The pair - who tied the knot at the luxury Aman-i-Khas resort in Rajasthan, India, last month - are determined to get fit and feel healthier, and so have cut out all wheat and dairy until the end of this year.
A friend of the couple said: "Katy and Russell are taking care of themselves and are on a mission to get fit. As well as cutting out wheat and diary, they`re on a diet of raw vegetables, tofu and pulses.
As well as their special diet, the British comedian - who is vegetarian - and his pop star wife also appear to have picked up some health tips during their recent stay in India.
The source explained to the Daily Star newspaper: "They`re both drinking loads of pineapple juice and are obsessed with Indian green tea which is full of anti-toxins and great for weight loss."
However Katy`s and Russell`s new health kick may not last until the New Year as the pair are said to be planning a fun filled family Christmas in London.
The insider revealed: "Katy and Russell are trying to get their families together for their first Christmas as husband and wife. Katy thinks a white Christmas in London would be romantic.
"They are hoping to have a proper knees-up with the whole of Russell`s family. Katy`s mum and dad are hoping to come to London as well."