Simplify the following text for a Grade 7 native English-speaking person.
Original: In some countries , staple foods such as milk , flour and margarine are artificially fortified with vitamin D , and it is also available as a supplement in pill form . Food sources such as fatty fish , eggs , and meat are rich in vitamin D and are often recommended for consumption to those suffering vitamin D deficiency .
Simplified: Vitamin D is a fat-soluble vitamin .