Gary Barlow reportedly eats separately from the other X Factor judges, so he is not tempted to stray from his strict diet.
The Take That star - who used to be mocked by bandmate Robbie Williams as "the fat one" of the band - sticks to special carrot, beetroot and lemongrass smoothies, given to him at specific times to stop him piling on the pounds.
Gary, 41, also has special meals of vegetables and lean protein prepared for him by The X Factor chef, an insider told The Sun, and eats away from fellow judges Louis Walsh and Tulisa Contostavlos to avoid any temptation from the less calorie controlled food they eat.
The singer also avoids sugar, caffeine and milk, only drinking peppermint tea, which is said to aid digestion.
Gary, who once piled on the pounds to 17 stone, has previously spoken of his battle with his weight and revealed when Take That split in 1996 he comfort ate to cope with depression.
He said: "For somebody so big, I felt incredibly small."
Since Take That reformed in 2005 Gary is thought to have lost five stone, and last year encouraged his X Factor acts to follow his example.
Contestant Craig Colton claimed he lost a stone in a week on Gary's diet.