Food intolerances are caused by a problem with your digestion. Your body has trouble breaking down a certain food. A food intolerance can cause some of the same symptoms as a food allergy, such as nausea, cramps, and diarrhea. But a food intolerance is less dangerous than a food allergy, so you may be able to eat small amounts of the food. You can learn about lactose intolerance, which is one of the more common types of food intolerances.

People can feel ill after eating specific foods for reasons other than food allergy. Though these disorders may have some symptoms in common, these illnesses should not be confused with food allergy.

A problem often confused with food allergy is food intolerance, which is also an abnormal response to a food product, but differs from an allergy. A common example is an intolerance to lactose, a sugar found in many milk products that can cause an uncomfortable buildup of gas in the gastrointestinal tract. Gluten intolerance, or celiac disease, occurs when the immune system responds abnormally to gluten, a component of barley, wheat, and rye. However, unlike food allergies, these disorders do not involve IgE antibodies.

Foodborne illness, or food poisoning, can also be confused with food allergy because of similar symptoms, such as abdominal cramping. Foodborne illness, however, is caused by microbes, microbial products, and other toxins that can contaminate foods that were improperly preserved or processed.​

Gluten is a protein found in wheat, rye, and barley. It is found mainly in foods but may also be in other products like medicines, vitamins, and supplements. People with gluten sensitivity have problems with gluten. It is different from celiac disease, an immune disease in which people can't eat gluten because it will damage their small intestine.

Some of the symptoms of gluten sensitivity are similar to celiac disease. They include tiredness and stomachaches. It can cause other symptoms too, including muscle cramps and leg numbness. But it does not damage the small intestine like celiac disease.

Researchers are still learning more about gluten sensitivity. If your health care provider thinks you have it, he or she may suggest that you stop eating gluten to see if your symptoms go away. However, you should first be tested to rule out celiac disease.

Lactose intolerance means that you cannot digest foods with lactose in them. Lactose is the sugar found in milk and foods made with milk. After eating foods with lactose in them, you may feel sick to your stomach. You may also have

• Gas
• Diarrhea
• Swelling in your stomach

Your doctor may do a blood, breath or stool test to find out if your problems are due to lactose intolerance.

Lactose intolerance is not serious. Eating less food with lactose, or using pills or drops to help you digest lactose usually helps. You may need to take a calcium supplement if you don't get enough of it from your diet, since milk and foods made with milk are the most common source of calcium for most people.

Lactose intolerance is a condition in which you have digestive symptoms—such as bloating, diarrhea, and gas—after you consume foods or drinks that contain lactose. Lactose is a sugar that is naturally found in milk and milk products, like cheese or ice cream.

In lactose intolerance, digestive symptoms are caused by lactose malabsorption. Lactose malabsorption is a condition in which your small intestine cannot digest, or break down, all the lactose you eat or drink.

Not everyone with lactose malabsorption has digestive symptoms after they consume lactose. Only people who have symptoms are lactose intolerant.

Most people with lactose intolerance can consume some amount of lactose without having symptoms. Different people can tolerate different amounts of lactose before having symptoms.

Lactose intolerance is different from a milk allergy. A milk allergy is an immune system disorder.

Lactose intolerance and milk allergies are different conditions with different causes. Lactose intolerance is caused by problems digesting lactose, the natural sugar in milk. In contrast, milk allergies are caused by your immune system’s response to one or more proteins in milk and milk products.

A milk allergy most often appears in the first year of life, while lactose intolerance typically appears later. Lactose intolerance can cause uncomfortable symptoms, while a serious allergic reaction to milk can be life threatening.

Congenital lactase deficiency is a rare disorder, though its exact incidence is unknown. This condition is most common in Finland, where it affects an estimated 1 in 60,000 newborns.

Approximately 65 percent of the human population has a reduced ability to digest lactose after infancy. Lactase nonpersistence is most prevalent in people of East Asian descent, with 70 to 100 percent of people affected in these communities. Lactase nonpersistence is also very common in people of West African, Arab, Jewish, Greek, and Italian descent.

The prevalence of lactose intolerance is lowest in populations with a long history of dependence on unfermented milk products as an important food source. For example, only about 5 percent of people of Northern European descent are lactase nonpersistent.

Celiac disease is a digestive disorder that’s triggered by eating foods containing gluten—a protein common in foods such as bread, pasta, cookies, and cakes. When a person with celiac disease eats or drinks anything with gluten, the body’s immune system attacks the inside of the small intestine. The damage from this attack keeps the body from absorbing needed nutrients. If left untreated, celiac disease can lead to malnutrition, anemia, weakened bones, and other problems.

Researchers don’t know exactly what triggers celiac disease. Certain genes and other factors, such as things in the environment, can lead to celiac disease. Viral infections have been suggested as a possible trigger.

A team led by Dr. Bana Jabri at the University of Chicago and Dr. Terence S. Dermody at the University of Pittsburgh School of Medicine investigated whether a common but harmless type of virus that people are frequently exposed to, called reoviruses, can prompt sensitivity to dietary proteins. The study was supported by NIH’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and National Institute of Allergy and Infectious Diseases (NIAID), among others. Results were published in Science on April 7, 2017.

The team studied two different strains of reoviruses (T1L, which can infect the intestine, and T3D-RV, which was engineered to infect the intestine) in mice. Both strains triggered an immune response that protected the mice against the infection. However, animals infected with TL1 also showed altered immune responses in the gut. Both type 1 interferon (IFN) signaling and interferon regulatory factor 1 (IRF1) expression increased.

IRF1 regulates gene transcription and plays a key role in oral tolerance, or the suppression of immune responses to an orally delivered substance, like food. After being infected with T1L, susceptible mice lost their oral tolerance to a dietary protein. A follow-up gene expression analysis showed this was triggered by specific changes in immune signaling molecules. These changes both block the mechanism that maintains tolerance to dietary proteins and promote the immune reaction against them.

The team next investigated whether patients with celiac disease had been exposed to reoviruses. Patients with celiac disease had higher levels of reovirus antibodies than controls. Patients on a gluten-free diet also had higher levels of reovirus antibodies and IRF1 levels in their small intestines, suggesting reovirus infection may lead to long-lasting gene expression changes.

“This study clearly shows that a virus that is not clinically symptomatic can still do bad things to the immune system and set the stage for an autoimmune disorder, and for celiac disease in particular,” Jabri says. “However, the specific virus and its genes, the interaction between the microbe and the host, and the health status of the host are all going to matter as well.”

More work will be needed to better understand how reovirus and possibly other viruses might provoke food intolerance. The results may lead to new prevention and treatment strategies.

―Tianna Hicklin, Ph.D.