What is infant formula, and how can scientists make it more like human milk?

There are many reasons that a new parent might choose to feed a baby infant formula—either exclusively or as a supplement to human milk. Many new parents find that their little one needs more than can be provided by nursing alone.

The World Health Organization (WHO) recommends “exclusive breastfeeding for the first 6 months of life” with “continued breastfeeding up to two years of age or beyond.” But Sarah Sobik, a pediatrician at Arkansas Children’s Nutrition Center, says that many new parents have trouble producing enough milk for their infants or don’t have the proper support to help them breastfeed or chest feed. “Breastfeeding is very hard,” she says. “It rarely comes easy to anybody.” And some infants have sensitive stomachs or metabolic illnesses that require a special diet. For all these reasons and more, infant formulas can be a useful solution.

Walk down the baby food aisle in any grocery store and it is hard to miss the colorful display of infant formula. According to data presented at a Nestlé investor seminar in 2019, the infant nutrition market, which includes formula, is expected to expand to more than $92 billion globally by 2023. Nestlé is one of the leading infant formula manufacturers internationally.

Infant formulas are sold as powders in a seemingly endless variety of formulations and are reconstituted with water before use. While many brands advertise specific ingredients, the majority contain the same basic components. The most important aspect is to ensure that infants get enough nutrition in a safe and stable formulation.

Infant formula is designed as a complete substitute for human milk to meet the full nutritional needs of babies under 12 months of age. Internationally, the required components for formulas are set by the Codex Alimentarius, a joint food standards program overseen by the Food and Agriculture Organization of the United Nations and WHO. Individual countries can also set additional guidelines.

In total, the codex lists more than 30 required nutritional ingredients for infant formula, including vitamins and minerals, but the three major constituents are fats, proteins, and carbohydrates—the primary building blocks that little humans need to grow and develop. “The most common formulas are going to be cow’s milk protein—with whey and casein as the most common proteins,” Sobik says. “Then they’ll have vegetable oils as the fat source and lactose as the carbohydrate source.”

The codex specifies that formulas should have 1.8–3.0 g of protein, 4.4–6.0 g of fat, and 9.0–14.0 g of carbohydrates (specifically lactose or glucose) per 100 kcal, a typical infant-sized serving of formula. The US Food and Drug Administration has similar recommendations.

In addition to the principal building blocks that compose their diet, infants need supplements of vitamins and minerals. Even babies fed human milk need a secondary source of vitamin D, which is a required nutrient in infant formulas, along with other cofactors and electrolytes.

Of those micronutrients, Sobik says, iron is one of the most important. Because their iron stores will run out within a few weeks of birth, infants need a supplement in their diet. In the US, the FDA requires that all formulas are fortified with between 0.15 and 3.0 mg of iron per 100 kcal, a range recommended by the American Academy of Pediatrics (Pediatrics 1999, DOI: 10.1542/peds.104.1.119). The codex sets the iron minimum at 0.45 mg per 100 kcal.

Sobik works with parents to find the best food source for their babies but says infant formula is still a long way from fully re-creating the composition of human-produced milk, which also contains growth hormones, immunological factors, and many other useful compounds to help an infant thrive. But manufacturers are developing ingredients to make formula milk more comparable to human milk.

“The thing about breast milk is that it’s going to be different for every infant” in terms of composition, Sobik says. That’s because human milk includes a parent’s microbes and antibodies, which change over the course of an infant’s life—and even in a single feeding. It is probably impossible to make a formula that fully recapitulates all the qualities of human milk, Sobik says. Yet manufacturers are actively working to close the gap between infant formula and human milk.

One way to do this is by including docosahexaenoic acid (DHA) and arachidonic acid (ARA), two long-chain polyunsaturated fatty acids found in breast milk, says Jim Richards, a nutritional scientist and vice president of nutritional science and advocacy at DSM, a company that produces some of the ingredients used in infant formulas. Researchers have found that both these compounds are important for brain development in the first year of life (Dev. Psychobiol. 2018, DOI: 10.1002/dev.21780) and are also associated with increased visual acuity in infants (Am. J. Clin. Nutr. 2010, DOI: 10.3945/ajcn.2009.28557).

DSM produces both lipids by fermentation from algae—a sustainable, vegetarian source of these molecules—which manufacturers then include in infant formulas. While DHA is often advertised directly on the label of formulas, the codex stipulates that ARA should be included in at least the same concentration.

Though these lipids are beneficial to infants, Sobik says formulas that include DHA and ARA are a “bonus” for parents. That’s because babies can also make their own DHA from α-linoleic acid. Both the codex and FDA require all formulas to contain α- and γ-linoleic acid.

Another additive that some brands also advertise is human milk oligosaccharide (HMO). Steven Townsend, an organic chemist at Vanderbilt University, led a team that showed that these complex carbohydrates act as prebiotics in human milk by promoting healthy bacterial communities in the guts of infants, which may contribute to a stronger immune system (ACS Infect. Dis. 2017, DOI: 10.1021/acsinfecdis.7b00183). Human milk has these different sugar molecules at concentrations that vary according to a number of factors, including a parent’s genetics and the infant’s age.

There are more than 200 known HMOs, Townsend says, but most are too complex to produce at scale for infant formulas. One HMO sometimes included is the trisaccharide 2′-fucosyllactose (2′-FL). 2′FL is one of the most commonly produced HMOs across all demographics, Townsend says. But he doesn’t think that choosing this one relatively simple sugar out of the full mix of HMOs is enough to fully close the gap between formula and human milk. Exploring new ways to include complex sugars in infant formula is the next step in making the food source more similar to human milk, according to Townsend. In addition, there is more research to be done on HMOs and their benefits to babies.

Whether the formula milk that parents feed to their babies is basic or one that contains extra fatty acids and sugars to make it more like human milk, one of the most important ingredients for formula is added by the parent—water, Sobik says. Dry infant formula needs to be added to purified, sterile water and must be mixed properly. The components of the powder formula are processed so that it forms a fat-in-water emulsion when combined with water, and emulsifiers and stabilizers like lecithins and mono- or diglycerides in the formulation prevent the oil phase from separating out. If the formula is not fully homogenized, an infant won’t be able to digest all the nutrients—undoing all the important work of formulation.

Whichever option a parent chooses to feed a child—human milk, formula, or a mix of both—Sobik says her job is to collaborate with new parents to find the best option. “My number 1 priority is making sure the infant is growing and developing. All of these formulas do that,” she says.