Federal food regulators said Tuesday they have received 125 reports of adverse health effects — mostly gastrointestinal problems — from consumers who ate gluten-free Cheerios.

After hearing of such complaints, General Mills Inc. on Monday recalled 1.8 million boxes nationwide of gluten-free Cheerios and Honey Nut Cheerios, concluding they inadvertently contained wheat, which contains gluten.

The company said Tuesday it has added another layer of testing at its cereal plants to ensure gluten contamination doesn't occur again.

The U.S. Food and Drug Administration said in an e-mail that since mid-September, it has gotten 125 reports of "adverse events from/about consumers who ate Cheerios labeled gluten-free." The FDA said it's "not aware of any reports of hospitalizations or deaths from the consumption of Cheerios labeled gluten-free."

People with celiac disease, who are estimated to make up about 1 percent of the population, get sick from eating gluten, a protein that can lead to damage in their small intestine. Another 6 to 7 percent of the population is believed to be allergic to wheat or have gluten sensitivity. For the gluten-sensitive, ingesting gluten can cause gastrointestinal issues such as bloating, diarrhea and constipation.

General Mills said Monday that it began investigating its Cheerios production system after becoming aware of illness reports to the FDA, as well as fielding a couple of consumer complaints directly.

The voluntary recall includes four days of production of Cheerios and 13 days of production of Honey Nut Cheerios made during July at the company's Lodi, Calif., plant. Cheerios made at other General Mills plants aren't subject to the recall.

The FDA said it tested 36 samples of Cheerios products labeled as gluten-free from different manufacturing facilities and lots. One sample of Honey Nut Cheerios contained 43 parts per million of gluten. The FDA limit for food labeled as gluten-free is 20 ppm.

General Mills, which is based in Golden Valley, launched gluten-free Cheerios in recent months as part of its biggest marketing offensive in years for cereals. Cheerios is the nation's biggest cereal brand, Honey Nut Cheerios is the top-selling U.S. cereal and gluten-free foods have become a hot trend in the packaged food industry.

The company said Monday the foul-up occurred when the Lodi plant lost rail service and its gluten-free flour was transferred from rail cars to trucks.

Cheerios are made primarily of oats, which naturally do not have gluten. But the cereal manufacturing process can still let through small amounts of gluten. General Mills has made a big investment in its plants and supply chain to ensure its gluten-free Cheerios are indeed gluten-free.

At its flour mill in Fridley, the company has built a multistory piece of equipment that sifts the oats it receives, shaking out traces of wheat and barley, which also contains gluten. After the sifting, General Mills tests the oats to ensure they are gluten-free, said Kirstie Foster, a company spokeswoman.

The oats are then milled into flour, which is tested again before it is shipped from the Fridley plant to the company's cereal factories, including the one in California.

General Mills said oat flour was contaminated when bulk rail cars full of the stuff were offloaded into bulk trucks, Foster said. The bulk trucks were believed to recently have carried wheat, she said.

Since the oat flour had been tested twice at the Fridley mill, General Mills didn't check the cereal itself as its rolled off the production line. Now, General Mills is testing gluten-free finished products for gluten at all of its cereal plants, Foster said.

Mike Hughlett • 612-673-7003