Baby formula is a manufactured food designed to mimic the nutritional profile of human breast milk. Every formula sold in the United States must meet federal requirements for at least 29 nutrients, including protein, fat, carbohydrates, 11 vitamins, and 9 minerals. Beyond those basics, modern formulas contain a growing list of bioactive ingredients meant to support immune function, brain development, and gut health. Here’s what’s actually inside the can.
Protein: Whey, Casein, or Both
Most standard formulas use cow’s milk protein as their base. Cow’s milk protein comes in two forms: whey (the liquid portion) and casein (the solid curds). In cow’s milk, whey makes up only about 18% of the total protein, with casein dominating. Human breast milk is the opposite, running between 70% and 80% whey in early lactation and settling closer to 50/50 as the baby grows older.
Traditional cow’s milk formulas are naturally high in casein, which makes them harder for infants to digest compared to breast milk. Many manufacturers now adjust the ratio by adding extra whey protein to better match what a baby’s gut expects. Federal regulations require between 1.8 and 4.5 grams of protein per 100 kilocalories of prepared formula.
Soy-based formulas replace cow’s milk protein entirely with soy protein isolate, while hydrolyzed formulas break the proteins down into smaller fragments called peptides. Partially hydrolyzed formulas contain peptides in the 3 to 10 kilodalton range. Extensively hydrolyzed formulas go further, breaking proteins below 3 kilodaltons, small enough that the immune system is less likely to react to them. These are designed for babies with cow’s milk protein allergy, though studies suggest that even extensively hydrolyzed formulas don’t resolve symptoms in every allergic infant.
Fat: A Blend of Plant Oils
Fat is the single largest source of calories in infant formula, required at 3.3 to 6.0 grams per 100 kilocalories. Since cow’s milk fat doesn’t match the fatty acid profile a baby needs, manufacturers replace it with carefully chosen blends of vegetable and seed oils. Common sources include soybean oil, coconut oil, sunflower oil, and safflower oil. Each oil contributes a different mix of fatty acids, and the blend is calibrated to approximate what’s found in breast milk.
Some brands avoid palm olein oil because it has been shown to reduce absorption of certain nutrients, particularly calcium. Linoleic acid, an essential fat the body can’t make on its own, must be present at a minimum of 300 milligrams per 100 kilocalories.
DHA and ARA for Brain and Eye Development
Two long-chain fatty acids get special attention: DHA (an omega-3) and ARA (an omega-6). DHA plays a critical role during the first 1,000 days of development, supporting healthy brain and eye growth. In formula, DHA is typically sourced from microalgae (often a species called Schizochytrium) rather than fish oil. Microalgal oil naturally provides a higher concentration of DHA relative to other omega-3s, making it well-suited for infant nutrition. ARA is usually derived from fungal fermentation. While not all formulas are required to include DHA and ARA, the vast majority of brands now add them voluntarily.
Carbohydrates: Lactose and Its Alternatives
Lactose is the primary carbohydrate in breast milk, and it’s the default sugar in most standard cow’s milk formulas. It digests slowly, producing a gradual rise in blood sugar, and serves as a fuel source for beneficial gut bacteria.
Some formulas replace lactose partially or entirely with corn syrup solids or maltodextrin. These substitutions are common in “sensitive” or “gentle” formulas marketed to parents of fussy or gassy babies. The swap matters more than it might seem: corn syrup solids are glucose-based, digest more rapidly than lactose, and produce a higher glycemic response. Research has raised concerns that this difference in carbohydrate source may affect metabolism and appetite regulation over time, and some studies have examined whether early exposure to these glucose-based formulas is linked to higher obesity risk. Sucrose (table sugar) was once used in some formulas but has been restricted in many markets.
