Leave the subject line blank. Put the line:
SUBSCRIBE DIABETIC Your name
(where Your name is your name!) as the body of the message. You should get back an email message, requesting a confirming reply to ensure the subscription request is not bogus.
You then will receive another email with more information about the list. Note that the list traffic is about 600 messages per week. Be prepared! You may want to consider receiving the postings in a daily "digest" format. The confirming message you receive once subscribed describes more about this option.