(CBS News) NEW YORK -- Gluten is a protein found in wheat and other grains and therefore in a lot of the food we eat. But it can make some people sick, which is why many products claim to be gluten-free.
Friday, for the first time, the FDA put out a standard definition.
67-year-old Stewart Levine believes a single bite from a cookie labeled "gluten-free" led to a painful reaction.
"Diarrhea, i just didn't feel right. Maybe it was those cookies, even though it is marked gluten-free. So I am not eating those cookies any more," said Levine.
Levine has Celiac disease. Even the tiniest amount of the wheat protein gluten can set off an immune reaction that damages the intestines and other parts of the body.
Until now there has been no standard definition of what it means for a product to be called "gluten-free." Sales of these products have exploded from $560 million in 2004 to $4.2 billion in 2012.
The rules require that a gluten-free product contain less than 20 parts per million of gluten, the lowest level that can be measured.
The FDA estimates five percent of products labeled gluten-free have more than that amount.
The government says these new labeling requirements will "help ensure that individuals with Celiac disease are not misled and are provided with truthful and accurate information."
Dietician Margaret Weiss runs the Kogan Celiac Center at Barnabas Health in Livingston, N.J.
She says of the new rules: "It's a happy day for Celiacs and people following the gluten-free diet. It really will help the problems and confusion when they are confronted with a label that doesn't make sense, is incomplete or has conflicting information on it."
Many people report feeling better under a gluten-free diet even if they do not have Celiac. This is called "gluten-sensitivity." Under the regulations, companies will have one year to comply.