Letter to the Editor

Abstract

9 March 2006 Dear Editor, CAN SUNLIGHT EXPOSURE AFFECT EARLY BREASTFEEDING CESSATION? A broad range of socio-cultural and physiologic factors, including regional differences,1 can influence early lactation success. To date, there are few detailed studies of seasonal variations in breastfeeding. It is something unexpected because it has been documented that short day exposure in mammals causes reproductive inhibition and concomitant reduction in plasma levels of prolactin;2 and blood concentrations of prolactin are highest in March–May and lowest in September–November in healthy women.3 Furthermore, lack of sunlight exposure is a risk factor for disease in children: purpura, multiple sclerosis, arthritis and diabetes, among others. Because of this lack of information, we undertook a retrospective study that explores the link between early breastfeeding cessation and the number of accumulated sunlight hours in Majorca (Latitude = 39°N; Longitude = 3°E), that is 1060 from October to March, and 1701 from April to September.4 Information extracted from medical records was used to examine the relationship between failure to breastfeed beyond 7 days of life and birth in September or in March. All singleton children attending the same paediatric clinic in Majorca, Spain, born in September 1999–2004 or in March 2000–2005 who met the following selection criteria were recruited: affiliation to this clinic within first month of life; follow-up more than 2 months; mother willing to attempt to breastfeed; single and full term infant 38–42 weeks’ gestation; birthweight 2500–4500 g; and no significant perinatal morbidity. In total, 181 mother–infant pairs were recruited. Of these, 63 were ineligible for the study. The reasons for ineligibility were: (i) did not plan to breastfeed (n = 43); (ii) preterm delivery (n = 4); (iii) perinatal morbidity (n = 9); and (iv) multiple birth (n = 4), birthweight <2500 g or >4500 g (n = 3). Of 118 eligible subjects, there were 59 born in March (BM) and 59 born in September (BS). Failure to breastfeed more than 1 week had been recorded in 10 (16.9%) babies BM and in 1 (1.6%) baby BS. The analysis showed that BM was associated with an increase in proportion of failure in onset of breastfeeding (odds ratio 11.83; 95% confidence intervals 1.46–259.49; P = 0.012) compared with BS. Patient characteristics are showed in Table 1. This paper has demonstrated that breastfeeding failure appears to be commoner in the period following the months of low sun exposure. The information available on this topic shows uncertainties. The Bedouin Infant Feeding Study reported that babies born during the wet cool months were exclusively breastfed for longer than those born in spring–summer; and more traditional women, living in tents rather than houses, were found to exclusively breastfeed for longer than the rest.5 These results disagree with our findings. One explanation for the discrepancy could be that Bedouins have a strong tradition of sun protection, meanwhile Spaniards are reckless with respect to sun. The North of Spain Study of Breastfeeding (a sample of 1175 women) has reported that cessation of breastfeeding at 1 month is less prevalent in southern regions (Aragon, Rioja, Soria, Navarra, Latitude 41°N) with more than 2000 sunlight hours per year than in northern regions (Santander, Basque Country, Latitude 43°N) with less than 2000 sunlight hours per year (64,8% vs. 71,0%; P = 0.05; 95% confidence intervals −0.12–0.00).6 Time of day, season and latitude all influence the zenith angle of the sun. Above 37° latitude during the months of November through February, there are marked decreases (80–100%, depending on latitude) in the number of UVB photons reaching the earth’s surface. Therefore, very little if any vitamin D3 is produced in the skin during the winter. Vitamin D can be a possible aetiology for our findings: it has a wide range of therapeutic and health-related benefits because it is present in most cells and tissues in the body and is one of the most potent regulators of cellular growth. Another possible explanation is prolactin fluctuation with season of birth. Our observation is worthy of further exploration in a temperate climate including biochemical measures of prolactin or vitamin D in the study design.