Most Canadians have enough vitamin D in their blood for bone health, but only one third are above the level increasingly believed necessary for overall health and disease prevention, Statistics Canada says.
More than 1.1 million Canadians or about four per cent of the population are vitamin D deficient, or low enough to cause nutritional rickets in children and osteoporosis in adults.
A report released Tuesday used data from the Canadian Health Measures Survey, which took direct physical measurements, including blood samples, to test levels of vitamin D and fats such as cholesterol.
Advocates of vitamin D believe it is important to human health, especially in more northerly climates such as Canada, where sunshine is often limited in winter.
Higher levels desirable to prevent disease
In the study, 90 per cent of Canadians aged six to 79 had concentrations of vitamin D in their blood that were considered adequate for bone health.
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But 10 per cent or roughly three million people had concentrations considered inadequate. Of these, 1.1 million were considered vitamin D deficient.
The agency defined vitamin D deficiency as a concentration below 27.5 nanomoles per litre (nmol/L) — a measurement of concentration based on the number of molecules per litre of blood.
The report's authors said there is growing consensus that much higher concentrations, above 75 nmol/L, are desirable for overall health and disease prevention.
"This higher level has been found to be associated with a lower risk of breast and colon cancer, some cardiovascular disease and other diseases like multiple sclerosis," said Kellie Langlois of Statistics Canada's health analysis division.
Between April and October, 38.6 per cent of Canadians tested had concentrations above 75. Between November and March, the percentage who met that concentration in their blood or serum fell to 30.3 per cent, according to the report.
"If you look at the reference values that your doctor would expect you to have when they measure your serum 25 hydroxy D, one-third of Canadians would have enough, and two-thirds of Canadians would be told they should probably be taking a vitamin D supplement," said Reinhold Vieth, a clinical chemist at Toronto's Mount Sinai Hospital.
Men were more likely than women to have inadequate concentrations. The highest prevalence of deficiency occurred among men aged 20 to 39 — about seven per cent of this group were considered vitamin D deficient.
"The factors related to low concentrations are winter season, racial backgrounds other than white, and less frequent intake of milk," Langlois and the co-authors concluded.
Levels under review
Future analysis of the data will investigate factors that may influence vitamin D concentrations, such as supplement consumption, body mass index, pregnancy, fish consumption and use of sunscreen, the researchers said.
Small amounts of vitamin D are found in fortified foods, such as milk, and naturally in oily cold-water fish such as salmon. For example, one cup of mild or fortified rice or soy beverage contains 100 IU of vitamin D and 85 grams of light canned tuna contains 200 IU, according to the B.C. Ministry of Health.
Men with inadequate blood levels of vitamin D of 30 nmol/L would need to consume about 3,000 international units (IU) of vitamin D supplements per day to reach the more desirable 71 nmol/L level, the authors of a 2008 U.S. study on heart attack risk said.
Vitamin D inadequacy for bone health was defined as a concentration below 37.5 nmol/L. The standards were set in 1997 by the Institute of Medicine in Washington, and are under joint review by Canada and the United States.
The institute expects to complete its vitamin D review and issue its final report by the end of this summer.
Health Canada said the current tolerable upper intake level for those over the age of one is 2,000 IU from all sources.