This page shows 250mL of milk having 45% of daily vitamin D. This is consistent with other brands like Neilson.
Health Canada recommends 20 micrograms of vitamin D daily.
However, a couple top results from a google search indicate that at most there would be 3 micrograms in 250mL of Canadian milk.
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So hopefully the math is clear, 20 micrograms recommended daily by health Canada, but nutrition labels indicate 6 micrograms is 90% of daily intake.
Milk does not naturally contain Vitamin D, or at least it contains only a very small amount. It’s fortified with Vitamin D. In some countries, including Canada, it’s a legal requirement for milk to have a certain amount of Vitamin D added to it and it’s common practice for other countries to do the same.
So the amount of Vitamin D in your Milk is the min requirement + however much the company chose to put in it.