Kids grow taller in the holidays than during school term, says health scientist

By Amanda Hoh

Did you have to buy new school uniforms or shoes this year because your child grew by one to two sizes over the summer break?

If so, you’re not alone.

Recent studies have shown that kids grow taller and put on more weight in the school holidays than in the school term, according to Professor Timothy Olds from the School of Health Sciences at the University of South Australia.

“During school term, kids get leaner,” Dr Olds said.

“In the holidays there’s change in diet, but also less stress.

“Stress has a big impact on hormones which affect growth.”

In general, Dr Olds said humans “have been getting bigger” over the past 150 years at a rate of about one centimetre per decade.

While the reasons are not clear, possible causes are changes in nutrition an a reduction in malnutrition and diseases that stunt growth, as well as better sanitation, hygiene and “looser fitting clothing”.

It also means kids are likely to grow at least three centimetres taller than their parents — something observed by a number of listeners.

“My son is 19 years old, and is seven foot tall (213cm) and wears size 19 shoes. It’s a tough life. He bends over, he sits badly. He can’t fit in chairs properly, can’t fly. We have been importing clothes and shoes since he was 16. No one sells size 19 shoes in Australia.” — Malcolm, Killara

“My nephew grew so much the year before and played so much sport, he ended up with osteocondritis in the top of the femur and needed surgery. He is just 17 years old and six foot six (202cm).” — Maggie, Loftus

“My great nephew is staying with us this weekend — just turned 19 and 203 cm (6ft 8″) tall. Greatest problem was finding a car with a seat that would go back far enough for him to drive safely!” — Anne, Bullaburra

“My son is in year 12 this year and is 17. He shot up at least five centimetres in five weeks. I used to look at him a week before school holidays and I was eye level just at the top of his head. He’s now looking at me eye to eye. My daughter, going into year 10, had to go two sizes bigger in pants to get the length in eight weeks.” — John, Moss Vale

When do kids grow fastest?

Dr Olds said he was not surprised by some of these stories.

He said at a child’s peak height velocity — the rate of fastest growth — kids can grow as much as 10 to 16 centimetres in a year.

“They can actually grow half a centimetre in a single night,” he said.

“Human growth hormone comes out in pulses, and those pulses come out when you’re sleeping.

“Early deep sleep is important; if you are sleep deprived, you’re likely to be growth deprived as well.”

The fastest rate of growth occurs in babyhood, then slows down until a child hits puberty and their peak height velocity.

The start of puberty is considered when girls first start menstruating, while for boys it is the age when their voices break.

Dr Olds said historical data showed puberty occurred when a child grew to about 150 centimetres.

“Evidence suggests there is some critical height and weight that triggers or allows puberty to happen,” he said.

“At the beginning of last century, puberty would start around 15 or 16.

“Now we’re seeing at 12 or even younger.”

Dr Olds said girls would stop growing around 15 or 16 years old, while boys would keep growing until their early 20s.

This post originally appeared on ABC News.