Season of Birth and Subsequent Body Size : The Potential Role of Prenatal Vitamin

Objectives: The relationship between season of birth and various physical and psychological outcomes was reported in many studies, although the underlying mechanism still remains unrecognized. The aim of this study was to explore the season-of-birth effect on body size in the sample of 1,148 eight-year-old Polish urban children and propose the mechanism responsible for this effect. Methods: The children were examined three times at their birthdays and at two cross-sectional surveys. Effects of the season of birth were checked by fitting the cosine function to empirical values and by comparison between two groups born in different periods of the year. Results: Data gathered at three examinations led to the same results: season-of-birth effect occurred only in boys and only in those relatively shortly breastfed and/or descended from the families of low-socioeconomic status. Specifically, the individuals born in October– April were taller (by 2–3 cm), heavier (by 2–3 kg), and fatter than those born in May–September. Conclusions: The following explanatory mechanism has been formulated: insolation in Poland is minimal in November–February (winter period), and so ultraviolet absorption and vitamin D production is then the lowest. Vitamin D regulates embryo’s cellular differentiation, and its deficiency triggers permanent developmental changes. Therefore, individuals conceived in autumn (i) are at the greatest risk of early vitamin D deficiency, (ii) are born in summer, and (iii) are relatively small in their further lives. The contribution of low-socioeconomic status, short breastfeeding, and being a male to the occurrence of the season-of-birth effect is also discussed. Am. J. Hum. Biol. 23:190–200, 2011. ' 2010 Wiley-Liss, Inc. Many seasonal changes of environment cause physiological and developmental seasonality in organisms (Benefice et al., 1996; Maleta et al., 2003). Short-term environmental impacts, especially during prenatal period, may exert long-lasting phenotypic changes (Cameron and Demerath, 2002). For example, the risk of many diseases in adulthood is increased by fetal undernutrition (Barker, 1995) or hormonal imbalance (Manning and Bundred, 2000). This phenomenon is called fetal programming and its mechanisms range from DNA and histone methylation, through membrane receptors organization, to an organ’s growth retardation (Waterland and Michels, 2007). If a factor responsible for programming early in life changes seasonally, then individuals born in various seasons would be programmed differently, and therefore the season of birth would be related to the later-life phenotype. Indeed, many characteristics have been revealed to correlate with the birth season; in humans, these are, for example, reproductive potential (Cagnacci et al., 2005; Lummaa and Tremblay, 2003; Smits et al., 1999), longevity (Doblhammer and Vaupel, 2001; Flouris et al., 2009; Lerchl, 2004), and psychological status (Chotai et al., 2003; Joinson and Nettle, 2005; Torrey et al., 1997). Human body size (height and weight) was also repeatedly shown to depend on the season of birth (Banegas et al., 2001; Henneberg and Louw, 1990, 1993; Kihlbom and Johansson, 2004; Kościński et al., 2004; McGrath et al., 2006; Puch et al., 2008; Puch and Kozłowska-Rajewicz, 2004; Shephard et al., 1979; Waldie et al., 2000; Weber et al., 1998). Various birth seasons were reported to be beneficial for further growth, however, most studies pointed to the winter period [see Table 3 in Puch et al. (2008)]. The seasonal factor exerting long-term developmental changes is unrecognized, although malnutrition (Doblhammer and Vaupel, 2001), ambient temperature (Murray et al., 2000), ultraviolet B (UVB)-dependent production of vitamin D (McGrath et al. 2006), photoperioddependent melatonin production that inhibits growth hormone secretion (Waldie et al., 2000; Weber et al., 1998), electromagnetic field or gravity (Henneberg and Louw, 1993), and family planning (Kościński et al., 2004) were suggested. Our study examines the relation between season of birth and body size and proportions in 8-year-old children from a Polish well-developed city. In comparison with previous research, this study has several advantages. First, children were measured close to their birthdays, so that they were in the same age irrespective of their month of birth (in cross-sectional surveys, children born in some months are larger than others just because of being older). Second, we collected data on many biosocial characteristics to check their role in the development of season-ofbirth effect. Finally, a comparison was made between the results of this study and those from our previous investigation of Polish rural children (Kościński et al., 2004), which proved helpful to elucidate the mechanism of the effect. Contract grant sponsor: Polish Ministry of Science and Higher Education; Contract grant number: 2P04C01429. *Correspondence to: Marta Krenz-Niedbała, Department of Human Evolutionary Biology, Adam Mickiewicz University, Umultowska 89, Poznań, 61-614, Poland. E-mail: martak@amu.edu. pl Received 5 February 2010; Revision received 10 August 2010; Accepted 16 August 2010 DOI 10.1002/ajhb.21101 Published online 30 November 2010 in Wiley Online Library (wileyonlinelibrary.com). AMERICAN JOURNAL OF HUMAN BIOLOGY 23:190–200 (2011) VC 2010 Wiley-Liss, Inc. MATERIALS AND METHODS