Whether you choose uppercase or lowercase letters has a strong effect on the legibility of your text. Indeed, words set in all uppercase letters should generally be avoided except perhaps for short headings because they are difficult to scan.
We read primarily by recognizing the overall shape of words, not by parsing each letter and then assembling a recognizable word:
Words formed with capital letters are monotonous rectangles that offer few distinctive shapes to catch the eye:
We recommend downstyle typing (capitalize only the first word and any proper nouns) for your headlines, subheads, and text. Downstyle is more legible because as we read we primarily scan the tops of words:
Notice how much harder it is to read the bottom half of the same sentence:
If you use initial capital letters in your headlines, you disrupt the reader's scanning of the word forms: