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Determining the osmolality of over-concentrated and supplemented infant formulas



J. Steele, Paediatric Dietetics, A Floor Brotherton Wing, Leeds General Infirmary, Leeds LS 1 3EX, UK.

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Hyperosmolar infant feeds can cause osmotic diarrhoea and may be a risk factor for necrotising enterocolitis; the osmolality of infant formula is therefore usually <400 mOsm kg−1. However, in fluid-restricted infants and those needing nutritional support, formulas may be over-concentrated or supplemented. The present study aimed to determine the effect of these practices on osmolality.


A clinical laboratory osmometer was used to measure the osmolality of infant formulas. The effect of over-concentration and supplementation on osmolality was then determined using three and seven different infant formulas, respectively. Osmolalities were measured in triplicate.


The effect of over-concentration was shown to be linear using Pepti Junior (Cow & Gate, Trowbridge, UK) at concentrations of 12.8% (standard), 17% and 19%. This linear relationship was also demonstrated with Enfamil A.R. (Mead Johnson Nutritionals, Uxbridge, UK) (15%) and Neocate (SHS International Ltd, Liverpool, UK) (21%). The effect of individual additives on osmolality was found to be similar for the seven infant formulas. All preparations of SMA High Energy (SMA Nutrition, Maidenhead, UK) and five of the 12 preparations of Nutriprem 1 (Cow & Gate) exceeded an osmolality of 400 mOsm kg−1.


The effect of over-concentrating infant formulas was shown to be linear, meaning that the osmolality at different concentrations can be predicted accurately. The over-concentrated infant formulas that were measured in the present study did not exceed 400 mOsm kg−1, with the exception of 21% Neocate, which would not be used in practice. When supplemented, some infant formulas exceeded an osmolality of 400 mOsm kg−1; this may be relevant in cases of feed intolerance or in those at risk of necrotising enterocolitis.