Vitamins and Minerals
Federal regulations mandate minimums (and in some cases, maximums) for a long list of micronutrients. A few stand out for their importance in infant health:
- Iron: Standard U.S. formulas contain 10 to 12 milligrams of iron per liter. The American Academy of Pediatrics has endorsed 12 mg/L as safe and effective for preventing iron-deficiency anemia through the first year of life. Low-iron formulas still exist but are not recommended.
- Vitamin D: Required at 40 to 100 International Units per 100 kilocalories, vitamin D supports bone growth and calcium absorption. Formula-fed babies typically get enough vitamin D from formula alone, unlike breastfed babies who often need a supplement.
- Calcium and phosphorus: Both are essential for bone development. The ratio of calcium to phosphorus must fall between 1.1 and 2.0 to ensure proper absorption.
- Vitamin A: Required between 250 and 750 IU per 100 kilocalories, supporting vision and immune function.
- Zinc, iodine, and selenium: Trace minerals needed for growth, thyroid function, and antioxidant defense. Iodine, for instance, is capped at 75 micrograms per 100 kilocalories to prevent excess.
Non-milk-based formulas (like soy formulas) must also include choline, inositol, and biotin, which milk-based formulas naturally contain from their dairy ingredients.
Human Milk Oligosaccharides
One of the most significant additions to formula in recent years is a class of complex sugars called human milk oligosaccharides, or HMOs. Breast milk contains over 200 different HMOs, which babies can’t digest directly. Instead, these sugars feed beneficial gut bacteria and play a surprisingly active role in immune defense.
The most commonly added HMO in formula is 2′-fucosyllactose (2′-FL). In the gut, HMOs are broken down into short-chain fatty acids that promote the growth of beneficial bacteria like Bifidobacterium while inhibiting harmful species. Research links HMOs to lower rates of diarrhea in infancy, reduced intestinal permeability (a marker of gut barrier health), and potential protection against infections and inflammation. Formula with added HMOs doesn’t replicate the full diversity of breast milk’s oligosaccharide profile, but the inclusion of even one or two HMOs represents a meaningful step toward closing the gap.
Probiotics and Nucleotides
Some formulas now include live probiotic bacteria, most often strains of Bifidobacterium or Lactobacillus. One well-studied strain, Bifidobacterium longum subspecies infantis, was specifically selected because it carries the full set of genes needed to digest HMOs from human milk. In clinical trials, supplementation with this strain increased beneficial Bifidobacteriaceae by 79%, reduced potentially harmful gut pathogens by 80%, and led to persistent colonization that was still detectable at one year of age.
Nucleotides are another common addition. These are the building blocks of DNA and RNA, present naturally in breast milk at higher concentrations than in unsupplemented formula. Animal studies and several clinical trials have shown that added nucleotides support immune response, promote growth and repair of the intestinal lining, improve the balance of gut bacteria, and reduce episodes of diarrhea. Some research has also found better catch-up growth in small-for-gestational-age infants receiving nucleotide-supplemented formula.
What’s Regulated and What to Watch For
The FDA regulates infant formula under the Infant Formula Act, which sets mandatory nutrient floors and ceilings. Manufacturers must register with the FDA before selling a new formula, provide nutrient analyses, and test for pathogens like Salmonella and Cronobacter. Every batch of formula sold in the U.S. must meet these baseline requirements regardless of brand or price point.
Contaminant limits are also tightening. As of January 2025, the FDA has set action levels for lead in processed foods for babies: 10 parts per billion for most baby foods and 20 ppb for root vegetables and dry infant cereals. These are not zero-tolerance thresholds but represent levels the FDA considers achievable with current manufacturing practices, intended to drive continued reduction in exposure over time.
The ingredient list on a formula can look intimidating, with dozens of chemical-sounding names. Most of those entries are simply the specific forms of vitamins and minerals required by law. The core architecture of every formula is the same: a protein source, a fat blend, a carbohydrate, and a carefully measured suite of micronutrients. What varies between brands is mostly the source of each component and whether optional ingredients like HMOs, probiotics, or DHA are included.