Miley Cyrus has come under fire by registered dieticians after she promoted her new gluten-free lifestyle on Twitter.

The 19-year-old had hit back at rumours that she was anorexic after she appeared to lose a substantial amount of weight in recent weeks.

But the performer said that she had a "gluten and lactose allergy" and told one fan that "everyone should try no gluten for a week."

"The change in your skin, physical and mental health is amazing," she wrote. "You won't go back!"

But registered dietitian Rachel Begun told Today's Health that only those with a gluten-related disorder should cut the food group out.

"People who go gluten-free may gain weight if they rely mostly on highly processed gluten free foods," Rachel said.

Another registered dietitian Karen Ansel also agreed.

"There's absolutely no evidence that a gluten-free diet promotes weight loss," Karen maintained.

"However, there is data that indicates that following a gluten-free diet can result in a diet that's low in key nutrients."