A film portraying New York City mother who starves her baby because she thinks he is saint-like and food contains impurities has caused a stir at the Venice Film Festival for its switch from light romance to painful psychosis.

"Hungry Hearts", by Italian director Saverio Costanzo, is one of two Italian films shown so far this week that are among 20 films competing for the top Golden Lion award at the world's oldest film festival.

It stars Adam Driver, who will be in the next "Star Wars" series, and Italian actress Alba Rohrwacher as his wife.

The film, which was shot for a budget of under 1 million euros ($1.3 million), starts off in rom-com style when Driver's character Jude, who works as an engineer, and Rohrwacher's Mina, who works at the Italian embassy, are both accidentally locked in the toilet of a Chinese restaurant.

They hit it off, get married and have a child, upon which Mina's fixation on cleanliness, which extends to having Jude wash his hands whenever he enters their apartment, becomes psychotic.

She only feeds vegetables she grows herself in a rooftop garden to the baby, whom a fortune teller has told her may be the reincarnation of a spirit from another world. She forbids any meat or dairy products, and will not let Jude take him to a doctor, whom she mistrusts.

When Jude finally gets the baby to the doctor one day when Mina is out, the doctor says the child's malnourishment is life threatening. This leads to a confrontation between mother and father that quickly escalates into near "Rosemary's Baby" horror-film territory.

Jude takes the baby out for a walk, to feed him meat on the sly, but on their return Mina gives the child a laxative to be sure the meat is expelled.

Francesco Bollorino, editor of psychiatryonline.it, said after a screening that although the film has a very "movie-like" ending, its portrayal of the mother, who thinks she is doing the best for the child even though those around her thinking she is killing it, was "very realistic".

"The borderline between health and insanity is difficult to see in this kind of case," he said, adding that diet fads and new lifestyles have made it even more complicated for people to determine what is or is not the right thing to eat.

Director Costanzo, whose screenplay is based on a novel by Marco Franzoso, said he had been drawn to the story because it seemed to be real.

"It tastes like a true story but I do not know if this has happened in reality or not," he said.

In an online review, trade publication Variety said the film "starts off with one of the more delightful opening scenes of recent years, but then, soon after the half-hour mark, the once-charming protags (protagonists) and their increasingly irrational behavior turn exasperating".

(Editing by Jeremy Gaunt)