The 29-year-old forward has netted 22 goals in 28 games to prove an astute purchase by Sir Alex Ferguson as his side look forward to a Sunday afternoon game against Everton with the chance of going 12 points clear of Manchester City.
And the Netherlands striker - who has struggled with injuries throughout his career - believes his current run this season is largely due to becoming heavier and being able to compete with opposition defenders with more strength.
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"My body weight used to always be around 77 kilos. Now I weigh 81 kilos. That is a difference of more than eight pounds - and it is all pure muscle.
"I haven't been spending more time in the gym lifting weights. I have got myself super-fit and as a result I am both heavier and stronger.
"It means I am stronger in battles with defenders, but I also feel like I am in the best condition of my life."
The prolific Dutchman has scored a number of vital goals against the likes of former club Arsenal, Liverpool, Tottenham and Chelsea this season, and has warned the rest of the league that he is coping well with his current workload.
He continued: "I also recover faster. The week before last, I played on Wednesday and Saturday.
"There was only two-and-a-half days between the games, yet I felt super-fit and had no trouble doing extra work in the final part of the game."