the cut, though training on depleted glycogen feels sluggish and exhausting.
Low-Fiber Diets to Reduce Gut Weight
A less obvious strategy targets the weight of food sitting in the digestive tract. Switching to a low-fiber diet in the final days before weigh-in reduces the volume and weight of stool in the gut. Research on healthy men found that dropping fiber intake from a normal 30 grams per day to under 10 grams per day reduced bowel movement frequency from about two per day to one, with noticeably firmer, smaller stools. Athletes typically eat low-residue foods like white rice, white bread, eggs, and lean protein while avoiding vegetables, whole grains, and fruit. Some fighters also reduce overall food volume significantly in the last 24 to 48 hours, eating only small, calorie-dense meals.
Sweating Out the Final Pounds
When water loading, sodium restriction, and glycogen depletion aren’t enough, athletes turn to active sweating. The two main approaches are exercising in heavy clothing (often wearing a sauna suit) and sitting in a hot sauna or bath.
Both methods work, but they aren’t equally safe. Research comparing sauna-induced, exercise-induced, and diuretic-induced dehydration to a target of about 4% body weight loss found that the method matters as much as the amount. Exercise-induced weight loss spread over 48 hours was less harmful to performance than the same amount lost rapidly through sauna bathing in 24 hours. Sauna dehydration and diuretics both reduced the body’s capacity for maximal exercise, including lower oxygen consumption and lower work output, while the slower exercise-based approach caused fewer measurable impairments.
In practice, fighters often combine both: low-intensity exercise in a sauna suit to start, then passive sauna sessions in the final hours if they still need to drop a pound or two. Sweat rates in a sauna can reach 0.5 to 1 kilogram per hour, making it possible to lose several pounds in a single session, though the physical toll is significant.
What Rapid Cutting Does to the Body
Rapid weight loss puts real stress on the kidneys. A review of studies on combat sport athletes found that markers of kidney function, specifically creatinine and blood urea nitrogen, were consistently and significantly elevated after rapid weight cuts. These increases indicate acute kidney stress and dehydration-related damage. In one study of Muay Thai fighters who lost an average of 4.1% of their body weight, blood urea nitrogen rose to levels indicating the kidneys were working hard to filter the byproducts of tissue breakdown. In elite wrestlers, there was a direct correlation between the amount of weight lost and the degree of kidney stress.
The mental effects are real but more nuanced than you might expect. Studies on wrestlers losing over 5% of body weight found clear mood disturbances, including increased negative emotions and decreased positive ones. However, cognitive performance, including reaction time and decision-making tests, wasn’t significantly impaired in controlled studies. The emotional toll may matter more than the mental sharpness toll, at least at the 5% threshold.
Rehydration After Weigh-In
The window between weigh-in and competition, usually 12 to 36 hours depending on the sport, is when athletes race to undo the dehydration. The goal is to replace not just water but also the sodium lost during the cut.
The standard recommendation is to drink 125 to 150% of the fluid volume lost. So an athlete who lost 4 liters of water needs to drink 5 to 6 liters during recovery, because some of that fluid will be lost to continued urine production. Drinking exactly what you lost isn’t enough since the kidneys will excrete a portion of the incoming fluid before the body fully rebalances.
The sodium concentration of the rehydration fluid matters significantly. Beverages containing at least 40 millimoles per liter of sodium promote meaningfully greater fluid retention than lower-sodium drinks or plain water. Plain water actually works against rehydration because it dilutes sodium levels in the blood, which triggers the kidneys to produce more urine, flushing out the very fluid you’re trying to retain. Oral rehydration solutions, the type used for treating dehydration in medical settings, outperform both water and standard sports drinks in the first two hours of recovery because they suppress urine output more effectively.
Athletes typically sip fluid in structured intervals rather than chugging large volumes at once. A common protocol involves drinking about 25% of the total rehydration volume every 10 minutes for the first 20 minutes, then smaller portions every 10 minutes after that. Alongside fluids, athletes begin eating carbohydrate-rich meals to replenish glycogen stores, which also pulls water back into the muscles. By competition time, a well-executed rehydration protocol can restore most of the lost weight, though whether it fully restores performance remains debated.