You may be shocked to learn that approximately 60%-70% of Americans living with celiac disease are undiagnosed, according to the Celiac Disease Foundation. A variety of reasons account for why people go undiagnosed: Celiac disease symptoms mirror those of other gastrointestinal (GI) conditions, and a reaction to gluten doesn't necessarily mean someone has celiac disease.

Let's look at the signs and symptoms of celiac disease to understand the condition that can develop at any age.

What is celiac disease?

Celiac disease is an autoimmune disorder that causes an immune system response after someone eats or drinks something made with gluten, a protein in barley, rye and wheat. The person's immune system kicks into action, creating antibodies that attack the villi, the tiny fingerlike projections that line the small intestine and aid in food absorption. Over time, the condition erodes the lining of the small intestine, which can cause nutritional deficiencies because the body cannot absorb the nutrients in food.

Who gets celiac disease?

"It develops in people who have a genetic susceptibility," says Dr. Rachel L. Schiesser, a gastroenterologist with Houston Methodist. "All the risk factors are not completely understood — it can develop at any time in a person's lifetime."

Gluten exposure is required to develop the disease. Other environmental factors may cause celiac disease to develop, including certain gastrointestinal infections, such as infectious gastroenteritis. People who have conditions such as Down syndrome, type 1 diabetes, autoimmune thyroid and adrenal disease, connective tissue disorders such as rheumatoid arthritis, Sjogren's and lupus, and other inflammatory gastrointestinal conditions are more likely to have celiac disease.

Because celiac disease is genetic, screening is recommended for all first-degree relatives of people with the condition. Those with type 1 diabetes can also benefit from celiac disease screening, even if they don't show symptoms.

"It is very common with a worldwide prevalence of roughly 1% of the population, many of whom are undiagnosed," she says.

Celiac disease symptoms

There are more than 200 known symptoms of celiac disease that can occur in the body, according to the Celiac Disease Foundation. People with celiac disease often experience gastrointestinal symptoms, including:

• Bloating
• Constipation
• Diarrhea
• Fatty stools
• Gas
• Stomach pain

While many experience GI distress after gluten exposure, some may have no symptoms. Those who go without the classic symptoms of indigestion and diarrhea after ingesting gluten nevertheless will eventually experience malabsorption, as the body has prolonged difficulty absorbing nutrients from food. This can cause signs of malnutrition, such as weight loss, low muscle tone, abnormal periods, dental issues and iron-deficiency anemia.

"There are many different symptoms, including vague complaints such as fatigue, joint pain, brain fog or headaches," Dr. Schiesser says. "There is dermatitis herpetiformis, which is an itchy rash and typically appears on the trunk and extensor surfaces of the body. Abnormal liver enzymes are another reason to test for celiac disease."

According to Dr. Schiesser, many people seek out an evaluation for celiac disease due to chronic diarrhea or iron deficiency anemia.

Celiac disease, gluten intolerance, gluten sensitivity or gluten allergy: are they the same?

There are multiple possible reasons to avoid gluten: gluten sensitivity, gluten allergy or celiac disease. Gluten sensitivity, also called gluten intolerance, is when a person has GI symptoms related to eating gluten without any other underlying reason, according to Dr. Schiesser. People with gluten sensitivity often feel the effects of eating or drinking gluten shortly after consuming it.

"Gluten avoidance at that point is mostly for comfort and prevention of symptoms," Dr. Schiesser says. "There is significant overlap between irritable bowel syndrome and gluten sensitivity, where many patients with IBS feel better avoiding gluten."

According to Dr. Schiesser, gluten allergy is a separate problem involving an allergic reaction to gluten. When exposed, someone with a gluten allergy can experience hives, swelling, abdominal pain, vomiting, and diarrhea.

"Celiac disease is a third separate entity where gluten may or may not make the patient feel bad after ingestion, but the inflammatory reaction to gluten causes damage to the small intestine over time and results in symptoms and signs," Dr. Schiesser says. "This is an important distinction because feeling bad after eating gluten does not make a diagnosis of celiac disease, and feeling fine after eating gluten does not exclude celiac disease."

How is celiac disease diagnosed?

In adults, celiac disease is diagnosed with a blood test that screens for celiac antibodies, along with an upper endoscopy with biopsies of the duodenum. Children may be diagnosed solely based on positive antibodies, which allows them to avoid upper endoscopy and biopsy.

"The symptoms of celiac disease vary greatly from patient to patient and overlap with almost all the other GI conditions," Dr. Schiesser says. "This is why a high index of suspicion is needed when GI symptoms are present. Fortunately, the screening blood test is simple and very reliable."

Dr. Schiesser says that someone experiencing classic symptoms may be diagnosed more quickly, while others with more vague symptoms can go for years without celiac disease being suspected by their providers.

Celiac disease is treated with lifestyle changes

The best — and only — treatment for celiac disease is to avoid anything and everything that contains gluten by adopting a gluten-free diet and lifestyle.

"In order to reduce the risk of malabsorption, malnutrition, osteopenia and GI cancers, persons with celiac disease have to be gluten-free for life," Dr. Schiesser says. "It is very important to work with a dietitian to cut out all exposure to gluten."

Even after adopting a gluten-free lifestyle, about half of people with diagnosed celiac disease still report having symptoms. As they age, those with celiac disease will need to be monitored for conditions that affect the bones, including osteopenia and osteoporosis. But Dr. Schiesser says that overall, people can expect relief from a gluten-free diet in the long term.

"The earlier the diagnosis and gluten-free diet is initiated, the better the outcome will be," Dr. Schiesser says. "Once a person with celiac disease is no longer exposed to gluten, the changes in the small intestine will generally reverse to normal over the course of one to two years. Additionally, the abnormal antibodies will become undetectable."

It can be difficult to avoid gluten in a society that loves bread, baked goods and beer — but it's easier to avoid gluten now than it was in the past.

"It is very fortunate that there are many more options today for gluten-free foods than there were in the past," Dr. Schiesser says. "Plus, there are medications under investigation which may help minimize effects of accidental gluten exposure in the future."