Your Diary will not show up no matter what privacy settings you choose.
As far as I can tell, the other settings can be made public/private so that people can view your statistics and then can help you with your weight loss/fitness goals.
When you think about it, a personal Diary really won't help in this regard. Plus it would be unmoderated comments so this in itself presents problems.
Think of food as fuel for the body instead of feeding emotions