I don’t think he did Ju-Jitsu for boxing. It’s literally pointless. I’ve seen Mayweather, Vitali and many more still chop wood.
My old pal Kirkland Laing told me he used to chew 10 pieces of gum at once for an hour at a time to strengthen his jaw. Not sure you’d find it in the boxers training manual 2022!
If I recall correctly, in marathons, overhydration tends to cause more serious issues than dehydration
Dempsey references chewing gum I believe in his book. Harold Johnson advocated it to Ali after a spar because Muhammad had a sore one