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Milk osmolality: does it matter?
  1. Freya Pearson1,
  2. Mark J Johnson2,
  3. Alison A Leaf1,2
  1. 1Department of Neonatal Medicine, Southampton University Hospitals Trust, Southampton, UK
  2. 2NIHR Nutrition, Diet and Lifestyle Biomedical Research Unit, Southampton General Hospital, Southampton, UK
  1. Correspondence to Dr Freya Pearson, Department of Neonatal Medicine, Southampton University Hospitals Trust, Mailpoint 105, Princess Anne Hospital, Coxford Road, Southampton SO16 5YA, UK; freya.pearson{at}


High osmolality of infant feed reflects a high concentration of solute particles and has been implicated as a cause of necrotising enterocolitis. Evidence for direct intestinal mucosal injury as a result of hyperosmolar feeds is scant, and no good evidence has been found to support such an association. High osmolality of enteral substrate may, however, slow down gastric emptying. Osmolality of current infant feeds ranges from around 300 mOsm/kg in human breast milk to just more than 400 mOsm/kg in fully fortified breast milk. Addition of mineral and vitamin supplements to small volumes of milk can increase osmolality significantly and should be avoided if possible.

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Establishing and maintaining adequate enteral feeding in the extremely preterm infant remains a major challenge within neonatology. There is considerable evidence to support a relationship between poor postnatal growth and long-term neuro-developmental sequelae within cohorts of these babies.1,,3 The spectre of necrotising enterocolitis (NEC) engenders a cautious approach to feeding, although a definitive cause remains elusive and the nutritional needs of this vulnerable group can be compromised by this strategy. One aspect of feeding which has been linked to the pathogenesis of NEC is osmolality of feeds. Concern over increasing feed osmolality is used as a reason to withhold fortification of human breast milk in an attempt to reduce the risk of NEC. Without fortification breast milk, even at very large volumes, will not supply the protein, energy and micronutrient requirements of growing preterm infants4 and extrauterine growth restriction is common.

This article reviews what is known about osmolality and considers the evidence base for the postulated link between feed osmolality and NEC.


Osmolality is a measure of osmolar concentration and is defined as the number of osmoles of solute per kilogram of solvent, expressed as mOsm/kg. Hyperosmolality describes osmolality greater than that of serum.

Osmolality is commonly measured by the reduction in freezing point (the more solute there is dissolved in a solution, the more the freezing point is reduced) using an osmometer. Measurements are given as total osmolality. A limiting factor in the laboratory assessment of osmolality is that some substances create an osmotic gradient in vivo whereas others do not.5 Thus, while laboratory measures of osmolality are useful, they may not truly represent what happens in the body.

Differences between osmolality and osmolarity

Osmolarity is a measure of the osmoles of solute per litre of solution. Since the volume of solution changes with the amount of solute added as well as with changes in temperature and pressure, osmolarity is difficult to determine. Since the amount of solvent will remain constant regardless of changes in temperature and pressure, osmolality is easier to evaluate and is more commonly used. As the solute takes up some volume in the total solution, generally, a 1 molar solution will have a higher solute concentration than a 1 molal solution. Consequently, the freezing point depression of a 1 molar solution will be lower when compared with the same 1 molal solution.

Osmolality depends on osmotically active particles dissolved in solution and, when considering milk feeds, these include electrolytes, oligo- and monosaccharides, amino and fatty acids. As such, it is a compound measurement with many different particles making up the total osmolality.


  • Solute – A substance that is dissolved in a liquid (solvent) to form a solution.

  • Osmole – A unit of osmotic pressure equivalent to the amount of solute that dissociates in solution to form one mole of particles.

  • Osmolality – The concentration of a solution in terms of osmoles of solute per kilogram of solvent.

  • Osmolarity – The concentration of a solution in terms of osmoles of solute per litre of solution.

