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Study suggests even mild weight cuts can affect performance 24h later

A recent study conducted in Australia suggests that weight cuts of 5% of body weight still affect performance after a day of rehydration.

MMA: UFC Fight Night-Weigh Ins Sean Pokorny-USA TODAY Sports

A recent post by Erik Magraken on the Combat Sports Law blog highlighted a recent study examining the effects of the dehydration on mixed martial arts athletes after various periods of rehydration. I dug into the data and spoke with one of the authors to find out what the study can actually tell us about the short-term effects of moderate weight-cuts.


The study found that following dehydration of 5% of body weight via sweating, most physiological measures of hydration had returned to normal after three hours of rehydration. Despite these normal tests, the ability to carry out high intensity exercise for a sustained period of time was significantly compromised. After 24 hours of rehydration, all physiological measures of hydration were normal, or extremely close to normal, but there was still a small but noticeable effect on sustained performance.

One of the study authors, Oliver Barley MSc, described the results:

“Our study showed clearly that acute dehydration impaired sustained high intensity effort but provided unclear results on the effects it had on short term explosive power.”

From examining the study, that seems like a reasonable characterization of the results. While there were hints that explosive performance may be somewhat compromised, the data were far from definitive on that matter. When it came to sustained effort there was a much clearer pattern, one which showed significant decreases in performance after three hours of rehydration, and less significant, but still noticeable, effects after 24 hours of rehydration.


The study had a few issues which need to be taken into account before drawing firm conclusions from the results. First, both the method of cutting weight and the amount of weight cut wasn’t necessarily equivalent to what most MMA athletes go through; there was apparently no water loading period, all weight was cut via sweat, and the participants only lost 5% of bodyweight, where the average in high level MMA is more like 8-10%.

Another potentially confounding factor is the lack of a rehydration protocol. The athletes were left to rehydrate however they wished, which raises the possibility that the recovery rate between athletes differed significantly due to varying methods of rehydration.

This decision makes sense in terms of trying to get the results as close as possible to the scenario of an actual mixed martial arts bout, but it does have the potential to affect the results in unpredictable ways.

The study also seems to find no correlation, or a weak correlation at best, between various physiological measures of hydration and the effect on athletic ability. I contacted one of the study authors about this discrepancy and was told there is a follow-up study in the works trying to look into that, and preliminary results suggest cognitive changes around dehydration may explain at least some of that discrepancy.

There was also a criticism in the paper about the reliability of various physiological markers of hydration when it comes to measuring the effects of acute dehydration and the after effects of such dehydration. The author noted that the existing hydration markers cannot directly measure cellular hydration but instead make inferences about possible cell hydration. The accuracy of this inference isn’t clear, and as hydration at the cellular level may be an important component in understanding the effect of dehydration on athletic performance, the limitations of existing hydration markers should be noted.


While the study was imperfect, the truth is it’s almost impossible to design a perfect study measuring the effect of weight cutting. There would be serious ethical considerations around allowing or asking participants to dehydrate themselves by 10% of their body weight, given the inherent dangers.

If a specific rehydration protocol was used for all participants, the study would necessarily be measuring the effects of that protocol as much as anything else. Since it’s unlikely most athletes use whatever protocol would be chosen, it would reflect real-world weight-cutting practices even less accurately if such a protocol was used.

One of the most interesting results in the study, in my opinion, was the lack of correlation between the athletic performance and the various hydration markers, which included blood osmolality, hematocrit, urine osmolarity and urine specific gravity. If nothing else, this study highlights that current physiological markers of hydration are an imperfect measure of hydration.

Since several athletic commissions are investigating hydration testing using some of these markers, it’s important that more research is done into the reliability and validity of such markers before they are used to definitively determine an athlete's hydration levels.

Outside of that, the findings seems to support the anecdotal evidence I have seen and heard, which is that a weight cut, even a relatively easy weight cut, can have a negative effect on a fighter’s “gas tank” in a fight. As more research is performed in the area, I suspect we will find a law of diminishing returns with weight cuts, which shows at some point the deleterious effects of a weight cut on stamina will outweigh any perceived advantage gained by cutting extreme amounts of weight.

The authors provided this statement about the results of the cited study, and the status of other studies currently in progress also examining the issue:

“This study raised a lot of questions, as we saw a clear effect of dehydration on repeat effort capacity (i.e sled push performance) both 3 and 24 h but the effects on short term explosive performance (i.e medicine ball chest throw, wrist grip and vertical jump) were less clear.

To better interpret these results I think it is important to understand the mechanisms behind these changes. Unfortunately there is minimal research investigating these mechanisms. However, a recent study we have just finished and will be publishing in the upcoming months investigated the influence of acute dehydration on markers of central and peripheral fatigue, from the preliminary review of these results it appears that maximal strength was not affected but strength-endurance was impaired as a result of dehydration.

Interestingly, none of the markers of central or peripheral fatigue seemed to be influenced. We did however observe dehydration influencing markers of mood. Which combined with the results from this study showing an increased perception of exertion during exercise could plausibly suggest that the influence acute dehydration has on performance could be at least in some small part caused by mental fatigue. But further research is definitely needed on this topic, before any confident assertions can be made.”