Cristiano Ronaldo is acting as a diet guru to Juventus players, and Arthur has revealed the "sick" dedication that has seen the Portuguese superstar train in the middle of the night.
Five-time Ballon d'Or winner Ronaldo turns 36 in February but is showing little sign of slowing down, scoring eight goals in five starts and one appearance off the bench in Serie A this season.
He is scoring at a rate of once every 58.75 minutes, taking his haul to 60 goals in 70 league appearances for the Italian champions.
Since joining from Real Madrid in 2018, Ronaldo's average minutes-per-goal record in Serie A stands at one strike every 101.3 minutes, with only Zlatan Ibrahimovic (94.65) scoring at a faster rate of players with 20 or more goals.
Ronaldo was already known to be a fitness fanatic, with his game centred not only on great skill but a phenomenal physique and great strength.
Now Brazilian midfielder Arthur, who joined from Barcelona at the end of last season, has explained exactly what it takes to make Ronaldo such a formidable player.
"I already knew it because people talk, the world of football is small and they tell you about it. They tell you what he does, but when you see it up close it is impressive," Arthur told Marca.
"There are days when we get back at two in the morning because we played late and he starts training.
"Who does that? Cristiano. I joke and tell him that he is sick, but what are you going to say to someone who has so many Ballons d'Or. Mentally he is very strong."
Beating Barcelona 3-0 at Camp Nou this week saw Juventus finish at the top of their Champions League group, with Ronaldo scoring a pair of penalties.
It is not just his own body that Ronaldo takes an interest in, offering advice to team-mates on what they should and should not be consuming.
"Cristiano is the best. Since I arrived, he has helped me a lot because we speak the same language," Arthur said.
"He is always close and helps me, for example, in things that I have not understood. In food too, he even tells me what to eat. He cares about others, always tries to help and contribute something. I've been very lucky with Cristiano and with everyone in the dressing room, they are very good people."
Arthur, 24, suggested it was daunting to join a club packed with globally recognised figures.
He said: "When I arrived at Barca the same thing happened to me. I met players with whom I had only played on the PlayStation and now I had them in front of me: Cristiano, [Gianluigi] Buffon, [Leonardo] Bonucci or [Giorgio] Chiellini, all of them are legends."
Arthur is gradually exerting an influence, with his passing accuracy in the opposition half standing at 93.24 per cent after his first six Serie A appearances - the highest on the team of players who have started at least one game.
He has won possession in the final third of the pitch five times - matching Ronaldo for the team high in that department.
Andrea Pirlo was appointed head coach after Arthur agreed to join Juventus and having the former Italy playmaker in charge has been a welcome boost.
"I have been fortunate to meet a coach who has played in the same position as me," Arthur said.
"He helps me and tells me what to do and what not to do. He has a lot of experience so I have no choice but to lower my head and listen to him."