Those who don't read well recognize words by their shapes. Words in all caps look alike. Minimum font size of 10 - 12 points, not too many different font styles. Line length of about 40 - 60 characters. It is harder for a reader's eyes to move all the way from one end of a long sentence to the other end.
Shorter lines help readers keep their place.
Use the active voice. Use concrete examples. In the above example, list examples of food that are high in calcium. Use appropriate graphics. Graphics should be relevant to the information you are conveying.