Day length may make geographical difference in body size and proportions: An ecological analysis of Japanese children and adolescents

Abstract

There is a north-south gradient in the body heights of Japanese children. A hypothesis had previously been proposed that differences in thyroid hormone activity induced by geographical differences in effective day length (duration of photoperiod exceeding a predetermined light intensity) might cause the differences in height. If thyroid hormone is involved, the effect should extend to body weight. This study examined whether geographical differences in body height and weight can be explained in terms of thyroid hormone activity induced by geographical differences in the photoperiodic environment using prefecture-level anatomical data and Japanese Mesh Climatic Data. Multiple regression analysis demonstrated that the combination of effective day length and weight was statistically significant as a predictor of height. Controlling for body weight revealed that effective day length was inversely correlated with height. Multiple regression analysis revealed that a combination of effective day length and height was statistically significant as a predictor of weight. Controlling for height demonstrated that effective day length was positively correlated with weight. Assuming an inverse correlation between effective day length and thyroid hormone activity, these results appear to show that short day-length will increase the activity of thyroid hormone and contribute to increasing height, but will inhibit weight gain; in contrast, long day-length will decrease the activity of thyroid hormone and contribute to increasing weight but will inhibit height gain. Geographical differences in height, and weight, and part of the prevalence of obesity in Japanese children and early adolescents may be explained by geographical differences in effective day length.

Stunting, wasting, overweight and their coexistence among children under 7 years in the context of the social rapidly developing: Findings from a population-based survey in nine cities of China in 2016

“…Studies have shown that the intake of enough milk can significantly promote the linear growth for malnourished and healthy children [ 41 ]. Besides, some reports had illustrated that the day length and temperature may be related to physical growth [ 42 , 43 ], and the statistics data also indicated the regional difference in yearly sunshine hours (2274h in northern, 1713h in central and 1622h in southern) and average temperature (12°Cin northern, 17°Cin central and 20°Cin southern). Therefore, by analyzing the population data from an eco-epidemiological perspective, we infer that the regional difference in stunted or overweight prevalence may also be tied to one’s natural environment, dietary patterns and lifestyles.…”

Regional Differences in Height, Weight, and Body Composition may Result from Photoperiodic Responses: An Ecological Analysis of Japanese Children and Adolescents

“…The physique of Japanese children tends to increase at higher latitudes and has a north-south gradient [ 1 2 3 4 5 ]. This trend has continued for at least 50 years, during which the national average of children’s physique has increased.…”

“…However, the north-south gradient remains almost unchanged. After consideration of the improvement of nutrient intake, hygienic environment, and intense migration that occurred in the last 50 years throughout Japan, the north-south gradient in physique is probably the result of environmental rather than nutritional or genetic factors [ 1 5 ].…”

“…The authors previously reported that geographical differences in effective day length may cause differences in thyroid hormone signaling in the endocrine system, resulting in geographical differences in physique [ 5 6 7 ]. The effective day length is the time for which the light intensity is considered as it indicates the duration of light exceeding a certain threshold of illuminance [ 7 ].…”

“…In short, the effective day length seems to affect both height and weight increase; however, the direction of the effect is the opposite. Although these characteristics of distribution appear to be inconsistent, it can be explained by assuming that it is the effect of thyroid hormone metabolism [ 5 ]. Thyroid hormone is an essential hormone for the growth of children and adolescents, with hyperthyroidism known to cause a tall and thin body composition [ 8 9 ], and hypothyroidism known to cause a short and fat body composition [ 10 11 ].…”