Evaluating the seasonality of growth in infants using a mobile phone application

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: ALAZ/day and AWAZ/day for length and weight, respectively. We analyzed 20,007 ALAZ/day (mean, −0.0022) and 33,236 AWAZ/day (mean, 0.0005) measurements, and found that ALAZ/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 ALAZ/day between winter and summer with a mean ALAZ/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 ALAZ/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.

Satoshi Narumi, Tetsu Ohnuma, Kenji Takehara, Naho Morisaki, Kevin Y. Urayama, Tomoyuki Hattori