Science of Mom reader Roxanne left a comment on my post about the recent peanut allergy study. She wondered about starting solid foods with her 4-month-old baby boy:

“Do you have an opinion on starting solids at 4 months versus 6 months? I noticed that many of the studies on allergy include babies in the 4-6 month range, but I think that the current recommendation is to wait until 6 months. I ask because my baby WILL NOT drink out of a bottle while I’m at work. He is miserable all day. I’m only gone 8-3 including travel time, so if he could just get a little something at 11am, I think he might actually nap and not cry all day. We have tried everything. If you know of any studies please let me know. He is 18 weeks old.”

I totally understand Roxanne’s confusion, because there’s lots of conflicting advice on this topic. This is a question that I tackled in-depth in my book (due out in July!), but I wanted to offer some of this information on my blog as well.

Let’s start by getting our terminology straight.

Starting solids is just the beginning of a slow transition from an exclusive milk diet to a diet of table foods. In some countries, this is also called “weaning,” which is confusing since the same term means stopping milk feeding in the U.S. (i.e. weaning from breastfeeding, weaning from a bottle). “Complementary feeding” is often used in the research and public health worlds. This is an apt term, because the goal with feeding solids to babies is to complement breast milk or formula, which will continue to provide most of babies’ calories through at least the end of the first year.

What is the history of starting solids?

There is a common assumption that longer exclusive breastfeeding – and longer delay in starting solids foods – must be more natural, and hence, healthier. But looking at traditional human cultures, with no access to commercial baby food, modern pediatricians, or divisive Internet forums, can give us valuable perspective on what is “natural.” A survey of childbirth and breastfeeding practices in 186 non-industrial cultures reported that solid foods were routinely introduced before 6 months, a finding that surprised the author:

“Contrary to the expectation of a prolonged period of breast-milk as the sole source of infant nutrition, solid foods were introduced before one month of age in one-third of the cultures, at between one and six months in another third, and was postponed more than six months for only one-third.”1

A more recent cross-cultural analysis of 113 nonindustrial populations from around the world found that parenting introduced solid foods before 6 months in more than half, with 5-6 months being the most common time for introduction.2

Human diets and infant care practices vary tremendously around the world, so it’s impossible to say if starting solids at 4 months or 6 months is more natural. As to which is healthier – well, that’s where we need to look at the science.

What is the official advice about starting solid foods?

Public health and professional organizations fall into two camps when it comes to recommendations about solids foods: they either recommend starting between 4 and 6 months OR at 6 months. There are well-respected organizations on both sides.

The World Health Organization (WHO), tasked with making recommendations for the entire world, “recommends that infants start receiving complementary foods at six months of age in addition to breast milk.” They also make it clear that solid foods are important, as the period of late infancy is a time of rapid growth and development, and breast milk alone just isn’t enough for most babies. “Complementary feeding should be timely, meaning that all infants should start receiving foods in addition to breast milk from 6 months onwards.” The government recommendations in Australia [PDF] and the U.K. mirror the WHO’s advice.

The European Society for Pediatric Gastroenterology, Hepatology, and Nutrition (ESPGHN) calls 6 months of EBF “a desirable goal.” But they also add, “In all infants… the introduction of complementary foods should not be before 17 weeks but should not be delayed beyond 26 weeks.”3 [PDF] In their statement on breastfeeding [PDF], the same committee writes, “In industrialized countries, there is at present no scientific evidence that introducing complementary foods to breastfed infants between 4 and 6 months of age is a disadvantage relative to introduction after 6 months.”4 The European Food Safety Authority came to similar conclusions [PDF].5

In the U.S., the American Academy of Pediatrics (AAP) 2012 Policy Statement, “Breastfeeding and the Use of Human Milk [PDF],” written by the AAP’s Section on Breastfeeding, recommends “exclusive breastfeeding for about 6 months, followed by continued breastfeeding as complementary foods are introduced…”6

However, scroll around the AAP’s website for parents, and you’ll find that the advice there focuses on developmental readiness (holding head up, sitting up, opening mouth for food, etc.) rather than age. Only later, in an italicized note, do they mention, “The AAP recommends breastfeeding as the sole source of nutrition for your baby for about 6 months.” There is no mention of what to do if you aren’t exclusively breastfeeding or of why it might be important to wait on solids until 6 months if you’re breastfeeding but not if you’re feeding formula (or some combination of the two).

Also within the U.S., the American Academy of Allergy, Asthma and Immunology recommends starting solids between 4 and 6 months based on some data (discussed below) that this may decrease the risk of developing food allergies. You can read their recommendations on preventing allergies here [PDF] – it’s a great summary of the current research. (This paragraph added 5/15/15 after reminder of this from a reader.)

