For decades, cow’s milk–based infant formulas like Similac and Enfamil have been widely used in neonatal intensive care units (NICUs) across the country. But the scientific evidence has long shown that these formulas pose a serious and specific danger to premature infants: a dramatically elevated risk of developing necrotizing enterocolitis (NEC), one of the most devastating intestinal diseases in neonatal medicine.
Understanding why formula causes NEC requires understanding the unique vulnerability of the premature infant’s digestive system, the specific biological mechanisms through which cow’s milk proteins trigger intestinal destruction, and the overwhelming body of research that formula manufacturers had access to — and chose to ignore.
Note: The medical information on this page is provided for educational purposes. It is not medical advice. Always consult your child’s healthcare provider for medical guidance.
The Premature Infant Gut: Why It’s So Vulnerable
A full-term baby is born with a digestive system that has had roughly 40 weeks to develop. A premature infant — born before 37 weeks of gestation, and often before 32 weeks — enters the world with intestines that are fundamentally unfinished. This immaturity is not a minor disadvantage. It is a critical vulnerability that makes the premature gut uniquely susceptible to injury, infection, and the cascade of tissue destruction that defines NEC.
The intestinal lining of a premature infant is made up of a single layer of epithelial cells that serves as the barrier between the contents of the gut (food, bacteria, enzymes) and the bloodstream. In a full-term baby, this barrier is relatively mature, with tight junctions between cells that prevent harmful substances from crossing. In a premature infant, these tight junctions are underdeveloped and abnormally permeable — a condition sometimes referred to as “leaky gut.”
This permeability means that bacteria, toxins, and undigested proteins can cross from the intestinal lumen into the surrounding tissue and bloodstream far more easily than they would in a mature gut. When this happens, the infant’s immature immune system responds with an exaggerated inflammatory reaction that, instead of containing the threat, actually amplifies the damage — leading to tissue death, bacterial invasion, and the rapidly progressive destruction of the intestinal wall that characterizes NEC.
Beyond the barrier itself, several other aspects of gut immaturity contribute to NEC vulnerability:
- Reduced intestinal blood flow: Premature infants often have compromised blood flow to the gut (mesenteric circulation), which means the intestinal tissue receives less oxygen and is more prone to ischemic injury.
- Immature motility: The coordinated muscular contractions (peristalsis) that move food through the intestines are poorly developed in premature infants, leading to stasis of gut contents, which promotes bacterial overgrowth.
- Deficient mucosal defense: The premature gut produces far less protective mucus and fewer antimicrobial peptides (natural germ-fighting molecules) than the mature gut, leaving the intestinal lining exposed and undefended.
- Dysregulated immune response: The premature immune system tends toward an exaggerated pro-inflammatory response. When the gut is challenged, this overreaction drives tissue destruction rather than controlled healing.
This is the starting point: a gut that is permeable, poorly defended, poorly perfused, and prone to overreaction. Into this fragile system, formula introduces a substance the premature infant was never designed to process.
How Cow’s Milk Formula Triggers NEC
The connection between cow’s milk–based formula and NEC is not a single mechanism but a convergence of multiple biological insults — each one compounding the others in the already-vulnerable premature gut. Researchers have identified five primary pathways through which formula feeding drives NEC development:
Foreign Proteins Trigger an Inflammatory Cascade
Cow’s milk proteins — particularly casein and whey — are structurally different from human milk proteins. The premature infant’s gut recognizes these proteins as foreign invaders. Because the intestinal barrier is permeable, incompletely digested cow’s milk proteins cross into the tissue, triggering a powerful inflammatory immune response. In the immature gut, this inflammatory response is poorly regulated and tends to escalate rather than resolve, causing direct damage to the intestinal lining and initiating the tissue necrosis that defines NEC.
Formula Lacks Critical Protective Factors
Human breast milk is not just nutrition — it is a biologically active immune defense system. It contains secretory IgA antibodies that coat the intestinal lining and block bacterial adhesion; lactoferrin, a protein that binds iron to starve pathogenic bacteria; human milk oligosaccharides (HMOs) that feed beneficial bacteria and directly inhibit harmful ones; growth factors that promote intestinal maturation; and anti-inflammatory cytokines that modulate the immune response. Cow’s milk–based formula contains none of these protective components. Feeding formula to a premature infant strips away the very defenses that the immature gut depends upon to survive.
