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Two years after tainted milk powder killed six infants and left some 300,000 more suffering from kidney stones, China is again roiled by allegations of health hazards in infant formula. This time the repercussions are less deadly but nonetheless disturbing. Three infant girls in the central city of Wuhan and a fourth in Beijing have shown signs of premature development, including growth of breasts, according to state media reports. Tests showed that their hormone levels were abnormally high, which some doctors suspect may be linked to drinking infant formula produced by Synutra International, a Nasdaq-listed infant formula maker based in Qingdao, in coastal northeastern China.

The company has denied the allegations, and on Aug. 9 CEO Liang Zhang said, "We are completely confident that our products are safe and our quality levels are industry-leading." Synutra has also threatened to take legal action against those who have accused it of selling tainted products.

New Zealand–based dairy producer Fonterra has acknowledged that it supplies milk powder to Synutra but says the Chinese company also buys milk from domestic sources and whey powder from Europe. "Fonterra remains 100% confident about the quality of its products," the company said in an Aug. 11 statement. Because of New Zealand's strict regulations for the use of hormones in dairy cattle, "it is not necessary for New Zealand milk or milk products to be routinely tested," the statement added.

Both companies were embroiled in the 2008 Chinese milk-powder scandal. Fonterra was a partial owner of Sanlu Group, the now defunct dairy producer that was a main source of the tainted formula. Synutra was one of the 21 other Chinese dairy companies also found to have products contaminated with melamine, a chemical used in making plastics that can make milk appear as if it has a higher protein content in certain tests. When consumed in large doses, it can lead to kidney damage, which caused the infant deaths. The level found in Synutra's products was small compared with that of other companies, like Sanlu, but as with many other Chinese infant-formula producers, Synutra suffered significant losses that year because of recalls and consumer fears.

The Chinese Ministry of Health has promised a full investigation into the hormone allegations. Ministry spokesman Deng Haihua said during an Aug. 10 news conference that "when it comes to early infant puberty, experts say the factors are complicated and the cause is often unknown."

Like in 2008, the latest milk-quality scare has prompted many parents to take their infants to hospitals for health checks. Perhaps more worrying than the hormone concerns is the fear of what is still unknown about China's food supply. Both the melamine scandal and the current hormone worries were prompted by readily obvious phenomena — kidney stones and premature development. But the possibility exists that there are other contaminants that have not yet come to light. What remains obvious is that despite steps to better regulate the food supply in China, many consumers here remain deeply concerned.

• Austin Ramzy / Beijing
• Two years after tainted milk powder killed six infants, China is again roiled by allegations of health hazards in infant formula. Is tainted milk to blame for early puberty signs?