Mike Brown has revealed that England's heat camp in north east Italy is causing players to shed half a stone in weight during gruelling training sessions.
Treviso was picked to host the 12-day visit due to the similarity between the conditions there and Japan, the setting for this autumn's World Cup where humidity for September is an average 73 per cent.
The level drops only slightly in October when the majority of the competition will be played and former Brave Blossoms coach Eddie Jones knows that central to a team's prospects will be their ability to acclimatise.
Brown has suffered along with his team-mates in the first of England's two Treviso camps, insisting they are more punishing than playing Test matches.
"It's incredibly tough. It's been about 80 per cent humidity so as soon as you step outside, everyone starts sweating," Brown said.
"You are absolutely dripping with sweat and that makes ball-handling very tough. It's also hard to keep your core temperature low because you are sweating all the time.
"It just sits on your skin and then heats up even more so you can't get your body temperature down. It's really sunny here as well so you're constantly feeling incredibly hot.
"We have guys coming on and spraying us with cold water and we're constantly trying to wipe the sweat off you so your skin gets the chance to cool down."
England's World Cup hopefuls, who will be trimmed to a final 31-man squad on August 12, are weighed before and after each session.
"On our first really hard session I lost 3kg of weight. Back in England, I would barely lose any weight from a normal session in normal conditions," Brown said.
"After a Test match, I would probably lose a maximum of 1kg. And that's playing at the highest level under massive fatigue. So that puts into perspective the weight loss that you can get over here.
"The nutrition guys make sure you get the right things after training to put that weight on. We have protein shakes and bars, fruit and liquids with proper salts to get the hydration back in.
"Once you have taken all the right things, we will go and eat lunch and you are pretty much back on it by the afternoon session. It's no problem as long as you do the right things."
Brown's presence in Treviso seemed improbable at the start of the month after England's most capped full-back was left out of Jones' initial World Cup training squad.
The Harlequin had been frozen out since winning the last of his 72 caps on the 2018 tour to South Africa but now has now been offered a glimpse of selection for Japan 2019.
"You have that doubt at the back of your mind when you're left out, but Eddie said just be ready," Brown said.
"I went straight back to Quins and was welcomed with a Bronco fitness test. That was pretty savage. I was over the moon to get the chance to come back in and continue.
"I am a competitive animal and I absolutely love playing for England, it means everything to me.
"It's great to be here and I am trying to show what I am about and show what I can bring to the team."