A nine-month, double-blind study found that women who ate the special rice developed by the Philippines' International Rice Research Institute (IRRI) had iron levels 20 per cent higher than peers who ate traditional rice.
"Although this sounds like a modest increase, it means that instead of 50 per cent of women getting adequate iron, 71 per cent of the women who consumed the biofortified rice, while eating a traditional Philippine diet, met the estimated average requirement for iron," said lead author Dr Jere Haas, based at Cornell University in the US.
However he told NutraIngredients.com that the rice still only provided 1.7mg of iron per day for the study subjects, much less than the recommended daily amount.
This means that the food is unlikely to compete with the fortified foods market, currently growing in the Philippines following a government programme to encourage vitamin and mineral enrichment.
"It's only part of a strategy. It's not the silver bullet that many people might be looking for," Dr Haas said.
For the past decade, plant breeders have been trying to boost the vitamin and mineral content of rice and other staples through traditional plant breeding and genetic engineering in order to combat widespread nutrient deficiencies in the developing world.
But Dr Haas said that none of the staple crops bred to contain higher nutrient levels have reached amounts that could replace fortified foods, where the doses added are significantly higher.
"There may be a limit to what conventional plant breeding can do," he said.
Nevertheless, the new research does show the merits of plant breeding for the very poor who are unable to afford fortified foods, even if they are subsidised by charities or public health initiatives, he added.
Known technically as IR68144-3B-2-2-3, the new rice has four to five times more iron than commercially available rice in the Philippines. Haas and his colleagues tested it on 192 nuns living in convents in the Philippines.
"The greatest improvements in iron status were in non-anaemic women who had the lowest body iron reserves at the beginning of the study and in women who consumed the most rice and, therefore, the most iron from rice," said Dr Haas.
He added: "Nobody has yet done a cost-benefit analysis to compare fortified foods with a nutrient enriched crop."
Lack of iron is the most common micronutrient deficiency in the world. Insufficient iron can slow growth and mental development in children and in adults it reduces the capacity for physical labour. Severe anaemia increases the risk of women dying in childbirth.
In the Philippines, as many as 60 per cent of the women are thought to be iron deficient.
In order to combat this, and other, nutrient deficiencies, the Philippines' Bureau of Food and Drugs (BFAD) has created a new seal, 'Sangkap Pinoy', for products enriched in vitamins and minerals.
After being introduced last November, 46 companies had already been given approval to use the seal by January of this year.
However many of the poorer people cannot afford or do not have access to commercially fortified foods and supplements. Resources to subsidise these products are limited.
In some areas, rice millers are being encouraged to use rice premixes in compliance with the food fortification law.
The new study is published in this month's issue of the Journal of Nutrition (vol 135, issue 12). It was previously presented at last year's Experimental Biology conference in San Diego, the American Society of Agronomy conference in Seattle and the World Rice Research Conference in Japan.