Updated: Mar 8
There is a persistent myth about infant gut development that comes up in every discussion on when to introduce solid foods/begin weaning. It’s the idea that babies have a “virgin” or “open” gut until 6 months of age. We see this argument on a daily basis on online forums and social media groups, claiming that introducing solid foods before 6 months of age is detrimental to the baby’s health, and by no means should weaning start before, unless directed by a health professional.
Is this bold claim correct? Has anyone ever really seen any documents or studies to back this claim? With this in mind, The Babyboo Blog set out to take a look at the science behind this claim, and finally put the debate to bed.
There is much controversy around when to begin weaning a baby. In the UK the recommendation is from 6 months. However, most baby food companies advertise their products from 4 to 6 months, leaving parents confused. Some health organisations such as the WHO recommend 6 months of exclusive breastfeeding, while many GPs and health visitors recommend starting to offer solids between 4 and 6 months, providing your baby is showing signs of readiness for solid food. Based on current evidence, we believe that the second approach is closest to the mark. It also encourages parents to focus on their baby’s unique development rather than watching the calendar. However, we also think that it’s just fine to wait until 6 months if that is your preference.
When the science is debated, reluctant mothers rush to link a document by kellymom.com who claim that a baby’s gut is open until 6 months of age. Here is what they say;
“From birth until somewhere between four and six months of age babies possess what is often referred to as an “open gut.” This means that the spaces between the cells of the small intestines will readily allow intact macromolecules, including whole proteins and pathogens, to pass directly into the bloodstream. This is great for your breastfed baby as it allows beneficial antibodies in breastmilk to pass more directly into baby’s bloodstream, but it also means that large proteins from other foods (which may predispose baby to allergies) and disease-causing pathogens can pass right through, too.”
Wow, that does sound scary! No wonder this “open gut” idea would worry parents approaching weaning stage. But wait… There are no references whatsoever to support these bold and scary claims, and in all our readings of the research literature on readiness for solids, we have not once encountered any science backing this concern. Yet somehow this myth of the open gut comes up over and over in online discussions and is often used to shame parents who have decided to wean their baby before 6 months of age.
What is meant by the term “an open gut”, and how do we know when it has closed?
The lining of the small intestine absorbs nutrients and immune protection. When we eat, food and bacteria enter the gastrointestinal tract, and the lining of the small intestine is what separates it from our bloodstreams. It is important that the lining can effectively absorb the good nutrients and keep out the bacteria. The small intestine is one of your baby’s most important barriers to infections.
The junctions in the small intestine which keep bacteria out and absorb nutrients are never completely sealed and let some particles through whilst excluding others, depending on size and energy.
Researchers have tested how easily particles can cross the lining of the small intestine and into the bloodstream. They call this intestinal permeability. When studying this, researchers give a person an oral dose of two sugars, usually mannitol and lactulose
Mannitol is the smaller of the two sugars with a molecular weight of 182 and is absorbed through pores in the intestine. Lactulose is much larger with a molecular weight of 342 and unable to fit through the pores. Some of it, however, will sneak through the junctions to get into the bloodstream. Once in the blood, neither lactulose or mannitol are further metabolised and are filtered out through the urine.
Whilst conducting this study, researchers give a person (or a baby) a dose of these two sugars and collect their urine. From the urine samples, they can measure how much of each was absorbed in the small intestine. Results are usually described as a ‘lactulose to mannitol ratio’, with higher values representing greater intestinal permeability and lower values representing less intestinal permeability or a more “closed gut”.
How quickly does intestinal permeability change in babies?
Intestinal permeability does change in infancy, but the timeline is quicker than 6 months of age, contradicting claims like those quoted above.
The sugar absorption test has been used to measure the process and timing of gut closure in babies.
Contrary to the ongoing claims from mothers on social media that babies guts are open until 6 months of age, studies show that the most important gut closure actually happens in the newborn period. For example, studies have shown that the biggest drop in intestinal permeability occurred within that first week of life. (one study conducted with 72 healthy newborns on days 1, 7, and 30 of life with the sugar test)
How does breastfeeding vs formula feeding affect intestinal permeability?
Breastfeeding Vs formula feeding slightly affects intestinal permeability in newborns, but this doesn’t last long. One study found no difference in intestinal permeability in exclusively breastfed and formula-fed babies on day 1 or even day 30. On day 7, however, intestinal permeability was slightly lower in breastfed compared with formula-fed babies, indicating that formula-feeding may slow the process of gut closure. By one month and beyond, there is no difference in intestinal permeability between breastfed or formula-fed babies.