Osmotic gradients

Osmotic gradients are found in most human organ systems to enable transfer of osmotically active particles across membranes. Osmotic gradients can be bi-directional. In health, these gradients are maintained by active solute transport. Osmotic gradients in the gut enable absorption of nutrients and water from the gut lumen into the bloodstream by transport molecules. Some compounds such as carrier molecules found in drugs and vitamin supplements and others such as alcohol diffuse across semi-permeable membranes without contributing to osmotic load in vivo.5

In the human gut, digestion of complex molecules within the small bowel may lead to production of increased numbers of soluble osmotically active particles and therefore increase the total osmolality of luminal contents. Particles which contribute to osmolality are absorbed in different parts of the gut; so some particles will exert this effect throughout the gut prior to distal absorption whereas others, more proximally absorbed, will not. Normal physiological processes reduce osmolality of luminal and gastric contents by secretion of quantities of hypo-osmolar fluid.6

Recommended osmolality of infant milk feeds

In 1976, the American Academy of Pediatrics (AAP) recommended that the osmolarity of infant formula should not exceed 400 mOsm/l. This was a consensus view based on the observation that the milk of most mammalian species has an osmolarity of around 300 mOsm/l and that ‘hyperosmolar formulas may be a factor in causing necrotising enterocolitis’.7 The AAP used osmolarity rather than osmolality as their unit of measurement, a factor which continues to cause confusion. Current recommendations suggest that the osmolality of enteral feeds should not exceed 450 mOsm/kg (which approximates to an osmolarity of 400 mOsm/l), a figure based on historical consensus view rather than experimental evidence. Currently, the standard measurement of feed concentration is osmolality. The values for human milk and infant feeds commonly used in the UK are shown in table 1. The addition of micronutrient supplements to expressed breast milk results in an increase in osmolality. Common examples include sodium and/or phosphate salts, iron and multivitamin preparations. Srinivasan and colleagues have examined the effect on osmolality of a number of therapeutic additives: a sodium chloride supplement of 2 mmol in 5 ml expressed breast milk increased the osmolality of the milk from 300 to more than 700 mOsm/kg and 1 ml sodium ironedetate resulted in an increase from 300 to 1 000 mOsm/kg.8

Table 1

Manufacturer's reported osmolality of commonly used (UK) infant formulae and human milk fortifier

The combination of micro- and macronutrients in human milk fortifiers increase total osmolality. De Curtis et al studied changes over time following addition of fortifier to human milk and suggested that some of the increase in osmolality may be due to continued hydrolysis of dextrin by milk amylase.9 In a further study, Janjindamai et al10 added fortifier at regular time intervals. They showed a rapid rise is osmolality by 10 min, a slight further increase at 1 h which then remained constant for 24 h. Both of these studies showed that, under standard conditions, increases all remained below the recommended level of 450 mOsm/kg.

Necrotising enterocolitis and feed osmolality

Risk factors for NEC

Necrotising enterocolitis is an acquired gastrointestinal disease with significant morbidity and mortality affecting approximately 5–7% of very low-birth-weight (VLBW) infants. Over the past 20 years, antenatal corticosteroid treatment has resulted in a significant reduction in risk but overall numbers of affected infants have not reduced, largely because of increased survival at shorter gestations. Prematurity remains the major risk factor.11

The pathogenesis of NEC remains unclear and a single cause has not been identified. Because of the complex nature of neonatal intensive care, there are many associated factors and confounding variables. Postulated elements in the causation supported by animal studies include inadequate oxygen delivery to the gut, potentially invasive pathogenic bacteria and substrate (ie, enteral feed) to enable bacterial proliferation.12

Newell identifies a total of 20 established and postulated risk factors12: prematurity, intrauterine growth restriction, placental abruption, premature prolonged rupture of membranes, perinatal asphyxia, low APGAR score, umbilical catheterisation, hypoxia and shock, hypothermia, patent ductus arteriosus (PDA), non-human milk feeding, hypertonic (or hyperosmolar) feeds, rapid introduction of enteral feeds, fluid overload, pathogenic bacteria, polycythaemia, thrombocytosis, anaemia, exchange transfusion and cyanotic congenital heart disease.

Gagliardi and colleagues,13 in their large Italian cohort, suggest that risk factors may be divided into nutritional and non-nutritional. Non-nutritional risk factors include mechanical ventilation, late onset sepsis and PDA – having a direct effect (ie, shunt of blood away from the gut) as well as an indirect effect of treatment with non-steroidal anti-inflammatory drugs. Ventilation may be a marker of conditions and diseases leading to hypoxia, acidosis and gut ischaemia. Surfactant treatment reduced the risk and may reflect a reduction in the number of factors described above.

Among nutritional risk factors, time of introduction, rate of increase and type of feed have all been studied with reference to NEC. The only confirmed association is the protective nature of human milk.14 ,15

Evidence base for link between feed osmolality and NEC

The suggestion that feed hyperosmolality may be a causative factor in necrotising enterocolitis came mainly from two studies during the 1970s.

