Abstract
It has been observed that growth velocity of toddlers and school children shows seasonal variation, while such seasonality is unknown in infants. The aim of this study was to examine whether growth velocity (length and weight) of infants differs by seasons. We assessed longitudinal measurement data obtained for 9,409 Japanese infants whose parents used the mobile phone application, “Papatto Ikuji”, during the period from January 2014 to October 2017. On average, each infant had 4.8 entries for length and 5.4 entries for weight. The mean daily change in sex- and age-adjusted z-scores between two time points was estimated as the growth velocity during that period: ΔLAZ/day and ΔWAZ/day for length and weight, respectively. We analyzed 20,007 ΔLAZ/day (mean, −0.0022) and 33,236 ΔWAZ/day (mean, 0.0005) measurements, and found that ΔLAZ/day showed seasonal differences with increases during summer. We conducted a multilevel linear regression analysis, in which effects of age, sex, nutrition and season of birth were adjusted, showing significant difference in ΔLAZ/day between winter and summer with a mean ΔLAZ/day difference of 0.0026 (95%CI 0.0015 to 0.0036; P < 0.001). This seasonal difference corresponded to 13% of the average linear growth velocity in 6-month-old infants. A modest effect of nutrition on linear growth was observed with a mean ΔLAZ/day difference of 0.0015 (95%CI 0.0006 to 0.0025; P < 0.001) between predominantly formula-fed infants and breastfed infants. In conclusion, we observed that linear growth, but not weight gain, of Japanese infants showed significant seasonality effects represented by increases in summer and decreases in winter.
Highlights
Growth and growth disorders in early life have been central themes in pediatrics, and there are several theories pertaining to concepts of human growth
There have been studies conducted in Ethiopia[5] and Timor-Leste[6] reporting the seasonality of infant growth, in which food availability was thought to be the major determinant of the seasonality pattern
Length-for-age and weight-for-age were converted into z-scores with age- and sex-specific references (LAZ and WAZ), and magnitude of change per day (ΔLAZ/day and ΔWAZ/day) was used as the primary outcomes
Summary
Growth and growth disorders in early life have been central themes in pediatrics, and there are several theories pertaining to concepts of human growth. The greatest growth velocity occurs during the first 6-months in both normal children and children born small-for-gestational age[3,4]. This rapid growth is thought to be influenced by nutritional features characteristic of this development period. Another known factor affecting infant growth is sex. There have been studies conducted in Ethiopia[5] and Timor-Leste[6] reporting the seasonality of infant growth, in which food availability was thought to be the major determinant of the seasonality pattern. Potential effects of the seasons on infant growth in developed countries have not been reported. The scarcity of previous studies may be due, in part, to the inability to collect measurements on child growth in a sufficiently detailed prospective fashion
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