The Truth About The “Open Gut” Theory And Introducing Allergy-Causing Foods

Learn why the “open gut” theory is misleading, and why all babies should be introduced to allergy-causing foods starting as early as 4 months of age.

A well-known theory called the “open gut” theory (also known as “virgin gut” or “leaky gut”) claims it is harmful to feed your baby any solid foods until they’re older than 6 months of age. 

But new feeding guidelines from leading medical organizations recommend introducing babies to allergy-causing foods as early as 4 months of age, to help prevent food allergies later in life. Many parents are confused by these competing claims.

What is the “open gut” theory? And is this theory supported by any scientific evidence, or is it just an unproven myth? 

Today, we’ll cover everything families need to know about the “open gut” theory. We’ll also cover how introducing common allergy-causing foods as early as 4 months of age offers proven benefits: it can reduce a baby’s risk of developing food allergies by up to 80%, based on landmark clinical studies.

The “Open Gut” theory: What is it?

The “open gut” theory claims that infants’ intestines aren’t ready to filter out bacteria that could harm their bodies.

According to this theory, a developed  small intestine’s lining is like a filter that knows the difference between helpful nutrients from food and harmful particles like bacteria. 

 It allows key nutrients from food to pass through into the bloodstream, but filters out harmful particles like bacteria that could cause infections.

But the “open gut” theory claims young babies’ intestinal linings aren’t developed enough to sort solid food and bacteria. Instead, infants younger than 6 months old have gaps between their small intestinal cells.

 Supposedly, these gaps are large enough for bigger molecules, like bacteria that could harm baby, to pass through into the bloodstream. 

Only once they reach 6 months of age, the theory claims, are babies’ intestines ready for solid food, because they’re ready to filter out the bacteria.

Four misconceptions from the “open gut” theory about starting solid foods 

  • Misconception #1: If a baby eats solid foods while their gut is “open,” their gut becomes “unsterile.” 
  • Misconception #2: For a baby’s gut to stay “sterile,” only breastmilk can pass through a baby’s intestinal openings while the gut is “open.”
  • Misconception #3: If a baby eats solid foods before 6 months of age, it increases their risk of developing food allergies and chronic illnesses, because the “open gut” lets bacteria, toxins and food proteins  pass through directly into the bloodstream.
  • Misconception #4: It’s only safe to introduce a baby to solid foods once their gut “closes up” around 6 months of age (once the gaps between the small intestines’ cells close up).

Is “open gut” supported by any science?

The “open gut” theory is widespread  online. Many parenting bloggers talk about it and caution against introducing solid foods “too early.”  But this theory doesn’t stand up to scientific evidence.

No scientific evidence supports the theory that babies’ guts are “open” until they are 6 months old.

Here’s how science can tell us whether a baby’s gut is “open” or “closed.”

According to Alice Callahan, PhD, infants are born with a “permeable” intestinal lining. This lining easily lets particles pass through into their bloodstream. 

Yes, this is similar to what the “open gut” theory describes. But the “open gut” theory is mistaken about when the gut “closes.” Scientific evidence shows gut “closure” happens far earlier than the “open gut” theory claims. 

  • Scientists can examine how permeable a baby’s gut is by giving them a dose of bigger and smaller sugar particles
  • Both of these sugar particles are able to pass through into the bloodstream, without breaking down. 
  • But the bigger sugar particle can’t fit through a fully developed intestinal lining, while the smaller sugar particle always gets absorbed into the bloodstream. 
  • When infants with a permeable intestinal lining consume the bigger and smaller particles, though, the bigger particles also pass through their lining. 
  • The greater the number of bigger particles that pass through the intestinal lining (relative to the number of smaller particles that pass through the lining), the more permeable a baby’s gut is. 

According to scientific research, how early do babies’ guts “close?”

  • One study from the National Institutes of Health (NIH) measured how permeable (“open”)  72 healthy newborns’ guts were using the same sugar particle test we explained above. 
  • The NIH researchers checked how permeable babies’ guts were on their first day of life, as well as when the babies were 7 days old and 30 days old.
  • The study found that the greatest reduction in gut permeability, or the most “gut closure,” happens during an infant’s first week of life.
  • By one month of age, the study showed that infants have “closed” guts that kept most of the sugar particles from passing through directly into the bloodstream. 
  • So, babies’ guts “closed up” much earlier than 6 months of age—much earlier than the “open gut” theory claims.
  •  And it didn’t matter whether infants were breastfed or formula-fed — all of the infants’ “guts” closed within 30 days.