Santulli et al

Santulli16 is widely quoted as suggesting a link between hyperosmolar feeds and direct mucosal injury. This statement arose from a published case series involving 64 surgical and autopsy specimens. The group did contain infants as small as 500 g, but there was considerable co-morbidity including trisomies and hypoplastic left heart syndrome together with a high incidence of both perinatal complications and maternal disease. Fourteen infants in the group weighed more than 2.5 kg.

Table 2

Characteristics of infants in Book's study

On reviewing Santulli's original article, this suggestion was on the basis that ‘a few term infants who developed NEC at a later stage received hyperosmolar feedings e.g. Nutramigen.’ At the time, this formula had an osmolarity of 750 mOsm/l. In his summary, however, Santulli states that indirect mucosal injury on the basis of selective mesenteric ischaemia in response to perinatal stress was thought to be more important than direct injury.

Despite this statement, evidence from this article was the basis for the proposed standards for osmolality of infant formula published by the AAP in 1976.7

Book et al

Also in 1975, Book17 published her prospective study of 16 infants weighing less than 1 200 g. These infants were randomised to receive a standard cow's milk formula or an elemental formula. The aim was to compare the nutritional efficacy as well as the incidence of necrotising enterocolitis. The background incidence of NEC in the unit was 27%. The clinical status of the two study groups was similar.

Seven of eight (87.5%) infants fed with the elemental formula and two of eight (25%) fed with the standard cow's milk formula developed necrotising enterocolitis.

There are clearly limitations to this study, particularly small sample size. Book suggested that ‘the hypertonicity of the elemental diet may have contributed to the increased incidence of necrotising enterocolitis in infants fed this formula.’ The elemental formula was Pregestimil (Mead Johnson), which, at the time, had an osmolarity of 650 mOsm/l, and the standard preterm formula (Mead Johnson) had an osmolarity of 359 mOsm/l.

Current elemental and amino acid formulas have osmolalities similar to that of preterm formula. However, their sodium content tends to be low and the need for supplementation of these formulas may increase osmolality significantly.

In an observational study, Willis et al noted an increase in cases of NEC coincident with administration of hyperosmolar calcium supplements (>1 700 mOsm/kg), which subsequently reduced once supplements were diluted down to an osmolality of 405 mOsm/kg with water.18

All the studies mentioned here are widely quoted, yet none of them are able to provide robust evidence linking feed osmolality to NEC. A criticism of both Book and Santulli's suggestions about feed osmolality is that there is little discussion of the many likely confounding variables in their populations, specifically the use of therapeutic additives and oral medicines. Both studies had methodological issues, and in each case the feed in question had an osmolality in excess of 500 mOsm/kg. Willis' study looked at an isolated supplement, rather than an additive to feeds. It is of note that none of the commonly used breast milk fortifiers increase osmolality above the recommended limit of 450 mOsm/kg and meta-analysis and review of trials of nutrient fortification have not shown evidence of an increase in NEC.11 ,19 Ryder and colleagues studied risk factors for NEC in 111 cases and matched controls19: milk feeding and PDA were significant; however, they found no relationship between osmolality of formula and risk of NEC.

Is hyperosmolality plausible as a mechanism for mucosal injury?

What are the effects of hyperosmolar solutions on gut architecture and function?

Increased osmolality delays gastric emptying which is mediated via osmoreceptors in the duodenum. This allows dilution of the gut contents by secretion into the lumen. The direct effects of hyperosmolar solutions on the gut mucosa and intestinal contents have been examined in animal models. Goldblum and colleagues studied the effect of hyper- and isotonic feeds on the osmolality of the intestinal contents of neonatal dogs.20 They found that the actual osmolality of the feed itself was not a major determinant of the osmolality of the contents of the stomach, proximal and distal intestine, which were similar in all feed groups. There was, however, a delay in gastric emptying in the hyperosmolar feed group. They concluded that the osmolality of formula could not therefore be a major determinant of potential intestinal mucosal injury. A study looking at the effect of hyperosmolar feeds on intestinal blood flow in newborn lambs concluded that while they resulted in an increase in blood flow, there was no effect on oxygen consumption and no evidence of any disturbances (locally or systemic) that might result in serious disease.21

