Presented at the Neonatal Society 2005 Spring Meeting.

McCrosson F, Smith J, Wilson V, Young D, Laing IA

Simpson Centre for Reproductive Health, Royal Infirmary of Edinburgh, Little France Crescent, Edinburgh, EH16 4SA, UK

Aim: The aim of the study was to determine values of sodium, chloride, potassium, glucose, total protein and water content of expressed breast milk from mothers of preterm and term infants over the first post-natal month, and to identify factors affecting the values.

Methods: Over a six-month period, seventy-five mothers were recruited and 439 1ml samples were obtained. Mothers were regularly expressing breast milk. The samples were analysed first for electrolyte content and then frozen and stored for protein and water content analysis at a later date. No mother was on diuretics. Ethical approval was obtained from the trusts ethics committee.

Results: The average value of each parameter from days 1-3 (colostrum), 4-15 (transitional milk) and 16-28 (mature milk) used in a repeated measures ANOVA including the variables gestation, age, parity and mode of delivery. There was a significant decrease in the average sodium for transitional and mature milk compared with colostrum (p<0.001). The same pattern was seen with chloride (p<0.001). There was a significant increase in potassium (p=0.005) and evidence that increasing gestation (p=0.083) and maternal age (p=0.063) were associated with increasing potassium. Glucose was significantly higher in transitional and mature milk than in colostrum (p=0.0004 and p=0.0063 respectively). There was an increase in glucose with increasing gestation (p=0.049). Total protein was significantly lower in mature milk compared with colostrum (p=0.049).Water content was significantly higher in transitional and mature milk than colostrum (p=0.0033 and p=0.0144 respectively). There was evidence that water content increased with gestation (p=0.090).

Conclusions: The downward trend in both sodium and chloride has been described before. To our knowledge, water content has not previously been studied. This shows an upward trend during the first postnatal month and may account for the dilution of sodium and chloride, yet the potassium and glucose is low despite low water content. These components have different physiological transport pathways for their secretion but have a common activation switch and seem to co-ordinate their activities.