Food is a source of nutrients for our bodies, and as we’ve learned, the GI tract functions to extract those nutrients from food and absorb them into the body. But sometimes, specific foods can cause problems for the GI tract and the body, including food intolerances, food allergies, and celiac disease. These conditions are often confused for one another, but they have different causes, symptoms, and approaches to treatment.

Food Intolerances

A food intolerance occurs when a person has difficulty digesting a specific food or nutrient, causing unpleasant GI symptoms such as gas, bloating, flatulence, cramping, and diarrhea. Food intolerances are commonly caused by the body not producing enough of a particular digestive enzyme, so the severity of symptoms usually correlates with how much of the food was eaten. Unlike food allergies, the immune system does not play a role in food intolerance, and while the symptoms are unpleasant, they are generally not dangerous and will subside once the food passes out of the GI tract. People with food intolerances can often consume small amounts of the offending food without symptoms.^[1]

Lactose intolerance is one of the most common food intolerances. People with lactose intolerance do not produce enough of the enzyme lactase, which is responsible for digesting the milk sugar lactose into single sugar molecules that can be absorbed in the small intestine. Undigested lactose cannot be absorbed, so it continues on to the large intestine where it draws in water and fermented by bacteria resulting in gas, bloating, cramping, and diarrhea, usually within 30 minutes to 2 hours after eating the offending food.

People with lactose intolerance can often consume some amount of lactose without discomfort, although this varies from person to person. Aged hard cheese and yogurt with live cultures are often easier to tolerate because they are lower in lactose, which is consumed by bacteria during fermentation and aging. Lactose-free milk and lactase enzyme supplements can also help reduce symptoms. Dairy products are important sources of calcium and vitamin D, so people who avoid dairy need to take special care to include other sources of these nutrients in their diets^[2].

The vast majority of humans are born with the ability to digest lactose since all mammalian milk, including human milk, contains lactose. In much of the world, it’s common for the activity of the lactase enzyme to decline after childhood. Worldwide, approximately 65 percent of adults have a reduction in the ability to digest lactose after infancy.^[3]. Lactose intolerance is less common among those of Northern European ancestry and more common among those of African, Asian, Hispanic/Latino, and Native American ancestry^[4].

Food Allergies

In addition to its role in digestion and absorption, the GI tract also plays an important role in immune defense. Intestinal cells form a barrier between the interior of the body and the lumen, or tube, of the GI tract, which is includes food, microbes, and other foreign substances and pathogens. Immune tissue in the GI tract and other parts of the body produce immune cells that target foreign invaders, in part through the production of antibodies, which are protective proteins that bind to foreign substances. This function requires the immune system to accurately distinguish between normal food proteins and invading pathogens. A food allergy occurs when the immune system mistakenly identifies a food protein as an invasive threat and mounts an immune response against it.

The most common type of food allergy involves immunoglobulin E (IgE), a type of antibody produced by the immune system in response to an allergen. Symptoms appear quickly – immediately to up to two hours after consumption – and can lead to symptoms all over the body, including skin rashes; swollen lips, face, or throat; wheezing and difficulty breathing; nausea and vomiting; cramping; diarrhea; and rarely, a dangerous drop in blood pressure. A severe allergic reaction involving more than one organ system—a rash coupled with difficulty breathing, for example—is called anaphylaxis. Anaphylaxis can be life-threatening and should be treated immediately with epinephrine, commonly administered by injection with a device such as an EpiPen, followed by immediate medical attention.^[5]

The most common food allergies in the U.S. are caused by proteins in peanuts, tree nuts, milk, shellfish, eggs, fish, wheat, soy and sesame. About 11 percent of adults and 8 percent of children have at least one food allergy.^[6]

Food allergies are especially common in childhood. Some children outgrow allergies to egg, dairy, wheat, or soy, but peanut, tree nut, fish and shellfish allergies often persist into adulthood.^[7] Research has found that introducing common food allergens to babies, particularly peanut products, can reduce the risk of developing allergies.^[8]

If someone thinks they may have a food allergy, they should see an allergist for evaluation. Diagnosis is based on symptoms after consuming a food, specific IgE blood tests, and/or skin prick tests, where a tiny amount of food protein is scratched onto the skin to test for a reaction. Blood and skin tests determine whether a person is sensitized to an allergen, meaning that they’re producing IgE antibodies to the food. However, the presence of IgE antibodies does not definitively mean a person has a food allergy; it’s common to have a positive IgE test but still be able to eat the food without symptoms. The gold standard test for diagnosing is an oral food challenge where the food is eaten under medical supervision while watching for signs of a reaction. If a food allergy is confirmed, treatment usually includes strict avoidance and carrying epinephrine in case of accidental exposure.^[9]

There are some promising new therapies for treating food allergies that involve exposing a person to small amounts of the allergen to try to teach the body to tolerate it. These don’t cure the allergy completely but may reduce the risk of a severe allergic reaction.

