The letters " R.D." after a person's name signify that he or she has completed at least a 4-year college degree in food science or human nutrition. 

After that degree, they must complete a minimum 900-hour internship, which trains and supervises their work in every area of dietetics and nutrition practice.  A national qualifying examination must be passed after the internship. To retain R.D. status, they must complete on going continuing education. 

Do you know the difference between a Registered Dietitian and a Nutritionist?

All Registered Dietitians who are providing nutritional advice are technically "nutritionists". But not all nutritionists are Registered Dietitians. Anyone can call themselves a nutritionist, and hang out a shingle, without meeting any education or training standards.