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Sports nutrition · Part 3
Sport · Combat sports · Evidence graded

Making Weight: What the Science Says About Cutting for a Fight, and What It Doesn’t Know

Most fighters lose weight for a fight. A lot of it is water, and it goes in the last few days. The sport’s own nutrition society now publishes a guide to doing it, sauna and hot baths included. Here’s what that guide rests on, what the trials show, and the question nobody has answered: what does doing it ten or twenty times do to you?

Stephen Duncan FDN-P MSc · Detective Health · October 2026 · 16 min read

The grades in this post: Trial means randomised or controlled trials, or meta-analyses of them. Observational means surveys, cohorts or weigh-in records: they show what happens, not what causes it. Mechanism means it makes physiological sense but hasn’t been tested as a strategy. Opinion means expert consensus or practitioners’ experience, not data. Risk marks a safety point.

Most combat-sport evidence is small. A “trial” here often means 10 to 20 men in a lab. I say so where it matters.

The short version

Chronic and acute: two different things

When fighters talk about “making weight“, they usually mean two separate processes.

Chronic weight loss happens over the weeks of a fight camp. It’s fat loss, done by eating less and training more. The ISSN’s 2025 position stand on combat sports calls this the “longitudinal weight descent” (Ricci 2025). It’s the same as any sensible diet, with a deadline.

Acute weight loss happens in the last few days, mostly the last 24 to 48 hours. Fighters drop water, glycogen (stored carbohydrate, which holds water) and the contents of the gut. Then they weigh in, and put it back on before the fight. That’s the “cut” and the “rehydration“. It’s what this post is mostly about.

The difference matters because the risks are different. Losing fat at a sensible rate is low-risk. Losing several percent of your body water in a day is not.

How common is it, and how much?

Very common. Observational questionnaire studies, mostly self-reported.

What happens after the weigh-in is also big. In 399 professional boxers weighed by the California State Athletic Commission, 97.5% gained weight before the fight. They gained an average of 7.2% (4.4 kg). One in five (20.6%) gained 10% or more (Murugappan 2021). You can’t regain 10% overnight unless you lost something like it first.

Harms: what’s documented

Deaths. Case reports In November and December 1997, three previously healthy US college wrestlers died within five weeks of each other. Each was on a rapid weight-loss programme before a weigh-in. They restricted food and fluid, wore vapour-impermeable suits under cotton tracksuits, and exercised hard in hot rooms. They developed hyperthermia (overheating) and dehydration (CDC 1998). A review of youth combat sports says weight-cutting deaths “still persist into recent times” (Lakicevic 2022).

Collapse and illness. Observational self-reported. In elite judo, 21% had collapsed during a cut (Štangar 2022). In a survey of 70 judo athletes, 71% reported weakness, 51% headaches and 46% dizziness while cutting (Staśkiewicz-Bartecka 2025).

Kidneys and blood. Small observational and experimental studies. In 12 wrestlers, a cut raised blood urea, uric acid and creatinine more than the same training without a cut (Trivic 2023). The authors say this suggests a higher risk of acute kidney injury. That’s a reasonable worry, but rising creatinine in a dehydrated person doesn’t by itself mean kidney damage, and 12 people can’t settle it. In a controlled hot-bath study in 13 MMA fighters, plasma volume fell by about 14%, and urea and creatinine rose. All returned to near baseline after a day of eating and drinking (Connor 2022). In 10 boxers who cut 5.6%, plasma volume fell 8.6%, and total haemoglobin mass fell 5.3%. Haemoglobin mass and blood volume were still below baseline a week later (Reljic 2013). The authors called that last finding surprising and said it needs further investigation.

Injury. Observational In 155 combat-sport competitors across 24 events, men who lost more weight in the 24 hours before weigh-in were more likely to report an injury a week after the fight. Each extra 1% raised the odds by a factor of about 1.2 (Doherty 2025). That’s an association, not proof.

Eating. Observational Four out of five combat-sport competitors scored moderate to very high for disordered eating (Doherty 2024). Athletes who used certain higher-risk methods were more likely to report disordered eating behaviours. Sauna use was linked to bigger cuts (Barley 2025). Two-thirds of judo athletes in one survey reported binge eating after weigh-in (Staśkiewicz-Bartecka 2025).

What isn’t documented. Nobody has followed fighters for decades to see what repeated cutting does to kidneys, heart, bone or brain. The ISSN says so plainly (Ricci 2025). More on that below.

What dehydration does to performance

In general. Trial evidence (meta-analysis of 28 studies). Being dehydrated cut muscular endurance by about 8%, strength by about 5.5%, and anaerobic power by about 6%. Losing the water through exercise hurt more than losing it passively, for example in heat. Trained people tended to lose less than untrained people, though that difference wasn’t statistically significant (P = 0.06) (Savoie 2015).

In combat athletes. Trial evidence, small studies. A meta-analysis compared performance before a cut, straight after it, and after 3 to 36 hours of recovery. Straight after the cut, there were small falls in maximal strength and in repeated high-intensity efforts. After recovery and regaining weight, performance was no different from before the cut overall (Brechney 2022). Another review of Olympic combat sports found no effect on strength or power from cuts of up to 5% done in under 7 days (Mauricio 2022).

