A leading US nutritionist today claimed that vegetarian and vegan parents are damaging their children's health by denying them meat.

UK experts immediately contested the findings of Professor Lindsay Allen, of the University of California at Davis, and Sir Paul McCartney told the BBC that the claims were "rubbish".

Prof Allen conducted a study of impoverished children in Kenya, and found that adding as little as two spoonfuls of meat a day to their starch-based diets dramatically improved muscle development and mental skills.

Speaking at the annual meeting of the American Association for the Advancement of Science, in Washington DC, she said: "Animal source foods have some nutrients which are not found anywhere else.

"If you're talking about feeding young children and pregnant women and lactating women, I would go as far as to say it is unethical to withhold these foods during that period of life. There's a lot of empirical research that will show the very adverse effects on child development of doing that."

Prof Allen was especially critical of parents who imposed a vegan lifestyle on their children, denying them milk, cheese, eggs and butter, as well as meat. "There's absolutely no question that it's unethical for parents to bring up their children as strict vegans," she said.

Sir Paul, a vegetarian for 20 years, telephoned the BBC to dismiss the claims as "rubbish", telling the BBC Radio 2 Jeremy Vine show: "I really do think this is rubbish. I think the medical profession itself, apart from this one person, come to the conclusion that a veggie diet is good for you and can help with colon cancer and stuff, so I suspect these things are engineered by livestock people who have seen sales fall off.

"From my own point of view, it has been a good thing for me and my children, who are no shorter than other children."

However, the British Dietetic Association said the study looked at impoverished, rural children with a poor background diet low in essential nutrients such as zinc, B12 and iron, and its findings were not applicable to vegan children in the developed world.

"There is no evidence that our vegan and vegetarian children in this country suffer impaired development," a spokeswoman for the association said.

She said Prof Allen's assertion that some nutrients could only be obtained from animal sources was incorrect. Even vitamin B12, which is only found in animal products, was used to fortify vegan alternatives such as Marmite.

Vegetarians could obtain sufficient calcium from sesame seeds, nuts and fortified soya milk, and iron from dried fruit and fortified breakfast cereals, the spokeswoman added.

The African study involved 544 children in Kenya, typically aged around seven, whose diet mainly consisted of starchy, low-nutrition corn and bean staples lacking these micronutrients. Over a period of two years, one group of the children was given a daily supplement of two ounces of meat - equivalent to roughly 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.

Prof Allen, the director of the US Agricultural Research Service's Western Human Nutrition Research Centre at Davis, said: "It was found that, compared with controls that had no intervention, the meat group had 80% more increase in muscle mass over the two years of the study, and the milk and energy group had 40% more increase in muscle mass.

"In terms of cognitive function, the group that received the meat supplement showed the biggest improvement in fluid intelligence over the two years, and those who had either milk or energy supplements were better than the controls. The group that received the meat supplements were more active in the playground, more talkative and playful, and showed more leadership skills," she said.

Test scores for mental skills improved by 35 points for the meat group and 14 for the milk, while remaining unchanged for the children who received no supplements, she said. 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.

"It's important to know that these important benefits to human function and human capital were seen in just two years," Prof Allen added. "Had these children received these foods earlier in life, or their mothers received them when they had been pregnant, or people could receive them throughout their lives, we think that the improvements in human capital development would be even more dramatic."

She said that, although the study had been conducted in a poor African community, its message was highly relevant to people in developed countries. Studies of vegetarians in both the US and Europe had shown that missing out on meat and dairy products could permanently impair a child's development.

Liz O'Neill, the head of communications at the Vegetarian Society, is seven months pregnant with her first child, and said nothing in Prof Allen's research had dissuaded her from the intention of raising her baby as a vegetarian.

"This study should not scare any parents in this country. It's just not relevant," Ms O'Neill said. "Certainly a balanced vegetarian diet will be much healthier than the average meat-eater's diet. As soon as you take something out of your diet, it makes you stop and think about how to replace what is missing."

However, Dr Alastair Sutcliffe, a senior lecturer in paediatrics at University College London, said human ancestry showed that natural diet includes meat and that a diet inclusive of meat was likely to produce a taller, stronger child.

"Whilst I respect a person's ethical decision to eat a vegan diet as an adult, it should be very carefully considered if that is the right decision for a child," Dr Sutcliffe said. He said he believed a vegetarian diet including dairy products would probably be fine for children.

The British Dietetic Association spokeswoman said the study raised the important point that pregnant and breastfeeding women and young children were vulnerable as vegans, and that parents should carefully review their children's diet to ensure they were eating a well-balanced diet.

However, she dismissed as "extreme" Prof Allen's assertion that feeding children a vegan diet was unethical, and recommended parents read the nutrition guides available from the Vegetarian Society to help them plan a balanced diet.