What’s the Difference Between a Food Allergy vs Food Intolerance? Dr. Will Bulsiewicz Explains

It’s not often we have repeat interviewees here at WellBe, but sometimes a person just has too much valuable expertise to unpack in one interview. One of those people is Dr. Will Bulsiewicz (aka Dr. B). Dr B is a double board-certified physician, gut health expert, and author of Fiber Fueled and the brand new Fiber Fueled Cookbook, which he describes as “the ‘how’ of gut health.” While Dr. B’s first WellBe interview focused on the importance of fiber and dietary diversity, this one is all about the causes of food intolerance. Read on to learn the difference between a food allergy vs food intolerance, the most common food intolerances, and much more. 

*This is a short clip from Adrienne’s interview with Dr. Will Bulsiewicz. Click here to watch the whole thing.*

You can also listen to Adrienne’s interview with Dr. Will Bulsiewicz on The WellBe Podcast.

Food Allergy vs Food Intolerance: Understanding the Difference

While many people use the terms “food allergy” and “food intolerance” interchangeably, Dr. B explains that they are actually “radically different things.” When they are conflated, either intentionally or unintentionally, it can prevent people from getting to the root cause of their issues or even have dangerous consequences. So let’s get into the differences between a food allergy vs food intolerance. 

Dr. B explains that a food allergy is when your immune system becomes activated in response to a food you consume. Just as seasonal allergies are your immune system reacting to pollen, and cat allergies are your immune system reacting to pet dander, a food allergy indicates that your immune system is reacting to something in your diet. 

“As a medical doctor, I’m actually quite scared of food allergies, because they can be very violent, they can be very dangerous,” says Dr. B.  “ A food allergy could have some digestive symptoms, but the manifestation of a food allergy is commonly going to be outside of the digestive system.” He explains that a food allergy can cause extreme reactions, like breaking into hives or — in the most dangerous cases — a person’s throat or lips swelling such that they can’t breathe.

On the other hand, a food intolerance (sometimes referred to as a food sensitivity), is very different and much less dangerous. This, Dr. B says, is a good thing, since food intolerances are much more common than food allergies, affecting about one in five people in the United States (for food allergies, it’s one in ten). With a food intolerance, the symptoms are primarily digestive — gas, bloating, diarrhea, constipation. “You eat the food, you manifest these symptoms that you don’t want, and because there’s a connection between those two things, that’s what a food intolerance is,” he says. 

When it comes to a food allergy vs food intolerance, quantity also matters a lot. With a food allergy, the tiniest amount of exposure can cause a severe reaction. “You’re not even necessarily trying to eat the food. You may accidentally come into contact with the food, and yet it will trigger this big, dangerous reaction,” says Dr. B, explaining that this is why airlines are no longer allowed to give out peanuts on planes: if a passenger with a peanut allergy is sitting several rows away from another passenger who opens up their bag of peanuts, even that kind of trivial exposure can activate an immune response.

With food intolerance, on the other hand, it’s all about tolerance, as the term implies. Having a food intolerance simply means that your body doesn’t do a good job of tolerating larger (or even “normal”) amounts of a food. So, says Dr. B, someone who is intolerant to soy might have severe digestive symptoms if they drink a whole glass of soy milk, but be okay if they have a couple cubes of tofu in their salad. “This is part of what separates a food intolerance from a food allergy,” he says. “There is an amount that you’re capable of tolerating.”

The last big difference between food allergy vs food intolerance is how you test for them. With allergies, the testing is pretty straightforward. If you have any of the symptoms of a food allergy — hives, swelling of the throat or face, itchy mouth, trouble breathing — then Dr. B strongly recommends that you consult with an allergist or immunologist. The specialist will then run tests, which could include blood tests or skin prick tests, to see what allergens your immune system reacts to. 

With food intolerances, there’s no such test. While there are at-home tests that claim to diagnose food sensitivities, these are as yet unproven (and pricey), and so the gold standard for identifying a food intolerance is an elimination diet. In short, an elimination diet involves cutting out the food or foods that you suspect you might be intolerant to for a period of three weeks or more, and noticing how your symptoms change. At the end of the three weeks, you reintroduce the foods one by one and observe your symptoms. Read our full guide to elimination diets here.

The Major Causes of Food Intolerance

When Dr. B meets with patients who think they might have a food intolerance, he’s all about root cause analysis. That means taking a thorough, systematic approach that considers every possible explanation for their symptoms. And he always starts with what he calls “the big three” of food intolerance: three conditions that are often underlying causes of food intolerance. 

“I call them the big three because if you come in with food intolerances, I need to know whether or not these three are there,” he says. “Because if they are there, and I fix them, you’re not even going to need to do an elimination diet. This will fix your food intolerances by itself.”

The big three are: constipation, celiac disease, and gallbladder dysfunction. “If I get you pooping — fix the constipation — if I take you off gluten — fix the celiac disease — or we address a dysfunctional gallbladder, my expectation is that, if your food intolerances aren’t completely gone, they’re at least radically better, and then we can pick up from there,” says Dr. B.