What about older babies?
Studies have shown how there is a very gradual decline (if at all) in intestinal permeability over the first several years of life. No magic switch is flipped, nor gut-doors closed when your baby turns 6 months old.
The website ‘KellyMom’ also claims that a baby’s “open gut” allows breast milk antibodies to pass directly into the bloodstream. This is also a myth, and is something that actually doesn’t happen in human babies, with the exception of the first few days of life when intestinal permeability is truly high and when mothers are only producing colostrum. The truth is that human babies get their antibodies passed into their bloodstream when they cross the placenta during pregnancy. It is only animals that get antibodies via breastmilk after the newborn stage.
This isn’t to say that human milk isn’t full of antibodies. The most important antibody type in human milk is called secretory IgA, which coats mucosal surfaces such as the lining of the GI tract and can protect against infection. However, IgA can’t be absorbed into the blood in human babies.
When is my baby’s gut ready for solid foods?
The European Society for Pediatric Gastroenterology, Hepatology, and Nutrition say:
“The available data suggest that both renal function and gastrointestinal function are sufficiently mature to metabolise nutrients from complementary foods by the age of 4 months. With respect to gastrointestinal function, it is known that exposure to solids and the transition from a high-fat to a high-carbohydrate diet is associated with hormonal responses (eg, insulin, adrenal hormones) that result in adaptation of digestive functions to the nature of the ingested foods, by increasing the maturation rate of some enzymatic functions and/or activities. Thus, to a large degree gastrointestinal maturation is driven by the foods ingested.” (14)
It is only by exposure to a type of food that the GI tract can actually become efficient at digesting it. So expect some interesting nappies, from very messy to very solid, when you start weaning as the GI tract is adapting to digesting these new foods. It’s a good reason to introduce new foods gradually and in small quantities at the beginning, but it isn’t a good reason to avoid feeding them at all.
In other words, don’t feel pressured to calendar watch if you think your baby is showing signs of readiness after they turn 4 months. Equally, don’t force a baby to take solids who shows no signs of readiness. Mum has a good idea, and her instincts are often extraordinarily accurate, but when it comes to feeding your baby- science really does know best.
Handy hints before you wean
Never feed a baby solid food before the age of 4 calendar months (1st-1st, not 16 weeks). Never crush anything and put it in your baby’s bottle to thicken their milk- their milk is all that should come through a bottle. Make sure your baby can sit unaided before you begin weaning with finger foods. Introduce your baby to one food at a time, blended veggies are perfect for 4-6 months, and steamed vegetables make great introductions to finger food after 6 months. Avoid meat until 6 months. Do not give honey to a baby under 12 months, or whole nuts to any child younger than 5. Always cut grapes lengthways and into 4. Don’t give your child too much sugar or salt. Remember to give water with each meal (boil and cool for under 6 months). Relax, and let your baby take the lead. They don’t NEED to finish a portion, nor do they NEED to cut out a breastfeed or a bottle for every meal they start eating. Enjoy the experience, and take lot’s of food-faced photos, you’ll treasure them forever!
https://kellymom.com/van Elburg, R. M. et al. Repeatability of the sugar-absorption test, using lactulose and mannitol, for measuring intestinal permeability for sugars. J. Pediatr. Gastroenterol. Nutr.20, 184–188 (1995).Corpeleijn, W. E., van Elburg, R. M., Kema, I. P. & van Goudoever, J. B. Assessment of intestinal permeability in (premature) neonates by sugar absorption tests. Methods Mol. Biol. Clifton NJ 763, 95–104 (2011).Data from Catassi et al. 1995Catassi, C., Bonucci, A., Coppa, G. V., Carlucci, A. & Giorgi, P. L. Intestinal permeability changes during the first month: effect of natural versus artificial feeding. J. Pediatr. Gastroenterol. Nutr. 21, 383–386 (1995).Colomé, G. et al. Intestinal permeability in different feedings in infancy. Acta Paediatr. 96,69–72 (2007).Udall, J. N. & Walker, W. A. The physiologic and pathologic basis for the transport of macromolecules across the intestinal tract. J. Pediatr. Gastroenterol. Nutr. 1, 295–301 (1982).