Editor's Note: Scientific literature generally uses the words "mother" and "maternal" in reference to a person with a uterus who has carried a child. To avoid inconsistencies, this article also uses that language.
"Breast milk contains all the nutrients a baby needs." At least, that is the oft-repeated claim that has spread online as lactation experts and "lactivists" encouraged mothers to breastfeed their babies. Videos like this one, viewed nearly 60,000 times, abound on social platforms:
This video on YouTube asserts the same thing.
This is not surprising. In fact, this claim has been made consistently by the world's top agency for public health, the World Health Organization:
Breastmilk provides all the energy and nutrients that the infant needs for the first months of life, and it continues to provide up to half or more of a child's nutritional needs during the second half of the first year, and up to one third during the second year of life.
The WHO has every interest in promoting breastfeeding and it recommends it as a first choice for up to two years. That's because the WHO's guidelines affect, most crucially, countries where formula is not readily available to local populations, and where potable water (with which parents mix formula powder) is a scarce resource.
But as we'll see, the claim that breast milk provides everything a baby needs is a bit of an oversimplification.
The Benefits of Breast Milk
If the circumstances allow, breastfeeding a baby is a great choice. Breast milk provides a baby with most of what it needs to grow, certainly more than what formula can offer. It is also a complex substance that evolves with time and even adjusts day to day to meet a baby's needs.
Breast milk is mostly water. It also contains around 7% carbohydrates, 4% fat and 1% protein, according to a 2020 article in Clinical and Experimental Pediatrics. Colostrum, which is the first "milk" produced immediately after birth, contains more proteins and less fat, however, attesting to the evolutive nature of breast milk.
Other elements pass, through breast milk, into a baby's body. These include "bioactive" factors such as stem cells, or cells that will promote the activation of defenses, antimicrobial, antibacterial and other anti-pathogenic agents, but also growth factors and hormones to support development. A 2013 article in Pediatric Clinics of North America provides a more exhaustive list of these elements.
What's remarkable is that breast milk, unlike formula, adjusts to more-immediate needs of babies. For example, in case of illness, breast milk changes its composition to provide more of what can help an infant recover. The mother's body "knows" this based on information communicated by the baby's saliva on the nipple: This is why some lactation experts recommend that mothers who pump rub a bit of the baby's saliva on their nipples so they will produce milk with the best possible composition.
Nutritionwise, breast milk has countless advantages. Logistically, it can make a lot of sense to do it, too. Breast milk is free — to the extent that a mother's time can be "free" — and it, more often than not, is readily available.
But when it comes to minerals, micronutrients and antibodies, a lot depends on the mother's diet, her vaccination status and exposure to illness, and a baby's own resources. Breast milk lacks some of these important elements, even with an optimal maternal diet.
Circumstantial Deficiencies
Vegetarian and vegan mothers will probably need to supplement their diet. While infants of mothers who eat meat typically get the amounts of vitamin B12 they need, those whose mothers don't eat meat are at risk of B12 deficiency. Vitamin B12 supports red cell and DNA formation. It is essential to the development of the brain and nerve cells, and not getting enough of it puts babies at risk of anemia and brain damage. If a mother is B12 deficient, a breastfed baby will be too.
This is also a risk for babies whose mothers have conditions that might reduce their ability to absorb nutrients. These include B12, but also folic acid, calcium and iron.
Iron deficiency also concerns prematurely born babies and babies born small who are exclusively breastfed. Average-size babies are born with enough iron to last them about six months, but breast milk does not contain much iron at all. While formula is fortified with iron, to prevent the risk of anemia for premature babies and babies born small who breastfeed only, pediatricians recommend an iron supplement until they start solids.
Iodine helps regulate the T3 and T4 hormones produced by a mother's thyroid. These hormones are key for a fetus' and a baby's growth and neural development. A mother who does not eat seafood or iodized salt, or a mother who smokes or eats high-goitrogen foods (kale and broccoli are good examples), should take iodine supplements.
It's also recommended that mothers receive all the appropriate vaccines to help boost antibodies that pass through breastmilk. For example, it was shown that breastfeeding mothers were able to pass on protective antibodies against COVID-19 if they had been vaccinated or exposed to the illness. The same is true for the flu and whooping cough (pertussis), which is why doctors recommend mothers receive the vaccine during pregnancy. Mothers without these antibodies cannot protect their babies through breastfeeding.
Pervasive Deficiencies
Even with an ideal maternal diet, a baby won't get everything they need from breastfeeding.
Vitamin D plays an essential role in calcium absorption. It helps to build bones and make them strong, but it also supports immunity, by helping to modulate immune response to pathogens. Humans can also get vitamin D from food, namely fish, meats, egg yolks and fortified foods (milk, cereals, bread, etc.), but a major source of vitamin D is sunlight: When ultraviolet B rays hit cholesterol in skin, the body uses it to synthesize vitamin D. This is why humans who live in higher latitudes (farther from the equator) are at a higher risk of vitamin D deficiency — especially those with darker skin. This deficiency concerns one in four people in the United States, 37% in Canada and 40% in Europe, according to a 2020 survey published in the European Journal of Clinical Nutrition.
It follows that breastfed babies in higher latitudes, too, are at high risk of vitamin D deficiency. Also, because of the risk of skin cancer, doctors recommend protecting babies from sunlight (the Food and Drug Administration also recommends not using sunscreen on babies). So, while formula provides vitamin D supplements, the American Academy of Pediatrics says babies who eat breast milk or a combination of breast milk and formula should receive 400 international units of vitamin D starting shortly after birth, and until they can eat solids.
Another essential nutrient is vitamin K, which supports blood clotting. Babies are born with very little of it, which puts them at risk of, in the worst cases, vitamin K deficiency bleeding. The Centers for Disease Control and Prevention warns that breastfed babies, regardless of what their mothers eat — and even if they take vitamin K supplements — do not get enough of it. This is why, according to the AAP, all babies should receive an intramuscular vitamin K shot within six hours of birth.