Children who are denied meat, milk and cheese early in life suffer mental and physical impairments that can cause permanent damage, said Lindsay Allen, Professor of International Nutrition at the University of California, Davis.

Research led by Professor Allen found that adding just two spoonfuls of meat to the daily diets of Kenyan children transformed their development, doubling increases in muscle mass over two years and improving their performance on tests of mental agility.

Although the study took place in a developing country among undernourished children, its findings are also relevant to parents in the developed world, she told the American Association for the Advancement of Science conference in Washington.

Comparisons with control groups given an energy supplement in the form of oil indicated strongly that the greatest benefits came from the nutrients and protein in meat rather than from the extra calories.

Studies in the US and the Netherlands have also shown that a vegetarian or vegan diet holds back children’s development, and that the damage cannot be redressed by restoring meat or dairy products after the age of 16. “Animal source foods have some nutrients which are not found anywhere else,” Professor Allen said. “If you’re talking about feeding young children and pregnant women and lactating women, I would go as far as to say it’s unethical to withhold these foods . . . there’s a lot of empirical research that will show the very adverse effects on child development of doing that.”

She was particularly critical of vegans who refuse to feed their children eggs and dairy products. “There’s absolutely no question that it’s unethical for parents to bring up their children as strict vegans,” she said.

The findings brought an angry reaction from vegetarians yesterday. Sir Paul McCartney, whose first wife Linda brought out a range of a meat-free foods, telephoned the Jeremy Vine show on BBC Radio 2 to say that the claims were “rubbish”. He said that he had been a vegetarian for 20 years and raised his children as vegetarians with no ill-effects.

In Professor Allen’s study, 544 Kenyan children with an average age of seven were split into four groups. One group received two ounces of meat each day while two others were given equivalent extra calories in the form of milk or oil. A fourth control group received no supplement.

After two years, the children receiving all three supplements had gained 400g more weight than those in the control group and had greater muscle development in the upper arms. The increase was greatest in the meat group.

The meat group also did significantly better in tests of intelligence, problem-solving and arithmetic: in one test, scores improved by 35 points for the meat group, compared with 14 points for milk and nothing for the controls. The milk and meat groups were also less likely to be deficient in micronutrients such as vitamin B12.

Professor Allen said: “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.”

ETHICS AND FOOD

• Vegans follow an extreme version of vegetarianism where any products derived from animals are deemed unethical
• This usually includes dairy products, eggs and honey
• Many vegans refuse to keep domestic animals bred as pets, though they will keep “rescued” animals from sanctuaries. Some switch cats and dogs to a vegan diet
• The vegan movement was founded in 1944 by Donald Watson in the US. There are an estimated 180,000 to 250,000 vegans in the UK.
• Celebrity vegans include the singer Bryan Adams, the actress Alicia Silverstone and poet Benjamin Zephaniah

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