One thing that’s certain is that what families actually do when it comes to starting solids is all over the map. According to the 2014 CDC Breastfeeding Report Card [PDF], just 19% of babies are exclusively breastfed at 6 months. A survey of 1,334 U.S. mothers, published in 2013, found that 40% introduced their babies to solids before 4 months, and only 7% waited until 6 months.7

Just about everyone agrees that is best to wait until 4 months to start solids. Studies have shown that introducing solids before 3-4 months can increase the risk of eczema at age 10, celiac disease, type 1 diabetes, wheezing in childhood, and increased body weight in childhood.8–11

However, the debate over starting solids at 4 versus 6 months continues. Which is better? Let’s take a look at what the science says.

How does starting solids affect nutrition?

As far as I can tell, there have only been three randomized controlled trials (RCTs) of starting solids at 4 versus 6 months. The RCT design is the gold standard for a clinical trial, and it gives us the best chance at determining how age of starting solids might cause different outcomes in babies’ health. (Most studies of infant feeding use observational study designs, which are always complicated by confounding factors and can only show correlations, not causation.)

The first two of the RCTs were conducted in Honduras in the 1990s.12,13 In both studies, mothers of infants were randomly split into two groups. Researchers asked one group to exclusively breastfeed (EBF) their babies until 6 months. They asked the other group to start solids at 4 months (while continuing to breastfeed) and gave them jarred commercial foods and advice on sanitary feeding practices, reducing the risk of foodborne illness. This was an important part of the study design in a setting where families might lack access to refrigeration or clean water, but we still have to be careful about applying these data to more developed countries, because factors such as maternal malnutrition and infant growth rates may be different. Happily, a third RCT with a similar design was conducted more recently in Iceland.14 It turns out that all three of these studies had similar findings:

Effects on infant growth: Whether babies started solids at 4 months or 6 months did not affect their growth during this time, suggesting that either method provides enough calories and nutrients for normal growth. The Iceland study also followed babies out to 18 months and 3 years, and they found no difference in growth at these ages (weight, length, head circumference, BMI, risk of overweight).15

Effects on energy intake: The Iceland trial included careful measurement 16 Similar results were seen in the Honduras studies. This is exactly what we’d expect given the similar growth between groups, and it shows that infants are good at self-regulating calorie intake to meet their needs.

Effects on nutrient status: The main nutrient of concern during the second half of the first year of life is iron. In one of the Honduran studies, starting solids at 4 months gave babies higher hemoglobin, hematocrit, and iron stores (measured by plasma ferritin) at 6 months of age compared to the 6 month EBF babies.17 In the Iceland study, the 4-month solids group also had higher iron stores than the 6-month group.14 However, ferritin levels were adequate in both groups of Icelandic babies, and the number of babies with iron deficiency or iron deficiency anemia was not significantly different. Thus, an earlier start at solids may give breastfed babies a little boost in iron, but it isn’t clear that this is really clinically relevant – that is, it may or may not make a difference to babies’ health. (The same cannot be said for delaying solids beyond 6 months; risk of iron deficiency does seem to increase in this situation.)

Observational studies support the conclusion that babies grow and develop well whether they start solids at 4 or 6 months. A 2012 Cochrane review looked at 23 studies (from both developing and developed countries) and concluded that 6 months of exclusive breastfeeding is usually sufficient to support healthy infant growth.18 The authors are cautious about applying this conclusion to every baby, though: “The data are insufficient to rule out a modest increase in risk of under nutrition with exclusive breastfeeding for six months and grossly inadequate to reach conclusions about the effects of prolonged (more than six months) exclusive breastfeeding.”

How does starting solids affect risk of infection?

In developing countries, where many families may lack access to clean water and/or refrigeration, the risk of illness dramatically increases when babies start solids. With the introduction of solid foods comes potential exposure to pathogens that may contaminate food, water, or utensils. For example, one study in the rural Phillipines found up to a 13-fold increased risk of diarrhea with feeding solids. This factor alone justifies the WHO’s recommendation for 6 months of exclusive breastfeeding in these settings.

But with access to clean food and careful feeding practices, the risk of infection with solids appears to be much lower and maybe not a concern at all. In a study of 16,000 U.K. infants, tracked for the first 8 months of life, starting solids didn’t increase their risk of hospitalization for diarrhea or lower respiratory tract infection, regardless of the age they took their first bite.19 Strangely, the Honduras RCTs found that the 6-month EBF babies were actually a little more likely to get sick (upper respiratory tract infections in one study12 and diarrhea in the other13) compared with those starting solids at 4 months.

