The effects of pre- and post-natal sunlight exposure on human growth: evidence from the Southern Hemisphere
Introduction
The intriguing finding that body size varies according to season of birth in both humans [1], [2] and animals [3] has led to speculation concerning the role of perinatal exposure to sunlight for development. The seasonal variation has been hypothesised to result from environmental exposure (e.g., sunlight [1], total energy reaching earth or fluctuations in temperature [3]) influencing hormonal expression between mother and developing foetus.
Some studies, however, have failed to find any seasonal effect on body size [4], [5]. This issue is complicated further by incongruities both between and within studies. For instance, Weber et al. [1] showed that height varied up to 6-mm depending on whether 18-year-old Austrian males were born in spring (i.e., maximum height in April) or autumn (minimum height in October). Although their results indicated that peak sunlight duration occurred in July (thereby corresponding to the first month of gestation and the fourth post-natal month in the tallest males), the authors speculated that the effect was due to “influences extending from the time of late pregnancy to the first post-natal year”. A further report [2] from the Northern Hemisphere found that the stature of a sample of infants was greatest both in the spring and in the autumn.
The initial aim of the present study was to determine if stature fluctuates with season of birth in a Southern Hemisphere population to the degree that has been reported in the above mentioned Northern Hemisphere studies. Further, by cross-correlating fluctuations in daily sunlight hours during pregnancy with the height and weight of infants at birth, the period of maximal influence of sunlight during gestation can be gauged.
Thus, in study 1 we sought to determine the solely pre-natal effects of sunlight on the birth size of a cohort of all infants born between 1967 and 1978 in Dunedin, New Zealand (a city with a population of approximately 120 000). To directly test this relationship, average hours of bright sunshine recorded for each month during the same period were obtained from the New Zealand meteorological service [6]. Variation in average monthly sunshine hours between consecutive years in Dunedin (e.g., April 1971=169 h versus April 1972=82 h) illustrates the importance of gathering data in this way. Typically the maximum duration of sunshine in Dunedin (latitude=45 54 South, longitude=170 31 East, 2 m above MSL) is in January (summer) and the minimum is during June (winter).
In study 2, data from a subset of this population (who were subsequently enrolled in the Dunedin Multidisciplinary Health and Development Study [DMHDS] [7]) were employed. The aim of study 2 was to investigate the influence of season of birth (and other meteorological and behaviour factors) on height and weight during development (up to age 26 years). Of particular importance was the longitudinal design of this study in that it allowed us to elucidate which period of sunshine duration (pre- versus post-natal) exerts the most influence on body size.
Section snippets
Study 1
Infants born at the major maternity hospital in Dunedin between 1st August 1967 and 31st July 1978 were included in this study (n=20 021). Data for births in 1974 were not available. Infants were examined with regard to anthropomorphic features (e.g., length, weight, gestational age, head circumference), and their primary caregivers were interviewed within 24–48 h of delivery.
Cross-correlation functions (CCF’s) [8] were calculated from the Fourier transforms of the mean monthly height, weight
Study 1
Spectral analyses [8] revealed that monthly means for neonate height and weight varied sinusoidally at the same frequency as that of the monthly variation in mean bright sunlight hours.
Peak height and weight lagged the month of peak sunlight hours by between 6 and 9 months (Fig. 1). This indicates that any facilitory effect of sunlight hours on birth height and weight was most evident when maximal sunlight was coincident with the early months of pregnancy (i.e. the first trimester).
Study 2
To determine
Discussion
Our findings revealed that the birth length and weight of this Dunedin cohort was strongly influenced by the amount of bright sunshine coincident with the early months of pregnancy. In the sub-sample of members enrolled in the DMHDS height varied up to 8-mm depending on month of birth, with maximum length in October (conception during mid-summer) and minimum length in January (conception during shorter autumn days). That we found significant seasonal variation in a relatively small sample size
Acknowledgements
The Dunedin Multidisciplinary Health and Development Study (DMHDS) is supported by the Health Research Council of New Zealand. The authors thank the late Dr. Patricia Buckfield for collecting the neonate data and the DMHDS Study members for their participation and continued support.
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