By Michelle Roberts
BBC News health reporter, in Washington DC
Putting children on strict vegan diets is "unethical" and could harm their development, a US scientist has argued.
Meat, milk, butter and cheese are all avoided by vegans
Lindsay Allen, of the US Agricultural Research Service, attacked parents who insisted their children lived by the maxim "meat is murder".
Animal source foods have some nutrients not found anywhere else, she told a Washington science conference.
The Vegan Society 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."
"If you're talking about feeding young children, pregnant women and lactating women, I would go as far as to say it is unethical to withhold these foods [animal source foods] during that period of life."
She was especially critical of parents who imposed a vegan lifestyle on their children, denying them milk, cheese, butter and meat.
"There's absolutely no question that it's unethical for parents to bring up their children as strict vegans," she told the annual meeting of the American Association for the Advancement of Science (AAAS).
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 suggests as little as two spoonfuls of meat each day is 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.
This meant they were already malnourished.
Over two years, some of the children were given 2oz 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.
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.
No quick fixes
Professor Allen stressed that although the study (which was partially supported by the National Cattleman's Beef Association) was conducted in a poor African community that was malnourished, its message was highly relevant to people in developed countries.
She accepted that adults could avoid animal foods if they took the right supplements, but she said adding animal source food into the diet was a better way to tackle malnutrition worldwide than quick fixes with supplements in the form of pills.
"Where feasible, it would be much better to do it through the diet than by giving pills," she said.
"With pills it's very hard to be certain that the quantity of nutrition is right for everybody and it's hard to sustain."
In Africa, good results had been obtained from giving people a dried meat on a stick snack which proved both nutritious and appealing.
Professor Montague Demment, from the University of California at Davis, said more emphasis should be placed on animal source food to combat global malnutrition.
However, the claims have been dismissed by the Vegan Society in the UK.
In a statement, it said increasing numbers of people were opting for a plant-based diet.
Kostana Azmi, the chief executive officer, said: "The vegan diet can provide you with more energy, nutrition, and is bursting with goodness."
She said plant sources were sometimes a safer, and cheaper source of nutrients.
For instance, animal sources of omega-3 oils, needed for the development of the brain and nervous system, were often contaminated with pollutants, such as mercury in fish.
In addition, the vegan diet was often a healthier alternative. She said dairy and meat products were rich in saturated fat, while plant based diets were low in it.
The society does recommend that vegans supplement their diet with vitamin B-12 pills.
The US Agricultural Research Service is part of the US Department of Agriculture.