One study, conducted in Belarus, found no difference in respiratory tract or ear infections but an increased risk of gastrointestinal infections in babies EBF to 3-4 months compared to those EBF for 6 months.20 The difference amounts to one extra case of diarrhea per 42 babies introduced to solids earlier. However, there was no difference in hospitalization for GI infections, so this increase seems to be in minor illnesses. It also includes some babies that were just 3 months old, and at least some of these infections could probably be prevented with more careful food prep practices.

So does starting solids increase the likelihood that your baby will get sick? Most studies say no, but one says that there may be a small increase in episodes of minor diarrhea.

How does starting solids affect the risk of allergies and chronic diseases?

When babies start eating solid foods, new proteins bombard their GI tracts. The immune system needs to learn about these proteins and recognize them as acceptable passengers through the GI tract rather than pathogenic invaders. Food allergies represent a failure to tolerate these food proteins.

For allergies and some chronic immune diseases, there appears to be a sweet spot for when to introduce solid foods. For example, one study found that children first exposed to wheat between 4 and 6 months (versus after 6 months) had a 4-fold decreased risk of wheat allergy.21 Another found that children who first had cooked egg at 4-6 months had the lowest incidence of egg allergy, whereas those starting egg at 10-12 months had a 6-fold increased risk.16 These studies were observational – not randomized – and there are a host of confounding factors that can muddle the waters of infant feeding research. However, the recent peanut study (an RCT, explained in my post here) clearly showed that exposure early in life reduces allergy, although timing and amounts of exposure will need to be defined for different allergens.

There is some evidence that a window of opportunity may exist for gluten as it affects the development of celiac disease. Earlier observational studies showed that infants who ate gluten after 3 months of age but before 6-7 months had lower risk of celiac disease. However, 2 RCTs (explained in my post here) published in 2014 showed that age of introduction of gluten (at 4 months, 6 months, or 12 months) didn’t ultimately affect the development of celiac. An observational study also showed lower risk of type 1 diabetes when cereals (including rice and gluten-containing grains) were introduced between 4 and 6 months.22 However, an RCT comparing gluten introduction at 6 or 12 months found no difference in risk of developing islet autoimmunity (ref 22a – this sentence added 5/15).

Either way, the research here suggests starting solids – particularly these potentially allergenic foods – by around 6 months may reduce allergy risk. Or it may not, but it doesn’t seem to hurt.

How does starting solids affect mom’s health?

In the Honduras studies, moms that exclusively breastfed their babies for 6 months were more likely to still have lactational amenorrhea (i.e., not have their periods) compared with those starting solids at 4 months. In one of these studies, 6 months of EBF resulted in more weight loss for moms (a difference of 0.6 kg or 1.3 lb between 4 and 6 months).23 Depending on your personal situation, these differences may or may not be benefits to you. Either way, exclusive breastfeeding or even amenorrhea is no guarantee that you won’t get pregnant, so don’t count on it as birth control.

What if I’m not breastfeeding?

Many of the studies I’ve discussed so far were specifically focused on when to introduce solids to breastfed babies. Is the decision different for a formula-fed baby? Not really. Some of the studies on infection and allergy risk included formula-fed babies, so those findings apply to both groups. If you’re feeding an iron-fortified formula, you don’t really need to worry about iron deficiency. Factors such as lactational amenorrhea and weight loss are obviously specific to breastfeeding.

We’ve covered a lot of ground, so let’s put these considerations in one place:

Reader Roxanne’s bottle-refusing baby inspired this post. In her situation, she has a baby who is hungry, and given the mix of risks and benefits of starting solids, it seems reasonable to give solids a try. Roxanne’s pediatrician told her the same thing, and she gradually started introducing some solids to her baby.

Each situation is different. Public health recommendations are intentionally simple and easy to understand, but babies and their families are complex. If you’ve read this far, then you understand that the research behind this question is also complex and doesn’t give us a clear right or wrong answer. This post focused on health outcomes related to starting solid foods, but it ignores what may be the most important factor of all: each baby’s developmental readiness and interest in solids. In my next post, I’ll write about how to tell if your baby is ready for solids.

Click here to read 4 Signs Your Baby Is Ready for Solid Foods.

Share your experience in the comments. What advice did you receive about starting solids? What did you actually do?

