WASHINGTON – For seven years, the Food and Drug Administration has been trying to answer this question: What does it mean to be “gluten-free”?

That is roughly the time it took to build a tunnel beneath the English Channel to connect Britain and France.

In the meantime, foodmakers have been deciding for themselves whether they can jump into a lucrative new niche and market their products as free from gluten, a protein found in wheat, rye and barley. As a result, some products labeled gluten-free contain no gluten, others may have a trace and still more may contain a sizeable amount.

That murkiness is creating a real problem for an increasing number of Americans whose health depends on avoiding even tiny amounts of gluten, which is commonly found in bread, pasta and other staples and even in soy sauce and blue cheese.

Celiac disease, an autoimmune disorder in which gluten damages the lining of the small intestine and causes it to malfunction, afflicts about 3 million people in the United States. Exposure to gluten can trigger problems ranging from gastrointestinal distress and infertility to an increased risk for certain cancers.

And new research suggests that an additional 17 million Americans are “gluten-sensitive,” which means that they, too, are sickened by the protein and can experience abdominal pain, fatigue, headaches, “foggy mind” or tingling extremities.

The treatment is straightforward: a lifelong commitment to a gluten-free diet.

Under a 2004 law, Congress gave the FDA until 2008 to establish a uniform definition for companies that want to label their products as gluten-free. But that deadline has come and gone.

“The FDA has spent years calling upon experts to have open-forum debates, town hall meetings — we’ve been having reiteration and reiteration,” said Alessio Fasano, medical director of the Center for Celiac Research at the University of Maryland School of Medicine. “I really don’t understand why it’s lingering up in the air when it really should be a no-brainer.”

Meanwhile, Fasano said, the prevalence of celiac disease in this country is soaring partly because changes in agricultural practices have increased gluten levels in crops. “We are in the midst of an epidemic,” he said.

And that has caused an explosion in gluten-free foods. The market is projected to reach $2.6 billion next year, up from $100 million in 2003.

But what exactly does gluten-free mean?

The FDA is still working on that question, a spokeswoman said.Meanwhile, Canada, Brazil, Australia and an international body have all set labeling standards for gluten-free items. In most cases, that standard is 20 parts per million.