Narrative review: Acute kidney injury associated with dehydration protocol used by combat sports athletes

Why this study matters

Combat sports use weight classes, and the official weigh-in is usually held 24 hours before the fight. The authors write that many fighters shed body fluid before the weigh-in to qualify for a lower class, a practice called rapid weight loss. They say this dehydration lowers blood volume and can affect the kidneys and the heart, and that deaths and acute kidney injury, a sudden loss of kidney function, have been reported in mixed martial arts. The study measured kidney function before the weigh-in, on the weigh-in day and on fight day.

What the researchers did

Thirty male fighters aged 18 to 37 took part: 15 from mixed martial arts (MMA) and 15 from Muay Thai. Each was assessed three times within 60 days: about 45 days before a fight, on the official weigh-in day and on fight day.

At the first assessment the fighters answered a questionnaire about their sport, their health and how they lose weight before competition. The researchers measured body weight, blood pressure and heart rate and took blood and urine samples. Blood was tested for creatinine and urea, two waste products that the kidneys clear. The researchers also estimated the glomerular filtration rate (GFR), a measure of how well the kidneys filter blood, with the CKD-EPI equation. Urine was checked with test strips for protein, blood, glucose and other signs, and its color was used to grade hydration.

What they found

By the weigh-in, fighters had lost a mean of 10% of their body weight in the MMA group and 7.7% in the Muay Thai group. Mean weight went from 74.7 kg to 67.3 kg (MMA) and from 70.6 kg to 64.9 kg (Muay Thai), and was 73.3 kg and 69.8 kg on fight day. Fighters reported restricting food and fluids and sweating through exercise, plastic clothing and hot, humid places. All of the MMA fighters and 40% of the Muay Thai fighters reported taking diuretics, drugs that increase urine output. At the weigh-in, the urine color scale classed 94% of the MMA group as dehydrated or severely dehydrated and all of the Muay Thai group as severely dehydrated.

Blood markers shifted at the weigh-in. In the MMA group, mean creatinine rose from 1.08 to 1.31 mg/dL and mean estimated GFR fell from 97.7 to 79.1 mL/min/1.73 m2. In the Muay Thai group, creatinine rose from 1.10 to 1.20 mg/dL, a change that was not statistically significant, and estimated GFR fell from 97.2 to 87.2. On fight day, mean creatinine was 1.09 and 1.12 mg/dL and estimated GFR was 96.5 and 102.2.

In the article’s second table, protein was present in the urine of 53% of the MMA group at the first assessment, 73% at the weigh-in and 40% on fight day. In the Muay Thai group the figures were 60%, 100% and 13%. The authors read the weigh-in results as a temporary impairment of the kidneys at the height of dehydration. They conclude that the findings suggest kidney damage and that the practice may lead to acute kidney injury.

Strengths and limits

The study measured fighters around real competitions, with blood and urine tests as well as body weight.

The sample was 30 men, which the authors acknowledge is small. There was no comparison group. The researchers had no control over the use of laxatives or diuretics and did not record how fighters rehydrated after the weigh-in. Kidney filtration was estimated with an equation and not measured directly, and the authors note that creatinine levels in athletes are also linked to muscle mass and activity. The last measurements were taken on fight day, and the authors write that longer monitoring is needed to tell reversible changes from a progression to chronic kidney disease.

No criteria for defining acute kidney injury are stated in the article, although the term is in its title. Its text and tables also disagree in places: the abstract speaks of an increase in filtration rate, while the results show a fall at the weigh-in. The figures in this review come from the tables.

What it adds to research on fighter health

The study adds measurements taken at an official weigh-in and on fight day in MMA and Muay Thai fighters. In this group, blood markers of kidney function shifted at the weigh-in, when almost all fighters were classed as dehydrated, and mean values had moved back towards their starting levels by fight day. The study followed each fighter through one fight and stopped on fight day, so it cannot show whether repeated weight cuts cause lasting harm. The authors call for guidance on safer methods of weight control.