In boxers specifically. Small trials. Sixteen amateur boxers who cut an average of 5.2% in a week did worse on a boxing circuit, with more anger, fatigue and tension, and less vigour (Hall 2001). In 7 novice boxers who lost 3.8% by sweating, the average drop in boxing performance wasn’t statistically significant, although a few boxers dropped a lot and one improved (Smith 2000).

How to read this. The studies are small, mostly men, and short. They measure strength, jumps and sprints in a lab, not three-minute rounds while being hit. None measures what dehydration does to how well you take a punch. The most honest summary: a moderate cut followed by a full day of recovery seems to cost little on the tests we have. A cut followed by a short recovery may cost more. Whether a cut ever helps depends on regaining more than your opponent, which is the next section. Mixed

Why the weigh-in timing matters so much

The time between weigh-in and fight is the single biggest factor. Consensus with observational support.

Regaining weight and winning. Observational In Australian national amateur boxing championships, boxers weighed about 2% more before each bout than at the weigh-in. But regaining more didn’t predict who won (Reale 2016). In professionals, with a day to recover, it did. Across 1,392 professional boxers, winners had regained 8.0% against losers’ 6.9%. Each extra 1% regained was linked to a 13% higher chance of winning (Baribeau 2022).

That’s the trap. If a bigger rebound predicts a win, everyone has a reason to cut harder. The authors of that study say their findings “raise fair-play and safety concerns“.

Putting weight back on isn’t the same as recovering. A 2026 meta-analysis found fighters regained on average 6.2% of body mass after weigh-in. In the few studies that measured it at the weigh-in itself (3 studies, 58 athletes), urine tests showed them dehydrated on average. The authors warn that weight regained should not be read as full rehydration or readiness to perform. No recovery protocol had enough evidence to call it the best (Sun 2026).

What one rule change showed

A caution on hydration tests. Urine specific gravity is the test most used to check hydration at weigh-ins. A review found it unreliable in combat athletes when diet and training aren’t controlled (Zubac 2018). A hydration test is better than nothing, but it can be wrong in both directions. Review evidence.

The ISSN position stand: what it says, and how to grade it

In 2025 the International Society of Sports Nutrition published a 16-point position stand on weight cutting for MMA and other combat sports (Ricci 2025). It’s the most detailed guide there is. It’s also the one a fighter or coach is most likely to find. So it’s worth reading carefully.

What it says. - Stay within 12% to 15% of your weight class between camps. - Lose fat gradually in camp, with protein prioritised. - In fight week, sodium restriction, water loading, cutting carbohydrate and a low-fibre diet can reduce body mass. - Sauna, hot baths and “mummy wraps” (being wrapped, apart from the head, in warm towels or foil blankets, often alongside other methods) “can be used effectively with appropriate supervision“, ideally for about 2% to 4% of body mass within 24 hours of weigh-in. - Rehydrate and refuel straight after the weigh-in. - Point 16: “The long-term effects of frequent weight cuts on health and performance are unknown, necessitating further research.”

What it gets right. It says diet pills, enemas, laxatives and bulimia are “not endorsed“. It describes diuretics as reported, potentially harmful and banned by the World Anti-Doping Agency. The wording of the “not endorsed” sentence is loose: the same list also includes saunas and fluid restriction, which the stand endorses later. It says losing 10% within 24 hours by sweating alone is “ill-advised and potentially dangerous to health“. It tells practitioners to monitor core temperature and stop before 40°C (104°F). It says athletes with same-day weigh-ins should rely on fat loss, not water. And it admits the long-term unknown in its own summary.

What the endorsements rest on. - Hot baths. Small trials. Two hot baths with wraps took about 2% to 2.8% of body mass off MMA fighters, with no extra loss from adding Epsom salts (Connor 2020; Connor 2022). The baths were around 39–40°C (102–104.5°F). In both studies the fighters had already restricted food and fluid for about 24 to 28 hours. In one study, performance was unaffected after a day’s recovery. The studies had 8 and 13 men. - Sauna. Mostly non-athlete data. The position stand says there’s “a paucity of data” in combat athletes. Its figures come from young adults sitting in saunas, who lost about 0.5% to 0.9% of body mass. - Mummy wraps. Almost none. The stand says researchers “have yet to thoroughly investigate this strategy“. The only data are the wraps used after the hot baths in the studies above, which is one way of using them, not the definition. - The fight-week percentages. Opinion and observational records. The stand says its targets for each weigh-in window are “based upon anecdotal observations from the authors“. The figures for losses at 72, 48 and 24 hours come from records of 600+ UFC fighters who made weight. That shows what successful fighters did, not what’s safe. The stand notes hydration status in those 24 hours wasn’t published. - The walk-around weight. Opinion The stand says the 12% to 15% range has “not yet been fully substantiated through research“.

