When you join MyFitnessPal you are joining a community-based program, and other members can view some of your information.
If you add a friend, they will be able to see your News Feed updates, including the amount of weight you've lost, but never your current weight. You may choose to limit the kinds of updates that appear on your News Feed.
Your Food diary is also private, by default, but you may choose to make this visible to your friends or to all users.
For the most part, members search for other members in order to find a fitness buddy to help motivate them toward their own goals. If you would prefer not to have your profile page visible to other users on the site, you can change your profile privacy settings. To do so, log in to your account on the site at www.myfitnesspal.com. Mobile app users should log in using the same username and password they use in the app. Once you've logged in click "My Home" then "Settings" then "Profile privacy settings". You can set this to "Myself Only," which leaves only your username and primary profile photo visible to other users.
Even with a profile set to "Myself Only," another member may send you a Friend Request. You may accept or refuse this request, or click the small triangle under their photo in the Friend Request screen, and choose "Block User" to prevent future contact.
You may also block a user who has sent you an email message on the site.