I don’t know how much you know about your niche, so I am treating this like you are a beginner, but you can adapt it according to your expertise.
First, you create a product something like “X secrets everyone in this niche needs to know” or some mystery they want to sign up to learn the answer. In weight loss, a big magnet seems to be “X foods that help you melt fat away.” or “X foods to avoid to lose weight.”
Now, if you have a product, when you get their email, direct them to the sales page. If you don’t have a product, then, send them followup emails that say more about whatever your report said. Then, after a few, 5-7 emails, tell them, “Now, I want to hear from you. What do you want to know more about? How can I continue to help you?” (rephrase it to your situation.)
That’s my two cents. Sean will eventually be here and give you better information.