Constipation can have a number of different causes (including, paradoxically, eating too much fiber), and Dr. B will work with patients to identify the issue and solve it. Gallbladder dysfunction is still poorly understood, though Dr. B believes there’s likely a connection to the gut microbiome. And celiac disease is a genetically motivated condition that is generally treated simply by cutting gluten out of the diet. “There is no option,” Dr. B says. “Unless something changes in science, there’s no option to bring gluten back into the diet. Not even in the most trivial amounts.” (Note that gluten intolerance is not the same thing as celiac disease.)

Once he’s ruled out the big three causes of food intolerance, Dr. B then turns to gut health. “With a food intolerance, it’s not the immune system. What’s happening is that your body is struggling to process and unpack food,” he says. “Most commonly, we’re going to find this takes place in a person who has damage to their gut microbiome. And this is because the gut microbiome is essential to digestion.” He explains that humans have around 40 trillion microbes living in our bodies, and that 38 trillion of those live in the colon alone, doing incredibly important work to help digestive processes. If those microbes were removed and your body had to rely only on digestive enzymes to break down food, says Dr. B, “we would all have food intolerances. We would all have massive food intolerances.”

Looking At the Most Common Food Intolerances

Now that we’ve looked at the causes of food intolerance, let’s look at the actual foods that people tend to have trouble tolerating (leading to the digestive symptoms we mentioned above). While a person could theoretically have an intolerance to any food, there are some more common culprits.

Dr. B points first to FODMAPs. FODMAP is an acronym that stands for fermentable oligosaccharides, disaccharides, monosaccharides, and polyols, and many people have trouble digesting foods that fall under this category. “It basically refers to the fermentable parts of our foods,” he explains.

Under the FODMAP umbrella is one of the most common food intolerances in the world: lactose. About 70% of the global population has an intolerance to lactose, which is a disaccharide (sugar) found in milk and other dairy products. “[People with lactose intolerance] can’t tolerate a normal bowl of ice cream or a normal glass of milk,” Dr. B says, “It will trigger symptoms for them. But they can tolerate some. It might be one scoop of ice cream or a half a glass of milk.”

Some of the other most common food intolerances include beans, whole grains, garlic, and fruits. With all of these, says Dr. B, “these specific foods have specific components that may cause you trouble.”

If you don’t know where to start with an elimination diet, it may make sense to begin with some of the most common food intolerances, though Dr. B recommends working with a doctor to figure out what elimination diet makes the most sense for you. “I’m not here to promise that everyone has universal results just by doing this,” he says. “What I’m promising is empowerment.”

The WellBe Takeaway from Food Intolerance Expert Dr. Will Bulsiewicz

We learned so many important things from our conversation with Dr. Will Bulsiewicz. Here’s what to remember about food allergies, food intolerance, and your health:

  • While the terms “food allergy” and “food intolerance” are often used interchangeably, they mean different things. A food allergy is an immune response that can result in severe and extreme reactions, like facial swelling or hives. A food intolerance is when your body has trouble digesting a certain food, and so you experience digestive symptoms as a result.
  • There are other differences when it comes to food allergy vs food intolerance. First, amount: while people with a food allergy can have a severe reaction if they even come close to a small amount of their trigger food, those with a food intolerance can generally consume some amount of the food without experiencing symptoms. Second, there are blood and skin tests that doctors can do to identify a food allergy. Identifying a food intolerance requires doing an elimination diet.
  • The “big three” causes of food intolerance are constipation, gallbladder dysfunction, and celiac disease. According to Dr. B, if someone has one or several of these issues and they are then treated, the food intolerance often goes away. When these conditions are not present, a damaged gut microbiome is often the cause.
  • Some of the most common food intolerances are FODMAPs, a category of foods that have fermentable components that can be difficult to digest. Lactose, the sugar in dairy, is a FODMAP, and also one of the most common food intolerances in the world. Beans, grains, garlic, and fruits are other common trigger foods.
  • While there is nothing that can be done about food allergies, people can actually increase their tolerance to foods that they don’t digest well by improving their gut health.

Watch our whole interview with Dr. Will Bulsiewicz to learn why nursing moms might want to avoid dairy products, the difference between genetic and acquired food intolerances, why it’s important to acknowledge the limitations of science, whether or not people can heal food intolerances, why fiber can sometimes cause constipation, his favorite recipes from the book, the link between dietary diversity in childhood and allergies later in life, the best sources of plant-based protein, and much more.

You can also listen to Adrienne’s interview with Dr. Will Bulsiewicz on The WellBe Podcast.



Do you have any food allergies or food intolerances? Tell us your experience in the comments below!


The information contained in this article comes from our interview with Dr. Will Bulsiewicz, MD, MSCI. His qualifications and training include graduating from Georgetown School of Medicine, a Master of Science in Clinical Investigation from Northwestern University, and a certificate in Nutrition from Cornell University. He trained in medicine at Northwestern Memorial Hospital and gastroenterology at the University of North Carolina Hospitals. He is double board-certified in internal medicine and gastroenterology. You can learn more about him here.


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