Elimination Diets: Purpose, Evidence and Limits

Elimination diets are among the most useful and most misused tools in clinical nutrition. Used well, they can identify food triggers that no laboratory test reveals, resolve symptoms that have troubled patients for years and allow a return to a varied diet with confidence. Used poorly, they become open ended restriction, nutritional inadequacy, social isolation and, for some patients, the beginning of a disordered relationship with food. The difference lies almost entirely in how they are designed and supervised.

The purpose

An elimination diet is a diagnostic tool before it is a treatment. Its purpose is to answer a question: does removing a food, or a group of foods, reliably improve symptoms, and does reintroducing it reliably bring them back? Because many reactions to food are not detectable by blood tests, and because symptoms such as bloating, abdominal pain, headaches and skin flares have many possible causes, a structured dietary trial is often the most reliable way to identify genuine triggers.

It is worth distinguishing the mechanisms involved. IgE mediated food allergy causes rapid reactions, sometimes severe, and is diagnosed by allergists using history and specific tests; foods suspected of causing such reactions should not be reintroduced at home. Non IgE mediated immune reactions include coeliac disease and eosinophilic oesophagitis. Food intolerances such as lactose intolerance and reactions to fermentable carbohydrates involve digestive mechanisms rather than the immune system. Other reactions involve pharmacologically active food components or are less well understood. Elimination and reintroduction is useful mainly for the non IgE mechanisms.

Types of elimination diet

Before starting

Several steps should precede any elimination diet.

1. Take a full history and timeline. What are the symptoms, when did they start, what patterns has the patient noticed? A food and symptom diary for 1 or 2 weeks can be revealing.
2. Exclude important conditions. Coeliac disease must be tested for before gluten is removed, because testing after a gluten free diet can produce false negatives. Red flags such as weight loss, bleeding or anaemia need medical assessment.
3. Screen for disordered eating. Ask about weight concerns, fear of food, previous eating disorders and how the patient feels about restriction. Elimination diets are generally not appropriate for people with, or at high risk of, eating disorders, including avoidant restrictive food intake disorder.
4. Check nutritional status and needs. Consider children, pregnancy, older age, vegetarian or vegan diets and existing deficiencies.
5. Choose the narrowest effective approach. Remove the fewest foods that the history supports.
6. Agree the plan and its duration. The patient should understand that elimination is temporary and that reintroduction is the most important phase.

The elimination phase

Elimination is usually maintained for 2 to 6 weeks, long enough for symptoms to improve if the removed foods are responsible. Key practical points:

• Provide lists of foods to include, not only foods to avoid.
• Replace nutrients from removed foods: calcium and iodine when dairy is removed, fibre and B vitamins when grains are restricted, protein when multiple protein sources are excluded.
• Plan meals, snacks, eating out and social events.
• Keep a symptom diary using simple ratings so change can be measured.
• Continue other parts of the plan, such as sleep and stress management, but avoid starting several new interventions at once, which makes results hard to interpret.

If symptoms do not improve after an adequate trial, the removed foods are unlikely to be the main cause, and the diet should be stopped rather than extended or made stricter.

The reintroduction phase

Reintroduction is where the diagnostic value lies, and it is the phase most often skipped. A systematic approach works best.

1. Reintroduce one food or food group at a time.
2. Start with a small portion, then increase over 1 to 3 days if no symptoms occur.
3. Watch for symptoms for 2 to 3 days after the final dose, since some reactions are delayed.
4. If a food produces symptoms, remove it again, wait for symptoms to settle, then continue with the next food. It may be retested later to confirm.
5. If no symptoms occur, the food returns to the diet permanently.
6. Continue until all removed foods have been tested.

Many patients find that they tolerate most foods, that some foods are tolerated in smaller amounts, and that only a few need to be limited. The goal is the most varied diet the patient can comfortably eat.

Risks and how to avoid them

• Nutritional inadequacy, particularly calcium, iodine, iron, B vitamins, fibre and protein. Avoided by planning replacements and limiting duration.
• Microbiome effects. Prolonged restriction of fermentable fibres may reduce beneficial bacteria, which is one reason the low FODMAP approach includes structured reintroduction.
• Disordered eating. Restriction can trigger or worsen eating disorders. Screening, supervision and prompt reintroduction reduce the risk.
• Social and psychological burden. Eating with others becomes harder; anxiety about food can grow.
• Missed diagnoses. Improvement on a gluten free diet may mask coeliac disease if not tested first.
• False conclusions. Symptoms fluctuate naturally, and expectation effects are strong. Reintroduction, and ideally retesting of apparent triggers, reduces false attributions.

An illustrative case

A 36 year old woman with bloating, abdominal pain and alternating bowel habit has already removed gluten, dairy and most fruit on her own, with partial improvement, and is anxious about eating. Her coeliac status is unknown.

The first step is to reintroduce gluten for testing, with her doctor arranging coeliac serology, which is negative. A screen does not suggest an eating disorder, but her anxiety about food is noted and discussed. Her history is typical of irritable bowel syndrome, so a structured low FODMAP approach is chosen instead of her self directed restrictions, with a clear 4 week elimination phase and reintroduction plan. Her symptoms improve substantially. During reintroduction she finds that fructans in wheat and onion and large servings of certain fruits trigger symptoms, while lactose, other fruits and dairy are well tolerated. She ends with a broader diet than she started with, and a clearer understanding of her triggers.

A tool with a beginning, a middle and an end

The best elimination diets are defined by what they are not: they are not permanent, not unsupervised and not open ended. They begin with a clear question and careful preparation, proceed through a time limited elimination, and end with systematic reintroduction and the most varied diet possible. Used this way, they are one of the most valuable diagnostic tools in clinical nutrition.