Preterm Nutrition: A Small Window With a Lifelong Impact
Being born early cuts short a critical growth phase. Here's a look at what the science says about feeding preterm infants well, and why those first weeks matter for years to come.
Specific nutritional needs
The third trimester is when a baby grows fastest and builds the nutrient stores it will rely on after birth. When a baby arrives early, that process is cut short; stores are low just as the body's needs peak.
That's why nutrition matters so much from day one for preterm infants. Mother's own milk remains the best option, as recommended by the World Health Organization (WHO), ideally continued until at least 6 months. But breast milk alone can't always supply enough energy and protein for a baby born early, so the WHO, the European Society for Paediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) and the American Academy of Pediatrics (AAP) recognize the value of fortifying it. Where breast milk isn't available, a preterm infant formula can help meet these babies' specific needs.
Researchers are also studying how preterm babies grow compared to healthy peers, and the microbes developing in their gut; both of which shape health well beyond the newborn period.
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The post-natal period is a unique window of opportunity during which appropriate nutritional care has the potential to set preterm infants on a trajectory for lifelong health, development and well-being.
— Summary of the scientific evidence on preterm nutrition
Beyond energy and protein, researchers are also looking closely at how preterm babies grow compared to healthy peers, and at the community of microbes developing in their gut, both of which turn out to shape health well beyond the newborn period.
Protein: enough to grow, not so much it overloads the system
Protein builds tissue, but it's a balancing act: too little slows growth, too much strains an immature metabolism. ESPGHAN guidelines put the minimum at 1.5 g per kg per day for infants fed intravenously, rising toward a target of 3.5 to 4.0 g per kg per day.
A US university hospital study (n=440) found infants given amino-acid supplementation within 24 hours reached full feeding sooner and gained more weight by discharge than those started later. Longer term, a study of very-low-birth-weight infants (n=103) linked higher energy and protein intake during hospitalization to more lean mass at discharge, and to better developmental outcomes at 12 and 24 months.
Energy: fueling a brain that needs half the calories
Calories do more than keep a preterm baby warm and growing; in the earliest weeks of life, brain development alone can claim up to half of everything the baby eats. According to the AAP and ESPGHAN, energy needs for preterm infants generally fall between 100 and 135 kcal per kg per day when fed by mouth or tube, and between 85 and 110 kcal per kg per day when fed intravenously.
To put that in perspective, a preterm infant's energy needs, relative to body weight, run 20 to 30% higher than those of an elite endurance cyclist during a multi-stage race. In practice, clinicians tailor these targets to each baby, and a 2021 ESPGHAN Nutrition Committee position paper notes that critically ill preterm infants in an acute phase should receive at least the lower end of these ranges to cover basic metabolic needs. Across the board, the guiding principle stays the same: start delivering adequate energy as early as possible.
The gut microbiome: a quiet influence on growth and immunity
The community of bacteria living in a baby's gut plays a bigger role than it might seem, it helps the body process nutrients and shapes the immune system, roughly 80% of which is rooted in the gut. This microbiome starts forming before birth and keeps developing afterward, shaped by factors like the type of delivery, antibiotic treatment, how early the baby was born, and diet.
A 2021 systematic review of 60 studies found a few consistent patterns: babies delivered by cesarean section tend to have less of a beneficial bacteria group called Bacteroides than those born vaginally; postnatal antibiotics, prescribed in up to 89% of preterm cases, often reduce helpful bacteria such as Bifidobacterium; and longer gestation is generally linked to a more diverse microbiome.
Breast milk stands out as the single most influential factor for a healthy microbiome in the first year of life, largely thanks to its natural oligosaccharides, which feed beneficial bacteria like Bifidobacterium directly. Researchers are exploring whether adding similar nutrients, such as human milk oligosaccharides (HMOs), lactoferrin, or probiotics, to preterm formulas could offer similar support, though neither ESPGHAN nor the AAP has yet established formal recommendations on this, citing insufficient data. What is clearer is the stakes: an imbalanced microbiome in preterm infants is linked to a higher short-term risk of necrotizing enterocolitis, and to microbial imbalance further down the line.
