Katy Perry keeps in shape by eating mushrooms.

The 'Hot N' Cold' singer says tasty fungi are her favourite food and the wide variety of earthy treats are very good for staying slim.

She said: "I love mushrooms. I could eat a ton of them. I really love truffles but I hardly ever get them. Mushrooms in general though are so healthy and good for you. I can't get enough."

Katy - who is engaged to vegetarian funnyman Russell Brand - admits she can't indulge in many of the waist-expanding delicacies she loves, which include Chinese food and pizza, because she puts on weight.

Listing many of her favourite calorific treats, she said: "My fave things include barbeque chicken chop at CPK Chinese, chicken salad at Chinchins, Rock shrimp and yellow tail sashimi at Nobu, Crispy beef at Mr.Chow's, four curry chicken salad at Urth Cafe, my beloved double double at In-N-Out Burger. All so good!"

Meanwhile, Katy is busy planning her wedding to Russell.

The 'I Kissed A Girl' singer - whose parents Mary and Keith Hudson are both Christian pastors - has reportedly told the British comedian she wants to tie the knot at a location close to her Los Angeles home.

A source said: "Russell has said she can have whatever she wants - as long as his mum's there. Katy's religious parents want her to get married in a church."