The former Miss World is now a vegan, having been a vegetarian for more than a decade.
The model is in training for an Ironman Triathalon in Galway in September, which she will compete in aid of the Irish Autism Action charity.
She tells the Irish Daily Mirror that her new diet gives her energy for the gruelling challenge, that involves a 1.9 km swim in Galway Bay followed by a 90 km bike ride finishing up with a run around Salthill.
“I start the day with a glass of warm water and lemon, then I make a smoothie with spinach, celery, banana, apple, lemon and pear.
“I know this may sound disgusting to most people but I’ve learned to love it and it gives you far more energy than you get from a cup of coffee.
Rosanna is studying n naturopathic nutrition and biochemistry and says her course has opened her eyes to what she puts into her body.
"It was enough to turn me off animal products," she says.
She will take part in the Galway Ironman challenge with her trainer Paul Byrne and fellow celebs Brian Kennedy and Keith Duffy on September 2nd.