Not all “food allergy” tests are legitimate. Some commercial blood tests measure IgG antibodies, but the presence of IgG does not indicate a food allergy. Therefore, these tests are not recommended by allergy experts, because they may cause a person to unnecessarily fear and avoid a long list of foods to which they are not allergic.^[10]

Celiac Disease

Celiac disease is an autoimmune disorder that affects around 1 percent of people in the U.S. In autoimmune diseases, the immune system mistakenly attacks the body’s own tissues. In celiac disease, eating gluten – a group of proteins found in wheat, barley, and rye – triggers an immune response that damages the lining of the small intestine. Over time, this damage blunts the villi (finger-like projections that increase surface area for nutrient absorption) and nutrient absorption is reduced. There is no cure for celiac disease, but it’s very effectively treated by eliminating gluten from the diet.^[11]

Symptoms of celiac disease can range from mild to severe and can include pale, fatty, loose stools, gastrointestinal upset, constipation, abdominal pain, skin conditions, and poor growth in children. Symptoms can appear in infancy or much later in life, as late as age seventy. Celiac disease is not always diagnosed, because the symptoms may be mild. Even without symptoms, the disease can still damage the small intestine and impair nutrient absorption. Nutrient deficiencies can cause health problems over time, particularly in children and the elderly. For example, poor absorption of iron and folic acid can cause anemia, which impairs oxygen transport to cells in the body. Calcium and vitamin D deficiencies can lead to osteoporosis, a disease in which bones become brittle.^[12]

Diagnosis of celiac disease begins with a blood test for specific antibodies that are elevated in those with the disease. If the blood test is positive, the diagnosis is confirmed with a biopsy of the small intestine, a procedure in which a small amount of tissue is removed for examination. These tests may not accurately detect celiac disease in people already consuming a gluten-free diet, because without gluten, antibody levels may be low and damage to the small intestine may not be visible. This is why it’s best to be tested for celiac before eliminating gluten from the diet.^[13]

It isn’t clear what causes celiac disease; genetics play a role, but other factors also seem to influence its development. It is more prevalent in women and in people with certain other conditions such as type 1 diabetes, autoimmune thyroid disease, and Down syndrome. Celiac disease is most common in people of European descent and is less common in people of African American, Japanese, and Chinese descent.^[14]

Celiac disease is treated by completely avoiding gluten, as consuming even small amounts can cause intestinal damage. People with celiac can consume grains that don’t contain gluten, including rice, corn, millet, buckwheat, and quinoa. Oats do not contain gluten themselves, although they are often contaminated during processing, so those with celiac disease should look for a “gluten-free” label on oats. After eliminating gluten from the diet, the tissue of the small intestine begins to heal within days to weeks.^[15]

Celiac disease is different from a wheat allergy in both cause and symptoms. Because celiac disease is an autoimmune condition, the cells of the small intestine come under attack, and damage causes chronic symptoms. A wheat allergy, on the other hand, is caused by antibodies attacking an allergen in the wheat itself, and the symptoms are usually immediate and acute.^[16]

Sometimes, people test negative for celiac disease but still believe that consuming gluten is causing symptoms. They may eliminate gluten from their diet and find that they feel better. This is often called non-celiac gluten sensitivity (NCGS). It’s not clear what causes NCGS or why a gluten-free diet is helpful. In some cases, symptoms may be related not to gluten itself, but to other components of wheat-containing foods.^[17]

Except in the case of celiac disease, wheat allergy, or confirmed NCGS, gluten-free foods or a gluten-free diet are not inherently more healthful. In fact, packaged gluten-free foods are often more highly processed, with more added sugar, salt, and fat, compared to foods containing wheat. A gluten-free diet can also be lower in fiber, so those following the diet should make room for naturally gluten-free whole grains, legumes, nuts, fruits, and vegetables.^[18]

This section is an adaptation “Food Intolerances, Allergies, and Celiac Disease” in Nutrition: Science and Everyday Application by Alice Callahan, PhD; Heather Leonard, PhD, RDN; and Tamberly Powell, MS, RDN licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.