Wales' Lee Selby says he was so drained at featherweight that he would chew but not swallow food to make weight.
The former world IBF featherweight champion has not fought since losing his title to Josh Warrington last May.
Selby, 32, said he will not consider making the 126lb limit any more, with his first fight at lightweight against American Omar Douglas at London's O2 Arena on Saturday.
"The weigh-in days I felt terrible. I could've been seriously hurt," he said.
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Selby said he will quit boxing if he is unable to make an impact in the lightweight division, which has a weight limit of 135lb.
"Nine pounds in weight is a big difference in boxing, but it is not a case of 'I get to pack on another nine pounds'," he told BBC Sport Wales before the IBF intercontinental title bout.
"The reality is it just means nine pounds less to take off.
"If you saw my body on the scales last week, you could never believe I was ever able to make featherweight. I don't know how I did it.
"If you offered me £1m to fight at featherweight in a month, I couldn't do it. It wouldn't be possible. I would cry thinking about it."
Selby said he has struggled on the scales since winning the world title against Evgeny Gradovich at the O2 Arena in 2015.
"From winning the world title, being honest, I should have vacated it and started moving up the weights.
"After working so hard to get the world title, though, that becomes a hard decision to make - especially as I was struggling to get the fights I wanted.
"The lengths I went to, to cut the weight, it was pretty bad.
"I got to the stage where I would have two meals. A normal meal that I wouldn't really eat - I would chew it up and then spit it out.
"On the other plate I would just have plain salad leaves. I would try and trick my mind into thinking it had eaten. It was dangerous."
Newport-trained Selby has opted against an easy fight on his return, as 28-year-old Douglas has lost only twice in 21 professional fights.
However, Selby said he is only interested in continuing in the sport if he can get back to his best.
"I don't want any time to adjust to the new weight - I don't see any point. I want to get straight back into the mix for a world title," Selby said.
"I am fighting a live opponent, he's a big puncher and it is a 12-round title fight. He's a really good test.
"But the way I see it is simple: if I can't beat him, I am not going to beat any of the champions.
"This is not a sport to be in unless you are fighting for top prizes. It is too tough a game to be second best.
"I will never be a gatekeeper. I will never be a fighter who is happy to lose or be second best. I am not in this game for that."
Nine of Selby's 26 wins have come via stoppage, but only one of the past seven have been inside the distance.
"I had a good run of knockouts in the middle of my career, some of those against fighters who had never been stopped before," he said.
"My power has always been there, but it has been reduced by having to make the weight.
"It's something I will surprise fighters with. I've always punched hard in the gym, but in the last two or three weeks of cutting the weight, suddenly the power is gone. Now my sparring is still full on."