Food allergies are caused by the body’s overreaction to an otherwise harmless protein.
When my older son was diagnosed with the first (of 10!) food allergies, we were stunned. No one in our family even has seasonal allergies.
Even though food allergies have become way more common, and now almost 10% of Americans have one, doctors still don’t know exactly why some people’s bodies make the mistake of reacting to harmless foods as if they were a threat, and they still cannot predict who will have a food allergy and who won’t.
Even without a clear way to predict food allergies, doctors have learned a lot about what causes a food allergy.
- We know that babies aren’t born with food allergies. They “develop,” usually when a baby’s immune system is learning.
- A few different things have been shown to increase a baby’s risk of developing a food allergy. Eczema is the biggest risk factor, but the environment, genetics, gut health, and avoiding foods in infancy can also cause food allergies.
Eczema is the Greatest Risk Factor for Developing Food Allergies
Having eczema (dry, itchy, cracked skin) increases your baby’s risk of developing an allergy.
Doctors think that the dry, cracked skin in babies with eczema accidentally lets food, pollen, and animal dander into the body, and this confuses the immune system, resulting in allergies to otherwise harmless things.
Imagine your body is like a castle with a big stone wall all around it (your skin) and one main entrance (your mouth). The stone wall is supposed to keep everything out. The knights that protect the wall (your immune system) think anything that comes through the wall is dangerous.
So if a wandering sheep sneaks through the wall, the knights might overreact and attack the innocent sheep because “it’s not supposed to be here!” The problem is that after the knights get scared of something once, they might continue to think it’s dangerous, even if it comes through the gate like it’s supposed to.
The same thing happens to babies with eczema. Food, pollen, and other particles in the air get in through the skin, which sometimes causes the immune system to overreact.
The graph above shows the results of a study that looked at babies without eczema and those with mild, medium, or bad eczema and tracked them to see who developed food allergies.
Babies who developed eczema very young (age 0-3 months) that required prescription-strength medicines to treat (severe eczema), had a 50% rate of food allergy. Babies who developed eczema later (age 10-12 months) that was pretty mild, had only a 5% rate of food allergy.
The more severe your baby’s eczema, meaning the more cracks they have in their skin, and the younger your baby is when eczema appears, the higher the chance they have of developing a food allergy (P. E. Martin et al 2014).
Why Does Eczema Cause Food Allergies?
It all comes down to the cracks in your babies skin, and eczema is a triple whammy.
First, eczema skin lacks a protein called filaggrin. Filaggrin is like the mortar that keeps the bricks of the skin together and stops food dust particles from entering the skin, or water from leaving the skin.
Second, eczema skin loses water faster, resulting in very dry skin. Dry skin can easily break and crack, allowing particles into the body.
Lastly, when a baby has eczema, their skin is really itchy. So they’ll scratch themselves. A recent study in mice done at Boston Children’s Hospital showed that scratching the skin promotes allergic reactions to foods, including anaphylaxis.
Scratching can create cuts in the skin that make it prone to infection, and scratching itself seems to call up the cells and chemicals that trigger an allergic reaction.
How the Environment Can Cause Food Allergies
All of us are exposed to millions of bacteria, viruses, chemicals, and other particles every single day. Our immune systems usually do an amazing job of ignoring the things that don’t matter.
But babies’ immune systems, just like their brains, are growing, learning, and changing a LOT in the first 3 years of their life. In the same way that lead exposure can mess up brain development, we know that air pollution can mess up immune system development.
If your baby’s immune system is exposed to the wrong chemicals and becomes hyper-active, it is more likely to make mistakes, like deciding that cheese is harmful.
While no study has been able to show directly that a particular chemical causes food allergies, many doctors believe that chemicals in our food, on surfaces, and in the air can cause our immune systems to be hyper-active and confused.
There is a lot more research to be done here, but we do know that babies who have more exposure to dirt, clean air, and animals instead of polluted air, water, and concrete are less likely to develop food allergies .
How the Gut Microbiome Can Cause Food Allergies
The same thing also happens to babies with the wrong microbiome, or bacteria, in their stomachs and intestines. Some bacteria thicken your baby’s stomach lining to make it stronger, and have a way of talking to the immune system to help it stay calm. Other, less helpful bacteria, can cause the stomach and intestines to become weak or inflamed.
A weak intestinal lining can cause leaky gut, which is exactly what it sounds like. In leaky gut, some food proteins can leak through the gut, and your baby’s immune system reacts like anyone might to finding something where it’s not supposed to be.
The microbiome is the hottest area of research today and has been linked to a head-spinning number of diseases. Doctors know that babies who develop food allergies have different bacteria in their guts than babies who don’t develop food allergies. Doctors are currently figuring out which bacteria are the problem, how the unhelpful bacteria got there, and how to give babies with unhelpful bacteria the right bacteria.
How Mom & Dad’s Genes Can Cause Food Allergies
There is some genetic component to food allergies. A baby is more likely to develop food allergies if a parent, specifically their mother, or a brother/sister has a food allergy.
But genetics is not a major cause of food allergy. Children with no family history still have a 10% chance of developing a food allergy.
There are some genes that doctors think are involved in developing a food allergy. But before you run off and get a genetic test from 23andme, you should know there is no known food allergy gene.
Even if you checked for a set of genes, it wouldn’t prove that your baby was going to have an allergy. Genes don’t work by themselves.
Some people have genes to be tall. But if they don’t get enough food when they are little, they might end up short. Genes increase the chance of something, but they don’t guarantee it.
How Avoiding Foods Can Cause a Food Allergies
Babies typically develop food allergies between 6-12 months old because you need to have an immune system in order for it to overreact. Newborn babies have weak immune systems that rapidly develop until age 3.
Ages 6 months to 3 years old is when the immune system is learning what is good and what is bad. What to eat and what to throw up. Every baby has to be taught what foods their bodies should eat and which ones they should reject.
The beauty of a baby's learning, changing, growing immune system is that no matter what is landing on their skin, or what their immune system is nervous about, the allergy isn't set in stone. Early allergen introduction, the practice of feeding babies 2g of things they might develop allergies to three times a week, properly trains their immune systems to stay calm.
Studies of early allergen introduction that compare babies with eczema and without show that it works the same amount in all of them. Studies of babies in high pollution areas, those with food-allergic parents, and even babies who already had antibodies against foods showed that early allergen introduction prevents 80% of food allergies from developing across the board.
So no matter what’s going on inside, every baby should be fed peanuts, tree nuts, eggs, wheat, and milk a couple times every week until they are about a year old to make sure that their immune system develops correctly and doesn’t make the mistake of a food allergy.