Food intolerance occurs when the body has difficulty digesting a specific component of a food, leading to uncomfortable but not life-threatening reactions. This adverse food reaction is fundamentally a digestive system problem, contrasting with a true food allergy, which involves the immune system. Gluten, a protein found in grains like wheat, barley, and rye, and lactose, the main sugar in dairy products, are two of the most common dietary triggers for this type of reaction.
Identifying Common Symptoms
The symptoms of food intolerance frequently begin in the digestive tract, though they can often extend to other body systems. Common digestive complaints for both gluten and dairy include abdominal pain, bloating, and excessive gas production. These issues occur because the undigested food component travels to the large intestine, where gut bacteria ferment it, creating gas and irritating the intestinal lining.
For dairy, the primary intolerance is usually lactose intolerance, which results from a deficiency of the lactase enzyme needed to break down lactose. This enzyme deficiency causes symptoms like acute cramping, nausea, and diarrhea, typically beginning 30 minutes to two hours after consuming milk products. The severity of the symptoms is often related to the amount of lactose consumed.
Non-celiac gluten sensitivity, or gluten intolerance, can present with a wider variety of systemic symptoms outside of the gastrointestinal tract. Individuals frequently report experiencing “brain fog,” which is characterized by difficulty concentrating or forgetfulness. Other non-digestive issues can include chronic fatigue, headaches, joint pain, and skin manifestations like rashes or eczema. The onset of gluten-related symptoms can be delayed, sometimes appearing hours or even days after the food is eaten, making it more challenging to connect the reaction to the specific meal.
The Role of Elimination Diets in Self-Assessment
A structured elimination diet is the most practical self-assessment tool for pinpointing a suspected food intolerance. This process is divided into two distinct phases: complete elimination and systematic reintroduction. The elimination phase requires removing all sources of the suspected food—gluten, dairy, or both—from the diet for a minimum period of two to four weeks. The goal of this phase is to establish a symptom-free baseline before proceeding.
During this time, it is necessary to maintain a detailed food and symptom journal, noting all foods and beverages consumed alongside any physical reactions. If symptoms improve significantly or disappear entirely during this elimination period, it suggests that the removed food group was the likely trigger.
The next step is the reintroduction or “challenge” phase, which must be done carefully and methodically. Reintroduce only one suspected food at a time, consuming a small portion on the first day and increasing the amount over a two- to three-day period. If you are testing for wheat, for example, use a pure form like a simple wheat cereal, rather than a mixed food like macaroni and cheese.
After the challenge period, remove the tested food again for two days while monitoring for the return of symptoms. If the original symptoms reappear, it confirms a sensitivity to that food; if no reaction occurs, the food is likely safe to consume, and you can move on to testing the next food group.
Professional Diagnosis and Medical Testing
While self-testing with an elimination diet is helpful, a formal medical diagnosis requires professional testing to rule out more serious underlying conditions. For suspected lactose intolerance, the standard diagnostic procedure is the hydrogen breath test. This test measures hydrogen gas levels in your breath after you consume a lactose-loaded liquid.
In individuals who cannot digest lactose, the undigested sugar ferments in the colon, producing hydrogen that is absorbed into the bloodstream and exhaled through the lungs. A significant rise in hydrogen levels, above 20 parts per million, is considered a positive result for lactose malabsorption.
For gluten, the diagnostic path is more complex because non-celiac gluten sensitivity (NCGS) is a diagnosis of exclusion, meaning it is only confirmed after other conditions are ruled out. The primary concern to exclude is Celiac Disease, an autoimmune disorder that causes damage to the small intestine. Testing for Celiac Disease involves a blood panel to check for specific antibodies, such as tissue transglutaminase (tTG-IgA).
It is necessary that you continue to eat a gluten-containing diet before undergoing these blood tests, as eliminating gluten beforehand can lead to a false-negative result. If the blood test is positive, a doctor will perform an endoscopy to take a biopsy of the small intestine to check for the characteristic villous atrophy, which confirms Celiac Disease.
If both Celiac Disease and a Wheat Allergy are ruled out, and symptoms still resolve upon gluten removal and return upon reintroduction, the diagnosis of NCGS is applied.
Understanding Related Conditions
Distinguishing a food intolerance from a food allergy or an autoimmune disease is important for long-term health management. A food intolerance, such as lactose intolerance, is caused by a lack of the digestive enzyme lactase and affects only the digestive system. This means that small amounts of the trigger food may often be tolerated, and the symptoms are limited to gastrointestinal discomfort.
In contrast, a food allergy, such as a Cow’s Milk Protein Allergy (CMPA) or a Wheat Allergy, involves an immediate, immune system reaction to a specific protein in the food. This immune response releases antibodies and can cause severe symptoms outside of the gut, including hives, swelling, wheezing, and in severe cases, anaphylaxis. Even microscopic amounts of the allergen can trigger a dangerous reaction, which is a difference from a non-immune intolerance.
Celiac Disease is an autoimmune condition where the immune system attacks the small intestine lining upon gluten consumption, damaging the villi. Unlike a food allergy, Celiac Disease does not cause anaphylaxis, but it requires strict, lifelong avoidance of gluten to prevent intestinal damage and long-term complications. Non-celiac gluten sensitivity (NCGS) is classified as an intolerance, as it produces symptoms without the immune-mediated intestinal damage seen in Celiac Disease.