To second what Jocelyn and Michelle said, there are many different organizations that grant certifications in the Nutrition area. Many of the organizations that certify Personal Trainers now offer Fitness Nutrition certs. I have heard that these are VERY basic courses, but it can’t hurt to look into.
I personally am enrolled int the Institute of Integrative Nutrition (www.IIN.com that Michelle mentioned) and loving it! It is very in depth, and you end the 48 week program as a Certified Health Coach.