Alecia Domer has had seasonal pollen allergies since she was a child. But she’s never had to carefully watch what she eats.

That is, until the age of 42, when she had lunch one day and shortly afterward, her throat and stomach felt like they were on fire and her face turned beet red.

“I didn’t know what I had eaten, it was so insane,” says Ms. Domer, 51 years old, a resident of Needham, Mass.

She went to an allergist and got tested, and learned that she was allergic to peanuts, tree nuts and soybeans.

Ms. Domer is among a little-researched group of people who develop food allergies as adults.

A study published in January in JAMA Network Open found that 10.8% of U.S. adults—more than 26 million—have a food allergy, and about half developed a new food allergy as an adult.

“It was definitely more than I expected,” says Ruchi Gupta, a professor of pediatrics at Northwestern University and Lurie Children’s Hospital and first author of the study.

The researchers say they can’t determine definitively if the number of adults with food allergies is increasing because it is the first comprehensive study on the topic and it is difficult to compare studies that use different methodologies. But anecdotally, doctors say they are seeing more adult patients with food allergies and it is important to know the scope of the problem to better understand and manage it.

A true food allergy is when the body’s immune system mistakes a food as harmful, triggering the immune system to release an antibody called immunoglobulin E, or IgE. When someone with an allergy eats that food, the IgE antibodies tell the immune system to release histamine, a chemical that causes symptoms such as hives, itching and, in extreme cases, anaphylaxis—in which the immune system affects multiple parts of the body at the same time, often leading to trouble breathing.

Increasing food-allergy rates in the Western world are a vexing problem for experts who theorize that they could be related to the increasing use of antibiotics, rising rates of C-sections which affect the microbiome of babies and increasingly sterile environments. All change the good bacteria in the intestinal tract, which alters the immune system.

The JAMA study follows a similar one done in children published in December in the journal Pediatrics, which found that about 8% of children have a food allergy. Food-allergy research typically focuses on children, but many children outgrow food allergies. One 2013 study found a quarter of children outgrow them by age 5.4, particularly those allergic to milk, eggs and soy. There is no research on whether adults outgrow the allergies later in life.

“Our study suggests that some particular allergies are more commonly developed during adulthood, while others are more likely to occur during childhood and then persist,” says Christopher Warren, co-author of the JAMA study.

For example, about half of adult shellfish and wheat allergies developed after age 17, while fewer than one in five peanut allergies first appeared during adulthood, he says.

The most common adult-onset allergy is shellfish, affecting about 3.5 million U.S. adults, followed by milk, wheat and tree nuts, each of which affects about a million adults.

Amanda Settos used to love shrimp and ate it all the time. After one meal of jumbo shrimp about 10 years ago, she broke out in hives across her face. Now, she can’t kiss her husband after he has eaten shrimp without having an allergic reaction.

“I love [shrimp] and sometimes I’m still tempted to try it, but it’s not worth it,” says Ms. Settos, 31, of Naples, Fla.

In the JAMA study, women were more likely to have developed a food allergy as adults, with 7.2% reporting an adult-onset food allergy compared with 3% of men. Researchers found adult-onset allergies affected every age group, race and geographical area.

Researchers asked respondents if they were aware of any triggers, asking about times of hormonal changes, such as pregnancy, puberty and menopause.

They also asked about geographical moves and viral infections or illnesses that could affect the immune system and contribute to the development of food allergies. They intend to analyze and publish the results of studying triggers.

The study found that while one in 10 adults has a food allergy, about twice as many—19% of adults—think they have a food allergy but didn’t report symptoms consistent with a diagnosable IgE-mediated allergy.

Doctors say adults who think they have a food allergy often suffer from other food-related conditions, such as lactose intolerance; oral allergy syndrome, or OAS; eosinophilic esophagitis, or EoE; or food protein induced enterocolitis syndrome, or FPIES.

“When you have a food-related reaction, it could be one of many things,” Dr. Gupta says. “The symptoms can easily be mislabeled. It is important to get diagnosed so you can best manage it.”

For example, EoE is an inflammation of the esophagus primarily triggered by food, says Emily McGowan, an assistant professor of allergy and immunology at the University of Virginia. The condition affects about 1 in 2,000 individuals, or .05% of the population, but appears to be more prevalent. The most common foods associated with it are milk, wheat, egg and soy. A reaction can occur up to a week after ingesting the food.

Meanwhile, FPIES is an immune reaction in the lining of the gut to specific food proteins, which causes severe stomach pain and vomiting, says John Costa, the allergy medical director at Brigham and Women’s Hospital. Symptoms usually occur two to four hours after a meal and can last up to eight hours. Seafood is the most common cause, though dairy and wheat can also trigger it.

For Ms. Domer, developing a food allergy later in life was life-changing. She now carries two EpiPens, which are emergency epinephrine treatments for allergic reactions. In the beginning, she had a few lapses in terms of reading labels and avoiding certain foods. “It took me a little while to become vigilant,” she says.

Write to Sumathi Reddy at sumathi.reddy@wsj.com