Formula Disrupts the Gut Microbiome
The gut microbiome — the community of bacteria that colonizes the intestine — plays a critical role in gut health and immune function. In breast-fed premature infants, the microbiome is dominated by beneficial bacteria such as Bifidobacterium and Lactobacillus, which help maintain the intestinal barrier and suppress harmful organisms. Formula feeding fundamentally alters this balance, promoting colonization by pathogenic bacteria such as Clostridium, Klebsiella, and Enterobacter. This dysbiosis (microbial imbalance) creates an environment where harmful bacteria can invade the intestinal wall, produce tissue-damaging toxins, and trigger the bacterial translocation that drives NEC progression.
Hyperosmolar Formulas Damage the Intestinal Lining
Many infant formulas used in NICUs — particularly those fortified with additional nutrients for premature infants — have a high osmolarity (concentration of dissolved particles). When a hyperosmolar solution enters the premature gut, it draws water from the intestinal tissue into the lumen through osmosis. This fluid shift can damage the delicate epithelial cells, disrupt the already-fragile tight junctions, and directly injure the intestinal mucosa. Studies have linked high-osmolarity feeds to an increased risk of NEC, yet many of the formulas marketed specifically for premature infants have osmolarity levels that exceed recommended thresholds.
Rapid Feeding Advancement Overwhelms the Immature Gut
In many NICUs, formula-fed premature infants have their feeding volumes increased on an aggressive schedule to reach full enteral nutrition as quickly as possible. But the premature gut cannot always keep pace. Rapid feeding advancement with formula introduces more foreign protein, more osmotic stress, and more substrate for pathogenic bacteria at a rate that exceeds the immature intestine’s capacity to cope. Research has consistently shown that slower, more cautious feeding advancement — particularly with human milk rather than formula — is associated with lower rates of NEC. Yet the standard protocols in many NICUs continued to advance formula feeds aggressively, driven in part by the influence of formula manufacturers’ marketing to hospitals.
Breast Milk vs. Formula: What the Research Shows
The scientific evidence comparing breast milk to cow’s milk–based formula for premature infants is not ambiguous. Decades of peer-reviewed research have established, with overwhelming consistency, that human milk dramatically reduces the risk of NEC while formula dramatically increases it.
Key Research Findings
Up to 77% reduction in NEC risk: A landmark 2010 study published in the Journal of Pediatrics found that an exclusive human milk diet reduced the incidence of NEC in very low birth weight infants by 77% compared to a diet that included cow’s milk–based formula or fortifier.
Donor milk as the recommended alternative: When a mother’s own milk is unavailable, pasteurized human donor milk is the recommended substitute. Studies show that donor milk, while not as protective as mother’s own milk, still significantly reduces NEC risk compared to formula.
AAP recommendation: The American Academy of Pediatrics (AAP) has recommended that all premature infants receive human milk, and that donor milk should be used when mother’s own milk is not available. The AAP has cited the reduced risk of NEC as a primary reason for this recommendation.
WHO endorsement: The World Health Organization (WHO) similarly recommends that preterm and low birth weight infants be fed mother’s own milk. Where this is not possible, donor human milk should be used. Formula should be a last resort.
Dose-dependent relationship: Research has shown that the more cow’s milk–based formula a premature infant receives, the higher the risk of NEC. Even partial formula supplementation significantly increases NEC incidence compared to an exclusive human milk diet.
This body of research was not hidden. It was published in major peer-reviewed medical journals, presented at neonatal conferences, and available to anyone in the infant nutrition industry. Formula manufacturers had access to every one of these studies. They knew their products were associated with a dramatically elevated risk of a disease that kills and permanently injures premature infants. And they continued to market them to NICUs anyway.
What Abbott and Mead Johnson Knew
The NEC baby formula litigation has revealed a deeply troubling pattern: both Abbott Laboratories (the maker of Similac) and Mead Johnson Nutrition (the maker of Enfamil) were aware of the scientific research linking their cow’s milk–based formulas to NEC in premature infants — and they chose not to adequately warn hospitals or parents.
