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“Here we show seasonality in standardized body mass index (BMIz), with children gaining height at a greater rate during the school year compared to the summer,” said Dr. Jennette P Moreno at the USDA/ARS Children’s Nutrition Research Center, Baylor College of Medicine in Houston, Texas, and the study’s first author.
Body Mass Index (BMI) is the ratio of body weight in kg and height in meters squared. BMIz is the individual BMI scaled to the population-specific average BMI so that a BMI of one is equal to a BMI one standard deviation above average. Typically, a BMIz of 1.04 is considered overweight, and a BMIz of 1.64 obese.
The children’s rate of weight gain was more consistent than their rate of gain in height across the year, except among children who started to transition to unhealthy weight status in the summer after completion of the second grade.
Researchers statistically reanalyzed the results from their 2013 study, where they followed 3588 children who entered kindergarten in September 2005, at any of 41 schools in the Fort Bend Independent School District around the city of Sugar Land in Texas.
They followed these children, who were five to six years old at the start, until the fall that followed the completion of the fourth grade of primary school, five years later.
Nurses measured each child’s height and weight twice a year, in mid-September and mid-April. The present reanalysis is more powerful than in 2013.
The authors divided the children into five ‘BMI trajectory groups’, based on their pattern of change in BMIz over the study. Here, 22.6% of children scored as ‘chronically overweight or obese, 8.2% ‘becoming healthy weight’, 8.5% as ‘late-onset overweight or obese, 8.2% as ‘early-onset overweight or obese, and 52.5% as ‘persistently healthy weight’.
Early-onset was defined as transitioning toward an above-average BMIz beginning the summer after kindergarten, and late-onset as beginning this transition the summer after 2nd grade.
Besides BMI trajectory group and season (fall or spring), the authors used the school, the child’s age in months, gender, race or ethnicity, and relative body weight or height compared to peers as explanatory variables to model seasonal changes in height, weight, and BMIz, as well as their modifying statistical interactions.
Differential Impact On Height And Weight
Vertical growth was seasonal. As a result, graphs for linear growth across time – corrected for other variables – show a jagged pattern, with peaks in spring and valleys in the fall.
This deficit in vertical growth over summer was greatest for ‘chronically overweight or obese with a total of approximately -0.1 cm/month less growth over summer than over the school year.
The rate of weight gain did not differ between seasons. But the latter combined patterns of height and weight meant that BMIz was highest in summer, while the probability of becoming overweight or obese increased sharply over every summer.
Despite the pattern of height gain showing a greater increase in height during the school year, children’s height gain influenced BMIz more strongly during the summer holiday year than during the school year, with weight gain showing a constant increase during the school year.
What causes the strong seasonality of vertical growth is not yet clear. The demands of the school year may alter children’s exposure to the daily light-dark cycle, which may cause a seasonal pattern in height. Additional studies on children who receive year-round schooling might help to answer this question.
Source: Medindia
