A source told the Daily Star newspaper: "The make-up they constantly wear on stage affects their skin - Harry in particular is prone to terrible spots.
"The way their management is coping with this is to give the boys three big vegetable drinks a day which are full of vitamins.
"The lads are already sick of their health drinks but they know they have to have them."
However, though the 'What Makes You Beautiful' hitmakers have reluctantly adopted the smoothie diet, but are adamant they won't be cutting out junk food any time soon.
The insider added: "When they're rehearsing for hours on end the boys almost always have takeaway pizza and Harry is a sucker for Chinese takeaways.
"The boys have agreed to have their smoothies to help with the spots, but they won't be giving up fast food."
Meanwhile, 20-year-old Louis complained of going deaf in one ear after a gig last week because of the band's screaming female fans.
Louis moaned:"I am actually going slightly deaf in my right ear. It's tinnitus... something like that.
"All our fans are always very loud."
The boys' management is making them wear special earpieces and filters to reduce the high-pitched noise as a result.