The Dutch forward leads the Premier League scoring charts with 18 goals this season, and the former Arsenal man has said that it is largely down to an alteration in his body mass.
"I am on a streak of matches and goals that has never happened to me before and it is all down to my fitness," Van Persie is quoted as saying in The Mirror.
"My bodyweight 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. I also recover faster. The week before last, I played on Wednesday and Saturday."
The Dutchman's goals have helped propel United to the top of the Premier League.