Two strict vegans have gone on trial in France charged with "neglect or food deprivation" after the death of their breastfed 11-month-old daughter who was found to be suffering from vitamin deficiency.

Sergine and Joel Le Moaligou called an ambulance to their home in the village of Saint-Maulvis – 90 miles north of Paris – in March 2008 after their daughter Louise became listless. By the time paramedics arrived, the baby had died.

The police were alerted after the ambulance crew noticed the baby was pale and thin and a doctor refused to issue a death certificate. A postmortem showed the child, who had been fed only on her 37-year-old mother's milk, weighed 5.7kg when she should have been about 8kg.

She was also suffering from deficiency of vitamins A and B12, which may have left her susceptible to infection. She died of a pneumonia-related illness.

Medical experts told the court in Amiens that the vitamin deficiency could have been caused by an unbalanced diet. While anxious not to call into question the couple's lifestyle, Anne-Laure Sandretto, the deputy prosecutor, admitted: "The problem with a vitamin B12 deficiency could be linked to the mother's eating habits."

The parents, who also have a 13-year-old daughter who was not found to be suffering from any deficiencies, said they became vegan after seeing a TV programme about "how animals were taken to abattoirs", said the mother's lawyer, Stephane Daquo.

At the time of their daughter's death, they were running an organic food business and refused to eat any animal products. Daquo said they had a mistrust of traditional medicine and preferred to treat their children's complaints with advice from books.

"The couple did not follow the doctor's advice to take the baby to hospital when they went for her nine-month checkup and found she was suffering from bronchitis and was losing weight," he said. Instead they treated her with cabbage poultices, mustard and camphor and washed her with earth and clay instead of giving her baths, the court heard.

Daquo said the couple had been reading "the wrong things at the wrong moment". "They preferred to use recipes [treatment] based on clay or cabbage poultices that they got from their books," he said.

Patrick Quenel, lawyer for the father, whose business has gone into liquidation, said the couple were "completely aware of the mistakes they made".

They have been charged with "neglect or food deprivation followed by death" and face a maximum sentence of 30 years in prison if convicted. The hearing is expected to last until Friday.