Is Food Intolerance Testing for Real?

Can a simple blood test reveal hidden food sensitivities and help you lose weight? Nutrition Diva sets the record straight onf food intolerance testing.

Monica Reinagel, MS, LD/N, CNS
4-minute read
Episode #202

Is Food Intolerance Testing for Real?

by Monica Reinagel, M.S., L.D./N.

David writes:

“The other day I was at my local pharmacy and saw a flyer for a food intolerance testing service. Because I listen to podcasts like yours, I have learned to be skeptical and to check out the science behind the claims. Normally I would instantly dismiss a test that promises to address everything from fatigue to migraines to weight loss—not to mention one where the results include a personalized list of recommended nutritional supplements! But the fact that a pharmacy was offering it made me less skeptical than usual. Perhaps you could set the record straight for your listeners.”

I’d be happy to, David. 

What’s a Food Allergy?

The first thing we need to do is distinguish between food allergies—which are quite specific and readily diagnosable—and food intolerance or sensitivities. A food allergy, such the type people commonly have to peanuts or shellfish, is what we call an IgE-mediated reaction. For whatever reason, your immune system has decided that a particular protein is a threat—a threat so dangerous that it has developed a special reconnaissance agent (the IgE-antibody) to be constantly on guard for it. Should that protein turn up in your blood stream, those IgE antibodies are going to sound the alarm and your body is going to react—sometimes quite violently.

Eating a food to which you have an IgE-mediated allergy could produce tingling or itching of the skin around or inside your mouth, flushing or hives, wheezing, or vomiting. In the most severe reactions, your throat might swell to the point that you can’t breathe or you might go into anaphylactic shock. Your doctor can test quite definitively for this type of allergy by looking for IgE antibodies in your blood. They are always there, regardless of whether you’ve just eaten the food in question.

What is a Food Intolerance?

But the type of blood test David saw advertised at the pharmacist’s is quite different. These type of tests claim to reveal food insensitivities or intolerances. According to promoters, most people suffer from undiagnosed food intolerances, which can be a hidden cause of everything from fatigue to acne to weight gain. The test, they claim, will reveal which foods are secretly to blame for whatever ails you. Avoiding these foods will clear up the problem. Magic!

The test is a blood test similar to the one used to diagnose classic food allergies, but instead of screening for IgE antibodies, it looks for a different class of antibody called IgG. If you have high levels of IgG antibodies to corn, for example, this indicates a corn intolerance. People who pay to have this test done (and it usually costs a several hundred dollars) usually get back a long list of foods to which they are allegedly either somewhat or extremely intolerant. The next step is to eliminate all of these foods from your diet and wait for your symptoms—and your spare tire—to magically disappear.

In the brochures and on the websites, there are always plenty of testimonials from people who suffered from all kinds of vague and persistent health issues, all of which improved once the test revealed their hidden food sensitivities and they revamped their diets. I’m very glad that these folks are feeling better. I really am. But I don’t want you spending $500 on a test based purely on anecdotal evidence. As I explained in my episode on raw milk, reporting bias, uncontrolled variables, and the placebo effect make anecdotal observations notoriously unreliable.

Some manufacturers have gone a step further and done studies in which they found that people who modified their diets according to their test results have lost more weight or had improvement in other symptoms. But all of the studies I’ve reviewed had serious methodological problems. There was no blinding of the subjects or researchers, no control groups, very poor control over variables, and many of the measures were extremely subjective. In other words, these published studies really aren’t that much better than anecdotal testimony.

Is the Test Valid?

Still, lots of people seem to think that the test helped them. Why am I so reluctant to give it the benefit of the doubt? Not because I don’t believe in food intolerance. But because I don’t think these blood tests are a valid diagnostic tool—primarily, because they have an unacceptably high rate of false positives.

The presence of IgG antibodies to certain foods does not necessarily suggest that these foods are causing a harmful or inappropriate immune reaction. It’s more likely to suggest that these are the foods you eat the most often or have eaten the most recently. In fact, in people who were allergic to milk or eggs as kids but eventually outgrew their allergies (which is fairly common), we see higher levels of IgG antibodies to these foods. In other words, the presences of those antibodies is actually linked to increased tolerance—not the opposite.

Food Intolerances Are Real, Food Intolerance Testing Isn’t

Food intolerances are a real thing. If you are lactose-intolerant, for example, you aren’t allergic to milk. But you do lack the enzymes that allow you to digest lactose. As a result, consuming dairy products may produce obvious symptoms like gas and cramping. Sometimes, food intolerance symptoms are not as obvious or immediate, which can make tracking down the culprit(s) very tricky.


The best way we have to diagnose authentic food sensitivities is by elimination/challenge diet, a lengthy and painstaking process that is best overseen by a qualified allergist or immunologist. It would be great if a simple blood test could offer a reliable shortcut. Unfortunately, it doesn’t. More likely, you’ll be out a few hundred dollars and working harder than you need to, avoiding tons of foods that aren’t actually a problem for you.

Keep in Touch

Thanks for David for suggesting today’s topic. As a thank you, I’ll be sending him a signed copy of my book, Secrets of a Healthy Diet: What to Eat, What to Avoid, and What to Stop Worrying About, available wherever you buy or download books. 

If you have a suggestion for a future show topic or would like to find out about having me speak at your conference or event, send an email to nutrition@quickanddirtytips.com You can also post comments and questions on my Nutrition Diva Facebook Page.

I answer a lot of listener questions in my free weekly newsletter, so if you’ve sent a question my way, be sure you’re signed up to receive that.


About the Author

Monica Reinagel, MS, LD/N, CNS

Monica Reinagel is a board-certified licensed nutritionist, author, and the creator of one of iTunes' most highly ranked health and fitness podcasts. Her advice is regularly featured on the TODAY show, Dr. Oz, NPR, and in the nation's leading newspapers, magazines, and websites. Do you have a nutrition question? Call the Nutrition Diva listener line at 443-961-6206. Your question could be featured on the show.