The "Open Gut" Theory and Early Allergen Introduction

Learn about common misconceptions on the "open gut" theory and why early allergen introduction is recommended for all babies starting at 4 months. 

The "open gut" theory, also known as "leaky gut" or "virgin gut," is a well-known theory that claims it is dangerous to feed babies solid foods until after 6 months of age. Meanwhile, new clinical guidelines recommend introducing allergenic foods as early as 4 - 6 months of age. These contradicting claims can be very confusing for parents. 

What exactly is "open gut," and is there any scientific evidence to support this theory? Or is "open gut" just an unsupported myth? 

Here’s what parents need to know, including the proven benefits of introducing allergenic foods as early as 4 - 6 months. 

What is the "open gut" theory?

Normally, the small intestine’s lining carefully sorts the food and bacteria that enter the body. It absorbs key nutrients and allows them to pass into the bloodstream, but filters out harmful particles that could cause infection.

According to the "open gut" theory, infants’ intestinal linings aren’t equipped to sort solid food and bacteria in this way until they reach 6 months of age. Instead, infants have spaces between the cells of their small intestines. Supposedly, these spaces are big enough for large molecules to enter the bloodstream directly, including particles that could harm an infant.

Here are common misconceptions on the "open gut" theory:

  • Misconception #1: When only breastmilk passes through a baby’s intestinal openings, the baby’s gut remains "sterile." 
  • Misconception #2: However, if babies eat solid foods while their gut is still "open," the gut becomes "unsterile." 
  • Misconception #3: This puts babies at risk for chronic illnesses and food allergies, as food proteins, bacteria and toxins are allowed to pass directly into the bloodstream.
  • Misconception #4: Only once the gut "closes" around 6 months of age, meaning that the spaces between the cells of the small intestines close up, is it safe to introduce solid foods. 

Is there any scientific evidence supporting "open gut?" 

The "open gut" theory is very popular online, and has been shared in many blog posts. But there is no scientific evidence to support the claim that babies have an "open gut" until 6 months of age. 

Using scientific evidence, how can we tell whether a baby’s gut is "open" or "closed?"

As Alice Callahan, Ph.D., explains, babies are born with a “permeable” intestinal lining that easily lets particles pass into the bloodstream, similar to the model described in the “open gut” theory. However, there are many common misconceptions about the timing of “gut closure”. Instead, this “closure” happens much sooner than the “open gut” theory claims. 

  • Researchers can examine someone’s gut permeability (including an infant’s) by giving them a dose of smaller and larger sugar particles that can enter the bloodstream without breaking down. 
  • Normally, the smaller sugar particle is absorbed into the bloodstream, but the larger one can’t fit through the intestinal lining. 
  • But, when infants with a permeable intestinal lining consume the larger and smaller particles, the larger particles also pass through their lining. 
  • The more larger particles that pass through (relative to the number of smaller particles that pass through), the more permeable an infant’s lining. 
According to scientific research, how early does gut “closure” occur?
  • An NIH study measured 72 healthy newborns’ gut permeability on days 1, 7, and 30 of life, using the sugar particle test described above.
  • The study found that the most “gut closure,” or the greatest reduction of gut permeability, happens within baby’s first week of life.
  • By one month, both breastfed and formula-fed infants were shown to have “closed” guts that keep the majority of particles from directly passing into the bloodstream. This is several months earlier than the 6-month mark used in the “open gut” theory. 

What does this mean for introducing solid foods, including allergenic foods?

  • The above NIH study demonstrates that the 6-month “open gut” theory is unsupported by scientific evidence. 
  • Also, the majority of studies have found that starting solids at 4-6 months does not pose any greater risk for GI infection than starting solids after 6 months.
  • Thus, there is no need for parents to worry that introducing solid foods prior to 6 months of age will put their baby at risk for food allergies or chronic diseases. 
  • Instead, parents should start introducing solid foods, including allergenic foods, when their baby is developmentally ready. In some babies, this is as early as 4 - 6 months of age. (Learn more here on identifying when your baby is ready for starting solids.) 
  • It is also safe for families to mix food-based powders into breastmilk or formula, starting at 4 months of age.

Early Allergen Introduction: Evidence-Based and Effective 

Introducing allergenic foods starting between 4 and 6 months is important for every baby. On the other hand, the delayed introduction of allergenic foods, which the unsupported “open gut” theory promotes, can actually make babies more likely to develop a food allergy.   

So, the benefits of introducing allergenic foods early and often far outweigh the risks. And unlike the “open gut” theory, early allergen introduction is evidence-based. 

Introducing allergenic foods starting between 4 and 6 months is important for every baby. The delayed introduction of allergenic foods can make babies more likely to develop a food allergy. 

“The most important thing is to introduce food allergens early and frequently, starting as early as 4 - 6 months,” says Dr. Andrew Matthew, Board Certified Pediatrician and Chief of Pediatrics at Los Robles Medical Center.  “According to recent landmark studies, early allergen introduction in infants has been proven effective. Parents should not worry that introducing allergens this early 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 introduction may put your child at a greater risk for developing food allergies. In addition, results from the recent clinical early allergen introduction trials should put parents at ease. 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.”

“According to recent landmark studies, early allergen introduction in infants has been proven effective. Parents should not worry that introducing allergens this early will put their baby at risk. After all, the “open gut” theory is not supported by any scientific evidence." ---Dr. Andrew Matthew, Chief of Pediatrics at Los Robles Medical Center

The NIAID Guidelines state: "early introduction of peanut will result in the prevention of peanut allergy in a large number of infants," based on findings from landmark clinical studies: LEAP (Learning Early About Peanut Allergy) and EAT (Enquiring About Tolerance). 

  • In the studies, babies were introduced to common allergens starting between 4 - 11 months of age. 
  • The babies continued to consume these allergenic foods multiple times per week for 3 - 6 months (or more; the LEAP study sustained exposure for 4 years). 
  • Findings from these studies show that introducing babies to common food allergens early and often is important for every baby

Ready, Set, Food!: A Simple Solution for Early Allergen Introduction 

Even though clinical guidelines recommend introducing allergenic foods starting at 4 - 6 months, some babies simply aren’t developmentally ready for solids at this early age. 

But you don’t have to wait until your baby is ready for solids to introduce allergenic foods. Ready, Set, Food! easily mixes with breastmilk or formula, and safely introduces peanut, egg, and milk to your infant in pre-measured doses, based directly on the amounts used in the recent landmark studies. So, our evidence-based system makes it easy to follow the clinical allergen introduction recommendations, even if your baby is not ready for solid foods.

Ready, Set, Food! can help you introduce allergens safely and easily. 


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.  If your infant has severe eczema, check with your infant’s healthcare provider before feeding foods containing ground peanuts.