need help with writing an expossitory essay on healthy eating

You mean healthful eating, right? I've never known of any "eating" that can be diseased!

http://www.powa.org/
click on Thesis/Support Essays across the top for specifics.

http://grammar.ccc.commnet.edu/grammar/composition/organization.htm
Organizing for exposition (expository essay)

Step-by-step approach to writing an essay

For your content:
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