John McGrath and colleagues are correct that seasonality could be a factor contributing to the higher birth weight we observed among infants born with a vitamin D deficiency.1 Proving a relationship between maternal exposure to sunshine, infant vitamin D status and infant size at birth is difficult because of the high cost of analyzing circulating 25-hydroxyvitamin D levels in a large cohort, quantifying skin exposure to ultraviolet B light and endogenous synthesis of vitamin D, and assessing the confounder, dietary vitamin D intake.
Other seasonally varying factors such as temperature are linked to size at birth.2 It is important to point out that the 299-g difference in birth weight between the infants with deficient and adequate vitamin D status in our study is 10 times greater than the weight difference that would be expected because of seasonal effects alone in industrialized countries.2 In the Winnipeg cohort, vitamin D status was highest in infants born in the summer (although season did not contribute to the regression analysis for bone mineral content in our study), but body weight did not follow the same pattern (Fig. 1).
Fig. 1: Infant vitamin D status (above) and weight at birth (below) categorized by season. Seasons are defined as winter: January through March (n = 14); spring: April through June (n = 14); summer: July through September (n = 12); fall: October through December (n = 10). Differences among groups were tested using ANOVA with Tukey's multiple comparison test. Bars with different letters indicate differences between groups (p < 0.05). (There were significant differences between the summer months and those in winter and spring. There were no significant differences between the fall months and the rest of the year.)
It is possible that the infants born with vitamin D deficiency might have had a higher birth weight because of maternal exposure to sunshine in the first trimester.2 The spring-born infants (n = 14) tended to weigh the most (difference of 244 g, p = 0.08, t test). These infants would have been conceived in the peak period of sunshine and warm temperatures in Winnipeg. Whether vitamin D status at the time of conception is behind these differences is unclear. In addition, melatonin should be included in future research on the relationships among seasonality, vitamin D, growth and bone mass.3
Hope Weiler Department of Human Nutritional Sciences University of Manitoba Winnipeg, Man.
References
- 1.Weiler H, Fitzpatrick-Wong S, Veitch R, Kovacs H, Schellenberg J, McCloy U, et al. Vitamin D deficiency and whole-body and femur bone mass relative to weight in healthy newborns. CMAJ 2005;172(6):757-61. [DOI] [PMC free article] [PubMed]
- 2.Tustin K, Gross J, Hayne H. Maternal exposure to first-trimester sunshine is associated with increased birth weight in human infants. Dev Psychobiol 2004;45:221-30. [DOI] [PubMed]
- 3.Cardinali DP, Ladizesky MG, Boggio V, Cutrera RA, Mautalen C. Melatonin effects on bone: experimental facts and clinical perspectives. J Pineal Res 2003;34:81-7 [DOI] [PubMed]