You can opt in or out of being notified of various forum interactions by clicking the “Bell” icon (Notifications) in the upper right corner, then by clicking the slider button in the opened drop-down menu. (Seen below)
On the following page, select your preferred notifications. Be sure to click "Save General Preferences" once you're done.
You can also customize your preferences by going here: http://community.myfitnesspal.com/en/profile/preference
Your notifications can also be found here: http://community.myfitnesspal.com/en/profile/notifications