I think what Carol is saying is that, while skim milk and low-fat yogurts are artifically supplemented with vitamin D (in the US and Canada - Europe doesn't), they cannot efficiently deliver the vitamins to your body. Vitamin D, as well as several others, are fat-soluble vitamins and need some fat in the recipe to serve as a carrier. So, if you're eating non-fat dairy sources, you're practically wasting the vitamin D. In addition, since vitamin D is needed to absorb calcium, fat-free milk and yogurt can deprive us of both. Instead of the above yogurt and milk, I use Balkan-style yogurt, 2% to 6% fat, which I consider to be some of the healthiest yogurt on the shelves. I omit the milk, or sometimes substitute for orange juice.

Hi Carol, all the sources I looked up list the same amount of vitamin D for all kinds of milk, including skim - like this one, Alberta Health Services.

Skim milk contains no vitamin D. Vitamins A,D,E and K need to be carried in some fat content.