How are food allergies diagnosed?

Various tests can be used to find out whether someone has allergic (atopic) eczema. The most common are a skin prick test for allergies and a blood test for IgE antibodies.

The results of these tests aren't always correct, though. If the results are normal, you can be quite sure that you don't have an allergy. Abnormal results are more difficult to interpret. They do show that the body is sensitive to the food. But the tests can't tell us whether it is causing the eczema or making the symptoms worse. Also, a lot of people react to some substances in these kinds of tests. But that doesn't automatically mean that this causes trouble in their everyday life too.

Because the results of allergy tests don't lead to clear conclusions, Germany's Dermatological Society only recommends doing them if you are thought to have a specific allergy. The test results also need to be interpreted with caution: Abnormal test results alone are no reason to exclude certain foods. Things are different if a test known as a "challenge test" confirms that the skin actually reacts to a certain food.

Research on elimination diets for eczema

Good scientific studies are needed to be able to find out whether or not elimination diets are effective for eczema. These studies look at what happens when people who have eczema leave certain foods out of their diets. To do this, volunteers with eczema are randomly assigned to two groups. The people in one group are asked to keep eating their usual diets, while those in the other group are asked to go on a special diet.

Researchers from the Cochrane Collaboration analyzed nine suitable studies that tested whether elimination diets had any effect on eczema symptoms. Six of the studies looked at diets that avoided eggs and milk. Two of them tested the effect of a liquid baby food reduced to a few nutrients without any allergens. One study looked at a diet made up of only a few foods. The participants were tested for food allergies in only one of these studies.

Most of the studies involved babies and children. One study only involved adults. The studies were relatively small (between 11 and 85 participants in each study). All of the studies also had weaknesses: For example, in some of them the participants did not stick to the strict diet properly. Only two studies followed the participants for longer than six months. Soy-based milk substitutes were used in half of the studies that excluded milk. This could limit the value of the study outcomes because other research has shown that soy milk can sometimes cause allergies itself.

Elimination diets probably only help in people who have a diagnosed food allergy

In 8 of the 9 studies, there was no clear difference between the groups of people who were on a special diet and those who were not. Most of the participants hadn't been tested to see whether they had a food allergy in the first place, though. So generally eliminating certain foods that you aren't thought to be allergic to probably doesn't help.

The outcome was different in the other study, which looked at babies who had been found to have an allergic reaction to eggs based on a blood test done before the study started. In this study, one group of babies had an egg-free diet for four weeks, whereas the other group had a normal diet. The results showed that avoiding eggs reduced the rashes in the babies. But there were only 62 babies in the study.

Sources

• Bath-Hextall F, Delamere FM, Williams HC. Dietary exclusions for established atopic eczema. Cochrane Database Syst Rev 2008; (1): CD005203. [PMC free article: PMC6885041] [PubMed: 18254073]
• Sidbury R, Tom WL, Bergman JN, Cooper KD, Silverman RA, Berger TG et al. Guidelines of care for the management of atopic dermatitis: Section 4. Prevention of disease flares and use of adjunctive therapies and approaches. J Am Acad Dermatol 2014; 71(6): 1218-1233. [PMC free article: PMC4430554] [PubMed: 25264237]

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