Adding to this site’s archived combat sports safety studies and weight cut reform articles, a study was recently published in the International Journal of Sports Physiology and Performance addressing the impact of rapid weight loss / gain and the outcome of boxing bouts.
In the study, titled “Weight Re-Gain is Not Linked to Success in a Real Life Multi-Day Boxing Tournament” the authors compared the weigh-in weight of 100 boxers competing in the Australian National Championships and then re-checked their weight one hour prior to competition. The amount of body mass increase was then contrasted with performance and the results showed that athletes who gained size over their opponents through rapid weight cut / rehydration practices did not enjoy competitive advantage.
The full abstract reads as follows:
Combat sport athletes acutely reduce body mass (BM) prior to weigh-in in an attempt to gain a size/strength advantage over smaller opponents. Few studies have investigated these practices among boxers and none have explored the impact of this practice on competitive success.
One hundred (30♀/70♂) elite boxers participating in the Australian National Championships were weighed at the official weigh-in and an hour before each competition bout. Re-gain in BM after weigh-in was compared between finalists and non-finalists, winners and losers of each fight, males and females and weight divisions. Boxers were surveyed on their pre and post weigh-in nutrition practices.
The lightest male weight category displayed significantly greater relative BM re-gain than all other divisions, with no difference between other divisions. BM pre-bout was higher than official weigh-in for males (2.12±1.62% (p < 0.001; ES=0.13)) and females (1.49±1.65% (p < 0.001; ES=0.11)). No differences in BM re-gain were found between finalists and non-finalists, winners and losers of individual bouts, or between preliminary or final bouts. BM re-gain was significantly greater (0.37% BM, p < 0.001; ES=0.25) prior to an afternoon bout compared to a morning bout.
Boxers engage in acute BM loss practices before the official competition weigh-in but this does not appear to affect competition outcomes, at least when weight re-gain between weigh-in and fighting is used as a proxy for the magnitude of acute loss. While boxers recognise the importance of recovering after weigh-in, current practice is not aligned with best practice guidance.
Combat Sports Law 
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