The most reliable test for vitamin D measures a form of the nutrient in blood known as 25-hydroxyvitamin D.

People are deficient if they have less than 27.5 nanomoles of 25-hydroxyvitamin D per litre of blood. Amounts under 37.5 nmol/litre are inadequate for good bone health.

Many experts believe that levels should be 75 nmol/litre or more to reduce the risk of cancer and other diseases. Only about a third of Canadians are above this threshold.

Most of the vitamin D people have is made in their skin after exposure to strong summertime sunshine.

Humans living near the equator would naturally have levels around 130 to 150 nmol/litre. Toxic levels are above 375.