Why Is Breast Milk So Low in Iron?
When I started feeling concerned about BabyC’s iron status (Does My Baby Get Enough Iron?), I did what most worried, sleep-deprived mamas do – a Google search. What I found were pages and pages of forums and blog posts full of comments from breastfeeding mothers who couldn’t fathom that their baby wasn’t getting everything she needed from breast milk. Feed an iron-fortified cereal or give my baby iron drops? Why? Isn’t breast milk the perfect food for my baby? I found many mothers fiercely defending breast milk and accordingly delaying the introduction of complementary foods and shunning the use of any fortified foods or supplements. I felt compelled to write about this, because I think that in some circles the enthusiasm for breastfeeding has swung a little too far outside of what is actually best and natural for babies.
After digging into the research on infant iron nutrition, I came to the conclusion that breastfed babies really are at higher risk for iron deficiency. Breastfed babies need to start eating high quality solid foods around 6 months, ideally while continuing to breastfeed, because certain nutrients (including iron, zinc, and vitamin D) are present at low concentrations in breast milk. The nutrient composition of breast milk is ideal for young infants (< 6 months), but in older infants, breast milk is really no longer the perfect food. It is an important part of a more complex diet, along with complementary foods.
Here is a very brief summary of infant iron nutrition (I’d like to tackle vitamin D and zinc in separate posts):
- Most babies are born with enough iron stores to meet their needs for about the first 6 months of life .
- Breast milk contains very little iron (~0.35 mg/liter). The Institute of Medicine recommends that infants 6-12 months old get 11 mg of iron per day . By this age, most babies’ iron stores have been depleted, so this iron needs to come from complementary foods, in addition to breast milk or formula. If you try to meet your infant’s iron requirement on breast milk alone, she would have to consume between 4 and 13 liters of breast milk per day, depending on your baby’s efficiency of iron absorption from breast milk (estimates range from 15-50% absorption). Most exclusively breastfed babies don’t consume much more than 1 liter of milk per day.
- Iron deficiency during infancy increases the risk of cognitive, motor, and behavioral deficits that may last into the teens, even with iron treatment. Specific deficits that have been identified include impaired motor development at 18 months , mental retardation at 10 years old , increased need to repeat a grade, and increased behavioral and attention problems . When I hear parents say that they declined the test for anemia at their baby’s 9 or 12-month check-up, I have to assume that they don’t know how serious iron deficiency can be for their child’s future.
- Babies that are exclusively breastfed beyond 6 months of age are at higher risk for iron deficiency [5-7] than those fed iron-fortified formula. This should not be taken as a deficiency of breast milk but rather as an indicator that it is important and natural for babies to start consuming solid foods around 6 months.
In light of these findings, I wondered:
Why would breast milk have evolved to be deficient in iron, putting babies at risk for iron deficiency?
What did the cave babies do before iron supplementation? This fascinated me.
We must remember that breast milk evolved over the last 2-2.5 million years to enhance infant survival (and also not put the mother at risk) in the context of the conditions of the time. In the developed world, there have been major changes in living conditions over just the last several hundred years, and evolution simply doesn’t happen that quickly. Researchers have proposed 3 hypotheses that could help explain why breast milk is low in iron:
Hypothesis 1. Babies used to get their iron from soil. Not too long ago in the history of the world, most people ate and slept on the ground, including babies. We all know that babies put everything in their mouths, and I doubt if cave babies were any different, especially as they became more mobile around 6 months of age. Iron in soil can be absorbed by humans . Most other mammals are similarly exposed to soil and also have low concentrations of iron in their breast milk . It is possible that breast milk evolved to have low iron concentrations because babies consumed plenty of iron through their environment (and why drain mom of iron if baby doesn’t need it?).
Hypothesis 2. Until recently in human history, the umbilical cord was not immediately clamped. In much of the modern world, it is common practice to clamp and cut the umbilical cord immediately after the birth of a child. Research has found that waiting just 2-3 minutes after birth before clamping the cord allows up to 50% more blood volume to pulse from the placenta to the newborn . A 2-minute delay has been shown to result in higher total body iron and plasma ferritin (reflecting iron storage) at 6 months of age, equating to about an extra month’s worth of iron stores [11, 12]. Certainly other mammals do not rush to clamp the cord immediately after birth and therefore also get that extra dose of iron to the baby before cutting her off from mom’s supply.Hypothesis 3. Breast milk may have evolved to have low iron as a mechanism for protecting infants from infection. Bacteria require iron to survive and reproduce, and many infections in young infants begin in the GI tract. What little iron is present in breast milk is bound to an iron-binding protein called lactoferrin. This limits the amount of free iron in a breastfed baby’s GI tract, which might also limit the growth of harmful bacteria. Older infants consuming iron-rich foods have more mature GI tracts that would be more resistant to infection. Although this hypothesis sounds plausible and is explained as if it is fact on sites like kellymom.com, there is actually not much hard data on it. Human milk inhibits the growth of E. coli in culture, but this effect is lost if enough iron is added to the culture to overwhelm the binding capacity of lactoferrin . However, this has only been shown in culture (in a petri dish) and in animals. Studies in real live babies have shown mixed results, some finding no effect of iron supplementation on rates of infectious disease and some finding a small effect . A 2002 review of 28 different randomized trials found an overall 11% increase in diarrhea in kids given iron supplements, but this small effect was not associated with iron-fortified foods, only iron drops . In a randomized trial of 4- to 9-month-old infants in Honduras and Sweden, Dewey et al.  found that in infants that initially had iron-deficiency anemia, supplementation helped – it reduced the incidence of diarrhea. However, in infants that were not anemic, iron supplementation increasedtheir incidence of diarrhea. This implies that too much iron can increase GI infections in infants, providing some support for the hypothesis that low iron in breast milk protects infants from infection.
Beyond these hypotheses, I have found no evidence that babies were breastfed exclusively much longer than 6 months throughout human history. Most 6-month-old babies are interested in eating because they are interested in putting EVERYTHING in their mouths. Given every mother’s interest in doing the best thing for her baby, I can imagine that babies have been given some of the best food available throughout human history, and in many cultures, that would be iron-rich meat (maybe initially pre-chewed by mom?). What about the babies that didn’t get enough iron in their diets? Before iron supplementation and awareness of the importance of dietary iron, babies probably were more likely to suffer from iron deficiency during the tender transition from breast milk to solid foods, and they probably suffered the consequences.
Thinking about breast milk from an evolutionary perspective helped me to realize that my breastfed baby, who is not raised in the dirt and had her cord clamped immediately after birth (not the plan, but that’s a story for another day), might need a little help getting enough iron in her diet. If she’ll eat some fortified oatmeal or Cheerios, I’ll take it! And if her iron tested low, I wouldn’t hesitate to give her an iron supplement to ensure that her brain and body were getting enough iron for normal development. Of course, it is possible to get enough iron into your baby’s diet without the help of fortified foods, but in practice this can be complicated by a baby’s taste and texture preferences and requires nutritional knowledge and time to prepare iron-rich foods.
For tips on ensuring that your baby gets enough iron, see my post 5 Practical Ways to Increase Iron in Your Baby’s Diet.
1. Institute of Medicine, Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. 2003, National Academies Press: Washington, D.C.
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