A diet endorsed by US pop singer Beyoncé "could be dangerous," the British Association for Nutrition and Lifestyle Medicine has told the BBC's Victoria Derbyshire programme.
Subscribers to the 22-day plan pay $14 (£11.50) to access plant-based recipes.
But nutritionist Daniel O'Shaughnessy says the diet could lead to "nutritional deficiencies".
Beyoncé's trainer Marco Borges said the singer was "mindful of the importance of proper nutrition and exercise".
The original diet was created in 2013 but the singer is now promoting Beyoncé's Kitchen, the plan she followed in preparation for her comeback performance at the 2018 Coachella festival after having twins.
A promotional video released on her YouTube channel last month opens with a clip from her Homecoming film, showing her stepping on to some scales and saying her weight is "every woman's worst nightmare".
The video has been viewed 1.7 million times.
Beyoncé also says to reach her goals she must limit herself to "no bread, no carbs, no sugar, no dairy, no meat, no fish, no alcohol… and I'm hungry". She followed the plan for 44 days.
Her plan was devised by 22 Days Nutrition founder Borges, whom she describes as her "friend, trainer, exercise physiologist and New York Times bestselling author".
Mr O'Shaughnessy questioned its stated nutritional values.
For example, one of the recipes, a "vegg sandwich" containing 36g (1oz) of protein, actually contained just 24g, he said.
While another, a green smoothie, contained eight teaspoons of sugar.
The NHS recommends men consume 2,500 calories a day and women 2,000 - but the diet supplies just 1,400.
"This is quite low for anyone, users will feel tired and exhausted particularly when adding in the exercise," Mr O'Shaughnessy said.
"It could be dangerous for the average person to follow without a team of nutritionists and trainers like Beyoncé has."
Excluding all animal products without any information on what nutritional issues the dieter may need to consider, such as replacing vitamin B12, iron or protein intake, was also problematic, Mr O'Shaughnessy said.
And celebrities should be encouraging women to be comfortable with their bodies.
"Beyoncé is selling a dream," Mr O'Shaughnessy said.
"This is worrying as she has a number of teenage followers who are easily susceptible.
"She is a gateway to millions of people."
Beyoncé did not respond to a request for comment.
In a statement, Mr Borges, said: "Beyoncé used a combination of a whole food plant-based diet and daily exercise as part of her discipline and hard work in order to reach her personal goals in preparation for her Coachella performance...
"She achieved her goals successfully and was able to show up and give 100% for a performance which required nothing less.
"She continues to be mindful of the importance of proper nutrition and exercise as part of a healthy and happy lifestyle.
"We applaud her and are humbled by her courage to share her journey with others."