Schmid and Ehrlein22 examined the effect of increasingly hypertonic solutions (saline, glucose and elemental diet) up to 1 520 mOsm/l on canine jejunal motility patterns. Increasing osmolality reduced jejunal motility both by local and inhibitory feedback, but the osmolality was only of minor importance compared with the role of the nutrient itself. Comparisons between different nutrients suggested a linkage between inhibitory control of motility and the absorptive capacity of the gut for the single nutrient. Hypertonic glucose evoked a significantly smaller level of motor activity compared with both saline (at given osmolarities) and an elemental diet (at given energy loads). Szabo and Fewell looked at the effect of hyperosmolar formula on the small intestinal motility of neonatal pigs and found that its use did not result in significant motor dysfunction.23

Other animal studies have looked at the effect of oral contrast media on the gut. Norris6 examined the effect of application of hypertonic dye (sodium diatrizoate) with an osmolality of 1 560 mOsm/kg on rabbit ileal mucosa. The hypertonic dye caused a rapid decrease in the height and width of villi, decrease in height of epithelial cells and closure of the intercellular space. Concomitantly, the tissue fluid content of the bowel wall and the volume of venous outflow from the ileal segment decreased in response to the osmotic gradient between ileal lumen and blood and hypotonic secretion into the lumen occurred.

Several other authors have cautioned against using contrast media in infants with suspected NEC,24 ,25 but in established disease, conclusions cannot be drawn about the possible roles of hyperosmolar solutions or bacterial translocation.

There is no clear evidence to support the hypothesis that hypertonic solutions cause direct disruption or damage of the bowel mucosa. However, studies were carried out in mature animals whose gastrointestinal tracts were able, presumably, to make normal adaptive responses.

Gastrointestinal development

The gut is a complex organ system which shows considerable growth and maturation during the final months of gestation. Growth is stimulated by trophic effects of numerous nutritional and non-nutritional factors mediated via changes in cellular metabolism and gene expression. Local nutrient supply is the most potent stimulus of mucosal growth via direct effects and by the triggering of neural impulses and release of gut hormones.25 The small intestine has constant contact with changing intraluminal contents: in swallowed amniotic fluid in utero and in milk ex utero. Both amniotic fluid and human milk contain growth factors, hormones, bacteria and their products. Moreover, amniotic fluid contains amino acids, carbohydrates (providing 10–15% of fetal nutritional requirements), proteins and enzymes (including lactase, sucrase-isomaltase, alkaline phosphatase and lysosomal enzymes).

After birth, enteral nutrition is critical in sustaining normal gut mucosal growth. Fasting, starvation and exclusive parenteral nutrition all deprive the gut of luminal nutrients resulting in a loss of gut tissue mass with blunted villi, net loss of protein, decreased cell proliferation, decreased brush border disaccharidase activity, increased intestinal permeability and increased catabolism. The effects are proportionally greater at earlier stages of development.25 Growth restriction during pregnancy compounds the effects of prematurity particularly affecting growth of the stomach and small intestine.26

Nutrient composition influences gut growth and function. Whole proteins within the diet appear to be important in maintaining gut health. There is evidence that, although elemental diets maintain the growth and morphological structure of the proximal gut, the effect is progressively diminished, such that the distal small intestine and colon become atrophied, as with total parenteral nutrition. Moreover, feeding elemental diets is associated with disruption of the microflora and bacterial translocation in the distal gut.25 This may provide an alternative explanation for the mechanism for the findings in the two original studies.


Enteral nutrition is vital for the newborn infant – both to enable growth and maturation of the intestine and to deliver optimal nutrients for infant growth. Breast milk is the preferred feed for both term and preterm infants.

Osmolality of feed varies, being higher in formula milk and milk with additives than in breast milk; however, we have found no evidence for a causal relationship between osmolality of feeds and the development of NEC. All milk feeds in common use have osmolality below 450 mOsm/kg. The normal physiological response to an increase in osmolality is to delay gastric emptying and allow dilution of the contents with gastric secretions. Physiological responses have been described in animal models to very hyperosmolar substances (other than milk); none of these studies showed direct mucosal injury as a response but the animals studied were all mature.

There remains a question about the addition to feeds of oral medications and electrolyte supplements which may have an extremely high osmolality. It would seem prudent to minimise the effect of this by diluting additives in the largest possible volume of feed and by using multi-component fortifiers in preference to multiple individual supplements.


We would like to acknowledge the contribution of Anita Emm, Neonatal Dietician, in providing information on osmolality of milk feeds.


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  • Funding No specific funding for this work, but Alison Leaf and Mark Johnson's salaries are supported by NIHR funding to the Biomedical Research Unit for Nutrition, Diet and Lifestyle.

  • Competing interests None.

  • Provenance and peer review Commissioned; externally peer reviewed.

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