The filmmaker has been shooting the boy band on tour for a documentary but made his name eating nothing but food from the world famous chain for a month for his documentary 'Super Size Me' and cataloguing the effects on his body.
Morgan admits the five guys - Niall Horan, Zayn Malik, Liam Payne, Harry Styles and Louis Tomlinson - have had a lot of fun munching burgers but they have a personal trainer who travels with them to keep them all fit and healthy.
Taking part in a Twitter Q&A under the hash tag #AskMorgan1D3D, he revealed: ''They love eating McD in front of me, but on the new tour they have a trainer who's keeping them off the Big Macs!''
Morgan also revealed he has captured many ''shirtless moments'' on the road but he isn't sure how many will make the final cut
He said: ''More shirtless moments than you can imagine, now how many of those make it into the final film we'll have to wait and see.''
As well as capturing the boys' ''wild side'', the 42-year-old filmmaker also promises many emotional moments in the final film.
He said: ''There have been some really lovely emotional moments that we've captured, I think fans may shed a tear or two while watching.''
Shooting on the 3D documentary began last November and it is set to be released in August.