Nutritional status of lactating New Zealand mothers and their breast milk concentrations of selenium and zinc

Abstract

Background: Lactating women may be at greater risk of suboptimal selenium and zinc status compared with the general population due to increased requirements from transfer of these nutrients into breast milk. In addition, selenium intakes in New Zealand are low in comparison with many other countries. To our knowledge, there are no recent studies focusing on the selenium and zinc status of lactating women.

Objective: To assess the prevalence of selenium and zinc inadequacy in lactating New Zealand women based on dietary intakes and status, and the relationship between these intakes and biochemical measures of blood and breast milk.

Design: Fifty-three exclusively breastfeeding mother-infant pairs at 8 weeks postpartum were recruited from Dunedin, New Zealand, from May 2012 to December 2013. Socio- demographic data, obstetric history, anthropometry (maternal height and weight, infant length and weight), dietary intake, breast milk sample and fasting blood collection were obtained from mother and infant. The prevalence of inadequate selenium and zinc intakes for the mothers were calculated using the Estimated Average Requirement (EAR) cut-point method. Selenium and zinc were determined in maternal serum and breast milk samples by inductively coupled plasma mass spectrometry. Glutathione peroxidase (GPx) activity was determined in whole blood samples for the mothers and infants using a commercially available assay. Serum selenium cut-offs of 70 μg/L and 110 μg/L were used to define suboptimal selenium status, and cut-offs of 7.65 μmol/L and 10.7 μmol/L were used for serum zinc.

Results: Mothers ranged in age from 18 to 42 years, with an average age of 32 years. The majority were New Zealand European (74%), and most women (43%) had a body mass index in the normal range (18 to 25 kg/m2). The mean maternal dietary selenium intake was 47.1 μg/day, with 58% (n=30) consuming below the EAR of 65 μg/day. The average serum selenium concentration was 74.6 μg/L, and 39% of women had serum selenium concentrations below 70 μg/L, while 98% were below 110 μg/L. The mean maternal whole blood GPx activity was 42.1 U/g Hb. After multivariate regression analysis, only dietary selenium was significantly associated with maternal serum selenium (p=0.027). Breast milk selenium concentrations ranged from 5.8 to 23.2 μg/L (mean 11.2 μg/L). The mean maternal dietary zinc intake was 11.2 mg/day, with 69% (n=36) consuming below the EAR of 10 mg/day. The average serum zinc concentration was 9.8 μmol/L, and 2% of women had serum zinc concentrations below 7.65 μmol/L, while 35% were below 10.7 μmol/L. None of the variables examined were significantly associated with serum zinc concentrations. Breast milk zinc concentrations ranged from 0.3 to 4.1 μmol/L (mean 1.8 μmol/L).

Conclusions: A large proportion of lactating women from Dunedin, New Zealand are not consuming enough selenium and zinc to meet their additional requirements, putting themselves and their exclusively breastfed infant at risk of inadequate intakes and therefore suboptimal status. More research is needed to assess a larger sample of lactating New Zealand women, and the response to dietary advice or supplementation on their intakes, status, breast milk concentrations, infant intakes, and maternal and infant health outcomes.