Bouts of high-intensity physical activity — such as running, swimming or playing soccer — are superior to longer periods of light and moderate exercise in reducing the risk factors that set kids on a path towards cardiovascular disease.
The findings were published online Monday by the Archives of Pediatrics & Adolescent Medicine.
In the study of 605 Alberta schoolchildren, aged nine to 17, researchers found that seven minutes a day of intense physical activity was associated with significant reductions in body weight and blood pressure levels, as well as increased fitness.
When it came to waist circumference, participants who did the highest amount of vigorous exercise each day pared their mid-sections by seven centimetres on average. Overweight subjects in that group reduced their waistlines by five centimetres.
"That is a huge difference," said principal researcher Jonathan McGavock, an exercise physiologist at the Manitoba Institute of Child Health.
"If we look at physical activity as the magic bullet or drug that is going to have health benefits, such as reducing the risk of overweight or reducing the risk of high blood pressure, a higher intensity is like taking a higher dose of that drug," he said Monday from Winnipeg.
"It says that the more intense the exercise is, the less likely (youth) are to be overweight or to have high blood pressure, compared to (those doing) lower-intensity exercise."
The cross-sectional study included 248 boys and 357 girls, about a quarter of whom were overweight or obese. Data was taken from the 2008 Healthy Hearts Prospective Cohort Study of Physical Activity and Cardiometabolic Health in Youth.
Participants wore accelerometers on their hips, which measure the amount and intensity of physical activity, for a maximum of seven days. The minimum time for inclusion in the study was three days, 10 hours.
Those who engaged in the lowest-intensity exercise, deemed light physical activity, clocked 133 minutes or just over two hours of easy walking. Participants who tallied the highest amount of light exercise walked for 244 minutes, almost twice as much.
"Despite the fact that they're getting over an hour — almost two hours — more light activity, we didn't see any differences in their waist, in their body weight, in their blood pressure or in their fitness," said McGavock.
Yet those engaging in vigorous activity, ranging from a low of a minute and a half to 8.7 minutes daily, appeared to reap greater rewards from a more intense — albeit much shorter — physical workout, he said.
"So what was really novel was that over a very short increment in time, you were seeing a lot of health benefits associated with it."
Canada's physical activity guidelines recommend that children aged five to 17 need 60 minutes of moderate- to vigorous-intensity exercise daily. This should include both vigorous-intensity activities and muscle and bone-strengthening activities at least three days a week.
"We need to start incorporating vigorous physical activity into those guidelines and emphasizing higher-intensity exercise," said McGavock.
"So the final message I would give to parents is try to get your children outside and into games they enjoy, but games in which they are more active, they are breathing hard, it's increasing their body temperature, they're red in the face — that kind of exercise — so we can see greater health benefits."
Mark Tremblay, director of active living and obesity research at the Children's Hospital of Eastern Ontario, said the Manitoba study is useful and supports the Canadian Physical Activity Guidelines.
Those guidelines stress that more is better, whether referring to duration, intensity, frequency or variety of physical activity.
"I think the particular value that this (study) adds is it throws a number out there," said Tremblay, referring to the seven-minute span.
"That's the sort of information that is severely lacking in the literature," he said from Ottawa. "It at least is putting a line in the sand, which I really value," said Tremblay, lead author of the activity guidelines.
"Hopefully there will be more (studies) like this so that when five years from now, when we're updating the guidelines again, we'd be able to say and include vigorous-intensity activities at least three days of the week for at least 'blank-blank' duration."