By Danny Halpin via SWNS
Kids grow in height more at during the school term time than during the summer break, new research showed.
The timing of this growth spurt could be behind why some kids appear to get fatter over the holiday as they continue growing out but slow down growing up.
The study from Baylor College of Medicine in Houston, Texas looked at 3,558 kids from 41 schools in the state.
Researchers discovered an increase in body mass index over the summer was not just down to lazing around and eating badly.
The rate of weight gain was found to be consistent all year but because BMI is the ratio of body weight in kg and height in meters squared, faster vertical growth in school means that BMI is highest in the summer.
Dr. Jennette Moreno, first author of the study at the USDA/ARS Children’s Nutrition Research Center at Baylor, said: "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."
BMIz is individual BMI but scaled up to a population-specific average, with a BMIz of one being equal to a BMI of one standard deviation above average.
Typically, a BMIz of 1.04 is considered overweight and a BMIz of 1.64 obese.
Moreno added: "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 an unhealthy weight status in the summer after completion of the second grade."
Professor Debbe (CORR) Thompson, a research nutritionist at Baylor said: "Despite the pattern of height gain showing greater increase during school, children’s height gain influenced BMIz more strongly during the summer holiday than during the school year, with weight gain showing a constant increase during school.
"This differential seasonal impact of height and weight on BMIz led to a healthier BMIz status during the school year."
For the study published in the journal Frontiers in Psychology, the team statistically reanalyzed the results from their 2013 study where they followed 3588 children across 41 schools in Sugar Land, Texas.
Nurses measured the children’s height and weight twice a year, beginning when they were five to six years old until they completed fourth grade of primary school five years later.
The children were divided into five "BMI trajectory groups" based on their pattern of change in BMIz over the study, with 22.6 percent of children scoring as ‘chronically overweight or obese’, 8.2 percent "becoming healthy weight," 8.5 percent as "late-onset overweight or obese," 8.2 percent as "early-onset overweight or obese" and 52.5 percent as "persistently healthy weight."
Early-onset was defined as moving into an above-average BMIz the summer after kindergarten and late-onset was defined as beginning this transition after second grade.
Age in months, gender, ethnicity and relative body weight or height in relation to peers were also considered as variables in explaining why children grow at different rates throughout the year.
Despite these, the researchers found a notable difference between seasons, with vertical growth being faster over the school months than the summer by an average of 0.55cm/month.
A higher BMI in summer driven by a growth in height means that children are much more likely to become obese during that time.
Speculating on why children grow at such varied rates between seasons, Dr. Craig Johnston, associate professor at the Department of Human Health and Performance at the University of Houston, said: “It's possible that the demands of the school year alter children's exposure to the daily light-dark cycle, which may cause the seasonal pattern in height.
“Additional studies on children who receive year-round schooling might help to answer this question.
"What is clear is that children at the greatest risk of becoming overweight and obese have a less pronounced seasonal impact of height gain on BMIz, indicating they would benefit from obesity prevention efforts throughout the year.”
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