A vegan couple in France who fed their 11-month-old daughter only breast milk and appeared to ignore signs of her poor health are now on trial for the girl's death.

The couple, Sergine and Joel Le Moaligou, are strict vegans who chose to feed their daughter, Louise, no solid food, giving her only breast milk.

After Louise died in March 2008, a post-mortem exam showed the child weighed just 5.7 kg (12.5 pounds) when she should have weighed about 8 kg (17.5 lbs).

The cause of death was listed as a pneumonia-related illness. But the autopsy also revealed the child suffered from a severe deficiency of vitamins A and B12, which may have left her susceptible to infection.

The vitamin B12 deficiency could be linked to the mother's eating habits, since the only source of the vitamin is meat, dairy or vitamin supplements.

The couple reportedly did not follow their doctor's advice to take their daughter to hospital when they went for her nine-month checkup and found she was suffering from bronchitis and was losing weight.

The court has heard that the parents chose instead to treat her with cabbage poultices, mustard, camphor and clay.

The couple has been charged with "neglect or food deprivation followed by death" and face up to 30 years in prison if convicted. The trial is expected to last until Friday.