What does this mean for introducing allergy-causing foods, and other solid foods?

  • The NIH study we covered above shows that there’s no scientific evidence to support the 6-month “open gut” theory. 
  • In addition, most studies have shown that introducing solid foods at 4-6 months does not put babies at greater risk for GI infection, compared to starting solids after 6 months.
  • So, you shouldn’t worry that introducing solid foods before 6 months of age will increase your baby’s risk for chronic diseases or food allergies. 
  • Instead, you should start introducing solid foods, including allergy-causing foods, whenever your baby is developmentally ready. For many babies, this will be as early as 4-6 months of age. As we’ll cover below, introducing allergy-causing foods early helps prevent future food allergies.
  • It is also safe to mix food-based powders into breastmilk or baby formula, starting as early as 4 months of age.

Early Allergen Introduction: An Evidence-Based Way To Prevent Food Allergies 

Introducing common allergy-causing foods starting between 4 and 6 months, and consistently feeding your baby these foods, can help reduce your baby’s food allergy risk rather than increasing it. 

In contrast, delaying the introduction of allergy-causing foods, which the unsupported “open gut” theory encourages, can actually increase your baby’s risk of developing food allergies.   

So, introducing allergy-causing foods early and often brings far more benefits than risks. And unlike the “open gut” theory, early allergen introduction is evidence-based. 

According to Dr. Andrew Matthew, Board Certified Pediatrician and Chief of Pediatrics at Los Robles Medical Center: 

  • “Parents shouldn’t worry that introducing allergens as early as 4-6 months of age will put their baby at risk. After all, the “open gut” theory is not supported by any scientific evidence. In fact, studies suggest that delaying allergen introduction may increase your child’s risk of developing food allergies.”
  • “The most important thing is to introduce allergy-causing foods early and frequently, starting as early as 4-6 months. According to recent landmark studies, early introduction of these foods in infants has been proven the most effective method for helping to prevent food allergies.” 
  • “Results from the recent clinical early allergen introduction trials should alleviate parens’ worries. These trials started early introduction as early as 4 months with over 2,000 babies, and there weren’t any cases of anaphylaxis or hospitalizations.”

Early allergen introduction is supported by several sets of recent clinical guidelines, which were written based on the results of three groundbreaking clinical studies: LEAP (Learning Early About Peanut Allergy), EAT (Enquiring About Tolerance), and PETIT (Prevention of Egg Allergy with Tiny Amount InTake). 

  • In the LEAP, EAT, and PETIT trials, babies were introduced to common allergy-causing foods starting between 4-11 months of age. 
  • The babies continued to consume these allergy-causing foods 2-7 times per week for 3-6 months (or more; the LEAP trial  sustained exposure for 4 years). 
  • Findings from these studies show that introducing babies to common allergy-causing foods (peanut, egg, milk) early and frequently can help reduce their food allergy risk by up to 80%. 
  • AAP and NIAID guidelines, developed based on these studies, recommend introducing your baby to peanut-containing foods as early as 4 to 6 months of age, to help reduce their risk of developing peanut allergies.
  • And new guidelines from the American Academy of Allergy, Asthma and Immunology (AAAAI), also based on these trials, state that “peanut and egg should be introduced” starting between 4 and 6 months of age “to prevent peanut and/or egg allergy.” 

So, if your baby is between 4-6 months old and developmentally ready for solids, don’t hesitate to introduce them to allergy-causing foods, as this will help prevent them from developing allergies to these foods in the future. And even if your baby is not ready for solids at this early age, new food powder options are available to mix with breastmilk or formula and introduce allergy-causing foods as early as 4 months.


All health-related content on this website is for informational purposes only and does not create a doctor-patient relationship. Always seek the advice of your own pediatrician in connection with any questions regarding your baby’s health.

These statements have not been evaluated by the Food and Drug Administration. Products are not intended to diagnose, treat, cure or prevent any disease.  

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