A woman looks at milk products under a notice board declaring them free of melamine, at a supermarket in China's Hubei province in October 2008. (Associated Press)

Fewer than one per cent of Chinese children likely to have consumed melamine-contaminated baby formula or food suffered kidney damage, and most of those who did have damage showed no symptoms, a study indicates.

In 2008, products contaminated with the industrial chemical from the Sanlu Dairy Company were recalled, and more than 50,000 children were hospitalized. Six died because of kidney damage.

Melamine, used to make plastics and fertilizers, was added to infant formula and other foods in China. It is high in nitrogen, and when added to foods it can mask low nutrient levels in standard tests.

In the study published in Monday's issue of the Canadian Medical Association Journal, Chinese researchers examined ultrasounds on 7,933 children whose mothers lived in rural areas around the industrial city of Shijiazhuang in northern China, where the now-bankrupt dairy company was located.

These children, all younger than three when the contamination became public knowledge in September 2008, probably had the highest melamine exposure worldwide, said study author Dr. Jian-meng Liu of the Peking University Institute of Reproductive and Child Health.

Among the 7,933 youngsters studied, the renal ultrasounds revealed evidence of kidney stones, enlarged kidneys or other urinary tract abnormalities in 48, or 0.6 per cent.

Of the 48 affected children, 43 showed no symptoms. Two had symptoms such as signs of kidney stones and swelling and were hospitalized, and three had symptoms but never sought treatment, the study found.

Among the 46 affected children on which follow-up information could be gathered after six months, five still had kidney problems, the team reported, while the abnormalities resolved in the other 41.

'Need for further follow-up'

"Without specific medical treatment, the majority of the affected children no longer showed evidence of renal abnormalities within six months after stopping consumption," the study's authors concluded. "However, renal abnormalities persisted in 12 per cent of the affected children, suggesting a need for further follow-up."

In a journal commentary accompanying the study, Dr. Jin-Ling Tang of the School of Public Health and Primary Care at the Chinese University of Hong Kong noted that the formula and technology Sanlu developed was awarded a prestigious Chinese science and technology award about eight months before the recall.

"It is difficult to assess the importance and reliability of a piece of research soon after … completion," Tang wrote. "Giving away prizes too quickly may result in mistakes, as in the case of Sanlu infant milk."

The award highlighted some of the barriers to high-quality research in China, such as how studies are evaluated, funded and rewarded, as well as the tarnished image of Chinese science that followed the isolated but widely publicized deceit.