The tension. The stand endorses passive sweating, with supervision, for fighters with a day to recover. It also says nobody knows what years of repeated cutting do. Both statements can be true. But they sit uneasily together. An endorsement in a position stand will be read by coaches and teenagers as “safe“. What the evidence actually supports is narrower. In a few dozen adult men, under supervision, one cut of a few percent, followed by a day’s recovery, didn’t measurably hurt short-term lab performance or leave blood markers abnormal the next day. Nothing beyond that has been tested. Not repeated cuts. Not women in any number. Not teenagers. Not long-term kidney or brain health.

Where I’d grade it. Pragmatic harm reduction for adults who are going to cut anyway. Built on small trials and practitioner experience. Silent, by its own admission, on long-term harm. Useful if you’re a professional with a supervised team. Not a reason for anyone else to start.

The gradual approach

Fight-camp weight management. Case study and observational evidence. In a well-known case study, a professional boxer made 59 kg over 12 weeks without any deliberate dehydration. He lost 9.4 kg at about 0.9 kg a week, and his body fat fell from 12.1% to 7.0%. He then refuelled after the weigh-in and entered the ring at 63.2 kg (Morton 2010). One boxer proves it can be done, not that it suits everyone. In 33 professional MMA fighters across 80 fights, slower weight loss in camp (rates under 0.5% of body mass; the abstract doesn’t state the time unit) best preserved lean mass. Body mass 7–10 days before the weigh-in, not dehydration in the last two days, best predicted how much they put back on afterwards (Reale 2024).

The practical lesson is simple. The closer you live to your fight weight, the smaller the water cut needs to be.

Low-residue (low-fibre) diets. One small trial, cited secondhand. Cutting fibre for a few days reduces the weight of food and water in the gut. The ISSN cites one study in 19 healthy men. Four days of low fibre reduced body mass by 0.4%, and by 0.74% on day five. The stand’s practical point says 1% to 2%, which is more than that study showed. I couldn’t find the original study to check it. Of all the fight-week methods, this one has the least obvious downside: the reported effects were more hunger and fewer bowel movements, and the diet was tolerated. Low risk; small effect; thin evidence.

Cutting carbohydrate. Mechanism with small studies. Glycogen holds water, so running glycogen down drops weight. The ISSN cites a 2% drop from a week of low carbohydrate. The cost is that carbohydrate is fuel for high-intensity work. That’s why it needs time to refuel.

Water loading. One small randomised trial; one smaller study. Water loading means drinking large volumes for several days, then restricting fluid. The idea is that the body keeps passing urine after you stop drinking. In the best study, 21 male athletes were randomised. Those who water loaded lost 0.6% more body mass after fluid restriction than controls. Blood sodium and other electrolytes stayed within normal ranges, and performance didn’t differ (Reale 2018). Small, but real under the study’s controlled conditions. The worry with water loading is low blood sodium (hyponatraemia). That didn’t happen in this trial, which used a standardised diet and tested blood. My view: an athlete doing this at home usually has no one checking their sodium. Effective in one trial; safety shown only under controlled conditions; Risk if improvised.

Youth and amateur boxers

This is where I’d draw the firmest line. Observational evidence; one review calling for caution; the under-18 line is my opinion.

Amateur boxing usually weighs in the same morning, so a big cut can’t be fully undone before the bout (Ricci 2025). The ISSN position stand is written mainly for professional MMA. The authors of the youth review call for a “conservative approach” with young athletes (Lakicevic 2022). I’d go further: no deliberate dehydration under 18. That’s my opinion, not a finding.

What my own experience adds

Experience, not evidence.

I boxed competitively from 17. I never went to extremes with water. My way of making weight was eating very little and training hard, and looking back, low calories plus intense exercise is its own kind of extreme. My thinking at the time was that I’d rather trade some muscle to lose fat, and keep dehydration as a last resort around the weigh-in if I’d got it wrong. As I got older I moved away from starving myself and ate more abstemiously instead.

What I’d tell a young boxer now:

Who it doesn’t apply to

Why there are no protocols here

Detailed fight-week protocols exist, including the ISSN’s own, but they’re written for supervised adult professionals, so this article deliberately doesn’t give them. If you’re under 18, they don’t apply to you.

Where I land

Making weight is part of combat sport, and it isn’t going away. The question is how much and how.

The fat-loss part is ordinary good practice: start closer to your weight, lose slowly, eat enough protein, and give yourself weeks, not days. The evidence for that is a case study, some observational data and plain physiology, and I’d still back it.

The water-cut part is where I’d be careful. A moderate cut, supervised, with a full day to recover, seems to cost little on the measures we have. That’s the most the evidence supports. The ISSN’s endorsement of sauna, hot baths and wraps is pragmatic harm reduction for professionals. It isn’t evidence that repeated cutting is safe, and the ISSN itself says nobody knows.

If you’re young, or an amateur with a same-day weigh-in, the answer is easier. Fight at a weight you can make without dehydrating.

Previously in the series: part 2 covers fuelling, and what happens when you don’t eat enough for what you’re doing.

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Continue reading
Sports nutrition · Part 2
Under-Fuelled
Sports nutrition · Part 1
Sports Supplements: The Short List That Works
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