Parents who put their children on a strict vegan diet, with no animal sourced food, could harm their development according to a US scientist quoted by the BBC.

“There’s absolutely no question that it’s unethical for parents to bring up their children as strict vegans,” Professor Lindsay Allen, director of the US Agricultural Research Service told a conference in Washington, reported by the BBC. Animal source foods had some nutrients not found anywhere else, she said.

The BBC said that the Vegan Society, campaigners for a diet with no animal products, dismissed the claims, saying its research showed vegans were often healthier than meat eaters.

Professor Allen said: “There have been sufficient studies clearly showing that when women avoid all animal foods, their babies are born small, they grow very slowly and they are developmentally retarded, possibly permanently.”

She said the damage to a child began while it was growing in the womb and continued once it had been born.

Research she carried out among African schoolchildren suggested that as little as two spoonfuls of meat each day was enough to provide nutrients such as vitamin B12, zinc and iron.

The 544 children studied had been raised on diets chiefly consisting of starchy, low-nutrition corn and bean staples lacking these micronutrients. Over two years, some of the children were given 2 ounce supplements of meat each day, equivalent to about two spoonfuls of mince.

Two other groups received either a cup of milk a day or an oil supplement containing the same amount of energy. The diet of a fourth group was left unaltered.

The changes seen in the children given the meat, and to a lesser extent the milk or oil, were dramatic.

These children grew more and performed better on problem-solving and intelligence tests than any of the other children at the end of the two years.

They also became more active, talkative and playful at school.

Adding either meat or milk to the diets also almost completely eliminated the very high rates of vitamin B12 deficiency previously seen in the children.

Professor Montague Demment, from the University of California at Davis, said more emphasis should be placed on animal source food to combat global malnutrition.