you even have to obtain a licence when you are a registered dietitian. Here is a sample from the website of the State of Georgia.
“I am a registered dietitian. Do I need a license to practice in Georgia?
Yes. You cannot use your registration to practice in Georgia. You must hold a valid license issued by the Georgia Board of Examiners of Licensed Dietitians in order to practice dietetics in Georgia.”
There are many resources to answer your question. Here is one link http://www.nutritioned.org/registered-dietitian-georgia.html.
Having a certification in fitness nutrition, no matter from which organization, probably does not suffice for the requirements for most states to obtain such a licence.