References:

Lozoff, B. Birth and Bonding in Non-Industrial Societies. Dev. Med. Child Neurol. 25, 595–600 (1983). Sellen, D. W. Comparison of infant feeding patterns reported for nonindustrial populations with current recommendations. J. Nutr. 131, 2707–2715 (2001). Agostoni, C. et al. Complementary feeding: a commentary by the ESPGHAN Committee on Nutrition. J. Pediatr. Gastroenterol. Nutr. 46, 99–110 (2008). ESPGHAN Committee on Nutrition: et al. Breast-feeding: A Commentary by the ESPGHAN Committee on Nutrition. J. Pediatr. Gastroenterol. 49, 112–125 (2009). EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on the appropriate age for introduction of complementary feeding of infants. Eur. Food Saf. Auth. J. 7, 2–38 (2009). American Academy of Pediatrics Section on Breastfeeding. Breastfeeding and the use of human milk. Pediatrics 129, e827–841 (2012). Clayton, H. B., Li, R., Perrine, C. G. & Scanlon, K. S. Prevalence and Reasons for Introducing Infants Early to Solid Foods: Variations by Milk Feeding Type. Pediatrics 131, e1108–e1114 (2013). Fergusson, D. M., Horwood, L. J. & Shannon, F. T. Early Solid Feeding and Recurrent Childhood Eczema: A 10-Year Longitudinal Study. Pediatrics 86, 541–546 (1990). Norris, J. M. et al. Risk of celiac disease autoimmunity and timing of gluten introduction in the diet of infants at increased risk of disease. J. Am. Med. Assoc. 293, 2343–2351 (2005). Norris, J. M. et al. Timing of initial cereal exposure in infancy and risk of islet autoimmunity. J. Am. Med. Assoc. 290, 1713–1720 (2003). Wilson, A. C. et al. Relation of infant diet to childhood health: seven year follow up of cohort of children in Dundee infant feeding study. BMJ 316, 21–25 (1998). Cohen, R. J., Brown, K. H., Dewey, K. G., Canahuati, J. & Landa Rivera, L. Effects of age of introduction of complementary foods on infant breast milk intake, total energy intake, and growth: a randomised intervention study in Honduras. The Lancet 344, 288–293 (1994). Dewey, K. G., Cohen, R. J., Brown, K. H. & Rivera, L. L. Age of introduction of complementary foods and growth of term, low-birth-weight, breast-fed infants: a randomized intervention study in Honduras. Am. J. Clin. Nutr. 69, 679–686 (1999). Jonsdottir, O. H. et al. Timing of the Introduction of Complementary Foods in Infancy: A Randomized Controlled Trial. Pediatrics 130, 1038–1045 (2012). Jonsdottir, O. H. et al. Exclusive breastfeeding for 4 versus 6 months and growth in early childhood. Acta Paediatr. 103, 105–111 (2013). Wells, J. C. et al. Randomized controlled trial of 4 compared with 6 mo of exclusive breastfeeding in Iceland: differences in breast-milk intake by stable-isotope probe. Am. J. Clin. Nutr. 96, 73–79 (2012). Dewey, K. G., Cohen, R. J., Rivera, L. L. & Brown, K. H. Effects of age of introduction of complementary foods on iron status of breast-fed infants in Honduras. Am. J. Clin. Nutr. 67, 878–884 (1998). Kramer, M. S. & Kakuma, R. Optimal duration of exclusive breastfeeding. Cochrane Database Syst. Rev. 8, CD003517 (2012). Quigley, M. A., Kelly, Y. J. & Sacker, A. Infant feeding, solid foods and hospitalisation in the first 8 months after birth. Arch. Dis. Child. 94, 148–150 (2009). Kramer, M. S. et al. Infant growth and health outcomes associated with 3 compared with 6 mo of exclusive breastfeeding. Am. J. Clin. Nutr. 78, 291–295 (2003). Poole, J. A. et al. Timing of Initial Exposure to Cereal Grains and the Risk of Wheat Allergy. Pediatrics 117, 2175–2182 (2006). Frederiksen, B. et al. Infant Exposures and Development of Type 1 Diabetes Mellitus: The Diabetes Autoimmunity Study in the Young (DAISY). JAMA Pediatr. 167, 808–815 (2013). Dewey, K. G., Cohen, R. J., Brown, K. H. & Rivera, L. L. Effects of exclusive breastfeeding for four versus six months on maternal nutritional status and infant motor development: results of two randomized trials in Honduras. J. Nutr. 131, 262–267 (2001).

(22a). Hummel, S., Pflüger, M., Hummel, M., Bonifacio, E. & Ziegler, A.-G. Primary dietary intervention study to reduce the risk of islet autoimmunity in children at increased risk for type 1 diabetes: the BABYDIET study. Diabetes Care 34, 1301–1305 (2011).

Here’s another post, from a blog I love, on this topic: When Is It Best To Introduce Solids at Expecting Science.

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