Becoming a RN and RD

I have googled the titles and have come up with a few RN's that are also RD's. I am currently studying to become a personal trainer through ACE and I would love to focus on preventive health and work with obease kids and helping them get fit and also educating those with health issues. I know having a RN degree a BSN is the best option to open many doors in the health care field that I want to focus on. I am hoping someone on the forum who is familiar with this could chime in on what would be the best order to persue this. I thought maybe getting th RD first and then going to a post-Bac Nursing program thats accelerated would be best. Either way I would like to know more about what options are available in the field of fitness/Nutrition in the Nursing field. Thanks!