Decades of Available Research
The link between formula and NEC has been studied since at least the 1990s, with landmark studies emerging throughout the 2000s and 2010s. As manufacturers of the most widely used infant formulas in NICUs, Abbott and Mead Johnson had every reason — and every resource — to be aware of this research. Internal documents and testimony in NEC lawsuits indicate that company scientists and medical affairs teams were indeed tracking this literature.
Aggressive Marketing to NICUs
Despite knowing the NEC risk, both companies marketed their cow’s milk–based formulas aggressively to NICUs. Sales representatives visited hospitals, provided free product samples, sponsored NICU events and educational programs, and cultivated relationships with neonatologists and hospital procurement departments. These marketing efforts directly influenced which products were used on premature infants — the exact population most vulnerable to formula-related NEC.
No Adequate Warning on Labels
At no point did Abbott or Mead Johnson place a clear warning on their formula labels indicating that use in premature infants was associated with a significantly increased risk of NEC. The labels did not inform NICU staff or parents that breast milk or donor milk was substantially safer for this population. The absence of this warning meant that healthcare providers and families made feeding decisions without access to critical safety information.
Knowledge Predating Current Lawsuits by Years
Evidence presented in NEC trials has shown that the manufacturers’ awareness of the NEC risk was not recent. Internal documents, research tracking, and communications within the companies demonstrate knowledge that predates the current wave of lawsuits by years — in some cases, by well over a decade. This is not a case of new science catching up with old products. The manufacturers knew, and they chose inaction.
Profits Over Infant Safety
The NICU formula market represents billions of dollars in annual revenue. Adding an NEC warning — or, more consequentially, recommending human milk over their own products — would have directly impacted sales. Juries have found that the manufacturers prioritized their financial interests over the safety of the most vulnerable patients imaginable: premature newborns fighting for survival in the NICU.
The Failure to Warn
At the heart of every NEC baby formula lawsuit is a straightforward legal principle: manufacturers have a duty to warn consumers about known risks associated with their products. This is a bedrock of product liability law in every state. When a company knows that its product can cause serious harm to a specific population, and it fails to communicate that risk, it is liable for the injuries that result.
In the context of NEC litigation, the failure to warn operates on multiple levels:
The Labels Were Inadequate
The product labels on Similac and Enfamil formulas marketed for use in NICUs did not contain a specific warning that cow’s milk–based formula was associated with an increased risk of NEC in premature infants. There was no statement advising that breast milk or donor milk was significantly safer for this population. The labels were, at best, silent on one of the most critical safety considerations for the product’s intended use.
Hospitals Relied on These Products
NICUs across the country stocked and used Similac and Enfamil products based on the reasonable assumption that the manufacturers had disclosed all known material risks. Hospital formulary committees, neonatologists, and neonatal nurses made feeding decisions for premature infants without knowing the full extent of the NEC risk. Had an adequate warning been present, hospitals could have implemented different feeding protocols — prioritizing human milk, establishing donor milk programs, or at minimum obtaining informed consent from parents before using formula.
Parents Had No Idea
Most parents of premature infants in the NICU trusted that the products being used on their babies were safe — or at least that they were being fully informed of any risks. They were not. Parents were not told that formula significantly increased the risk of NEC. They were not given the opportunity to make an informed choice between formula and the safer alternatives. Many parents only learned about the connection between formula and NEC after their child had already been diagnosed.
This Failure Is the Core of the Litigation
The NEC baby formula lawsuits are fundamentally about this failure. Abbott and Mead Johnson had the knowledge, they had the duty to warn, and they chose not to. Juries have found manufacturers liable for this failure, and the litigation continues to expand as more families come forward. If your premature infant developed NEC, this failure may be directly connected to what happened to your child.
Learn More About NEC Lawsuits
What Is NEC?
A parent’s guide to necrotizing enterocolitis.
Signs of NEC in Premature Babies
Warning signs every parent should know.
Do I Qualify for an NEC Lawsuit?
Find out if you have a case.
Similac NEC Lawsuit
Latest on the Abbott litigation.
Enfamil NEC Lawsuit
Latest on the Mead Johnson litigation.
All NEC Lawsuit Topics
Browse all resources for NEC families.