Everyone who cuts weight knows it is unpleasant. Almost nobody has been told it is an injury.
The belief is that water weight comes back, so nothing was lost. The water comes back. The kidney damage does not fully reverse.
What the bloodwork actually shows
When researchers measure kidney function in combat athletes before and after a rapid weight cut, the findings are consistent across studies: creatinine rises. Blood urea nitrogen rises. Urine specific gravity rises.
Those are not markers of being thirsty. They are the laboratory signature of acute kidney injury.
A systematic review of Olympic and non-Olympic combat sport athletes losing roughly 5% of body mass in a week or less found these markers significantly elevated in the majority of included studies. A separate study in wrestlers measured cystatin C, a more sensitive marker than creatinine, and found the same pattern.
Roughly 80% of combat athletes use deliberate methods to cut body mass. Many do it several times a year. Some do it up to ten times a year.
Board-certified nephrology at Remix Medical in Houston. Call (713) 597-5131 or book online.
Why repetition is the thing that matters
A single episode of acute kidney injury usually resolves. Creatinine comes back down. The athlete feels fine, makes weight, fights, and does it again in three months.
Here is what the recovery is hiding. An episode of acute kidney injury is not an event that ended. It is a risk that started. Even when creatinine returns to baseline, nephrons were lost, and the kidney that recovered did not recover completely.
Now repeat that, deliberately, ten times a year, for a decade.
We have seen exactly what this does in another population. Sugarcane cutters in Central America develop a form of chronic kidney disease with no diabetes and no hypertension — an epidemic now called Mesoamerican nephropathy. The proposed mechanism is recurrent heat stress and dehydration, producing repeated subclinical acute kidney injury, which produces interstitial fibrosis and tubular atrophy that does not reverse.
Field laborers do it for work. Fighters do it on purpose.
The electrolytes are how it hurts you on the night
Dehydration is only part of it. The methods used to shed water — saunas, sweatsuits, fluid restriction, diuretics, laxatives — move electrolytes in ways that can be dangerous.
Potassium. Falls with sweating, vomiting, laxative use, and diuretics. Low potassium causes muscle weakness, cramping, and cardiac arrhythmia.
Magnesium. Lost the same ways, and its depletion makes low potassium refractory to replacement.
Rhabdomyolysis. Hard training in a dehydrated, glycogen-depleted state breaks down muscle and releases myoglobin, which is directly toxic to the kidney tubules. This is the mechanism behind the fighters who end up in an intensive care unit rather than a cage.
Refeeding. After a severe cut, aggressive eating and drinking shifts phosphate, potassium, and magnesium into cells rapidly. In its severe form this is refeeding syndrome.
The NSAID problem
Fighters take ibuprofen and naproxen for the ordinary aches of training. Frequently. Often during the cut.
NSAIDs reduce blood flow to the kidney. In a well-hydrated person, that is tolerated. In a dehydrated person losing 5% of body mass in a week, it removes the kidney's principal defense against low perfusion.
Dehydration plus NSAIDs is a recognized combination for precipitating acute kidney injury. If you take nothing else from this page: do not take ibuprofen during a cut.
Supplements and creatine
Creatine gets blamed for kidney damage more than the evidence supports. In healthy people with normal kidney function, standard doses do not appear to harm the kidney.
Two caveats that matter.
Creatine raises your creatinine without harming the kidney, because creatine is metabolized to creatinine. A fighter on creatine who gets a blood test may look like they have kidney disease and not have it — or may have real injury masked by an assumed explanation. Tell whoever draws your blood.
High-dose anything is different. Excessive vitamin D, excessive vitamin A, anabolic steroids, and diuretics used to make weight all carry real kidney risk, particularly stacked on dehydration.
And it does not even work
This is the part worth sitting with.
In studies of MMA competition, the amount of weight cut did not predict winning. One analysis found no difference between winners and losers in weight lost or regained. Another found that fighters who cut more weight were more likely to lose.
You are injuring an organ for an advantage that the data does not show exists.
Trauma is the smaller risk
Kidneys are among the more commonly injured abdominal organs in sport, and strikes to the kidney are legal in MMA where they are prohibited in boxing.
Blunt trauma is real and it is worth knowing about. But it is not the main way combat sports damage kidneys. The weight cut is.
What to do instead, and what to watch for
Compete closer to your walking weight. Cut earlier and slower, with body composition rather than water. Work with someone who measures rather than guesses.
During and after a cut, get medical attention for confusion, seizure, chest pain or palpitations, muscle pain with dark or cola-colored urine, or producing very little urine.
And if you have cut weight repeatedly for years — get your kidney function checked. An eGFR and a urine albumin-to-creatinine ratio. Not because something is certainly wrong, but because the damage this causes is silent until it is not, and chronic kidney disease produces no symptoms until most of the function is gone.
If you have cut weight more than a few times, you have had episodes of kidney injury nobody measured. Two tests will tell you where you stand.
Call (713) 597-5131 or book online.