A vegan couple in Atlanta have been sentenced to life in prison for the death of their malnourished six-week-old boy, who was fed a diet largely consisting of soy milk and apple juice.

Superior Court Judge L.A. McConnell imposed the automatic life sentences Tuesday on 27-year-old Jade Sanders and 31-year-old Lamont Thomas.

Their son, Crown Shakur, weighed only 3½ pounds when he died of starvation on April 25, 2004.

Sanders and Thomas were convicted on May 2 of charges of murder and cruelty to children.

Defence lawyers said the first-time parents did the best they could while adhering to the lifestyle of vegans, who typically don't consume or use any animal products.

They said Sanders and Thomas did not realize the baby, who was born at home, was in danger until minutes before he died.

But the prosecution persuaded the jury that the couple had deliberately underfed and neglected their baby, then tried to use veganism as an excuse.

"The vegan diet is fine," prosecutor Chuck Boring told the jury during the trial. "These parents lied about what they fed him. He just was not fed enough."

He added: "They’re not vegans. They’re baby-killers."

The Canadian Pediatric Society says a vegan diet may compromise the growth of infants and children. The group suggests a less restrictive lacto-ovo diet of non-animal foods plus eggs and dairy products for children to meet their nutrition needs.