Being a registered dietician is really more about preventing deficiencies.
Most likely you'll end up working in a hospital, helping plan out tomorrow's "low sodium" meal plan for the CHF patients.
You might make rounds, and tell the old people who are hypokalemic to eat more potatoes and bananas...you might talk to a 500lb man about portion control....and if you're lucky, you'll explain to an old couple that drinking 2 cups of coffee a day isn't enough fluid consumption, and is the reason they're dehydrated.