More than 200 million children around the world suffer from insufficient intake of vitamin A, and 250,000 of them go blind every year as a result, with half of those children dying, Grossman said, citing World Health Organization figures.

The study was partly sponsored by the US National Institutes of Health, she said, and the results were published in the August edition of The American Journal of Clinical Nutrition.

On Saturday, government officials in Hunan denied claims that children were used as guinea pigs in the US study.

They were responding to a Greenpeace article which said the environmental protection group had discovered a study backed by the US Department of Agriculture involving feeding GM rice to children.

A spokesman for Hengyang City, where Greenpeace said the study was conducted, said initial findings indicated there had been no such research project. He said that there was a study on the transformation of carotene in vegetables to vitamin A in children's bodies, but denied it was the same one.

"The food given to the children did not involve GM rice or other GM food," the spokesman said. "Parents were notified of the experiment in advance."

The government spokesman also said the study had not involved any American institute.