Only a small percentage of Canadian children and adolescents have high blood pressure, despite their high levels of excess weight, Statistics Canada reported Wednesday.

Automated blood pressure measures taken from 2007 to 2009 showed 0.8 per cent of young people aged six to 19 had elevated blood pressure, while 2.1 per cent had borderline high levels.

For each age and sex category, average child and adolescent systolic blood pressure in Canada was about 10 millimetres of mercury lower than the most recent United States survey, Statistics Canada said

"The main finding of this analysis is the remarkably low overall prevalence of borderline or elevated BP among Canadian children and adolescents," the study's authors concluded in the said in its June issue of Statistics Canada's Health Reports.

When blood pressure is too high, it puts stress on the body's entire vascular system, forcing the heart to work harder, and increasing the risk of death from a variety of causes including heart disease and stroke

In adolescence, hypertension is also associated with these heart problems, and elevated blood pressure at young ages is a risk factor for development of hypertension in adulthood, according to the report.

Obesity paradox

"It is scary to think you could start to develop very high blood pressure levels even at very young ages," said study author Ian Janssen, a professor in the school of kinesiology and health studies at Queen's University in Kingston, Ont.

"The longer you have high blood pressure, the earlier you're going to develop the actual diseases associated with high blood pressure like heart disease and stroke."

As part of the broader Canadian Health Measures Survey, researchers surveyed and tested 2, 079 Canadians aged six to 19 between March 2007 and February 2009 at mobile examination centres.

The generally low levels of blood pressure in the survey "appear inconsistent with the rise of childhood and adolescent obesity in Canada," Gilles Paradis of the department of epidemiology, biostatistics and occupational health at McGill University in Montreal and his co-authors wrote.

"And despite a trend toward excess weight among youth in other countries, BP levels have not shown consistent increases. Hence, population-level increases in BP may not necessarily be a consequence of rising weight. More research is required to explain this apparent paradox."

Part of the paradox might be the "white coat effect," of having a high reading only when blood pressure is measured by a doctor, nurse or technician in a clinical setting, Janssen explained. Blood pressure guidelines were based on these type of measurements, but the Statistics Canada used automated measurements taken in a quiet location.

A previous study published in Quebec in 1999 suggested about 20 per cent of children had blood pressure readings that were either elevated or borderline elevated, but those were taken with different instruments in a noisy school setting, the authors noted.

Janssen said while there may be improvements in some risk factors, Canadian children show high levels of obesity and overweight and poor physical activity. Knowing these differences could help set targets and priorities for tackling different risk factors, he said.