When it comes to learning about food to prevent or help treat an illness (such as diabetes), an RDN should be your first stop. The national exam ensures that they're able to practice anywhere in the U.S., and the continuing education requirement keeps them up to date with current ideas about nutrition. But those with a CN degree are also qualified to assess your nutritional needs, help you plan your meals for the activities you're doing, and give nutritional counseling. The biggest difference between the two is the hands-on training requirement: RDNs need to log at least 1200 hours, but CNs usually require much less time.