Before I answer this question, I want to mention the differences between a dietitian and a nutritionist. "Nutritionist" is a blanket term. There is no regulation regarding its use (unfortunate, because I happen to like the word better). As such, anyone may call his/herself a nutritionist, whether knowledgeable about nutrition or not.
In contrast, a registered dietitians (RDs) have spent years studying biology, chemistry, clinical nutrition and food science, and are required to pass a national exam administered by the American Dietetic Association. Throughout their careers, RDs are required to accrue continuing education credits in order to keep up to date with the latest nutrition advancements. Someone with a Masters degree in nutrition has likely received additional training in clinical nutrition, research interpretation/evaluation and behavioral theory.
- Prevention of Disease. Visit a dietitian for a general nutrition check-in. Are you eating as well as you think you are? Are you bored with your food? Can't decipher nutrition labels and health claims? A dietitian can help you sort fact from fiction and identify ways to improve (and inspire) your diet and lifestyle. You can't change your genetic susceptibility to illnesses such as diabetes, heart disease and cancer, but you can give yourself a better chance at beating the odds.
- Support during Disease. Visit a dietitian if you are diagnosed with a medical condition, especially:
- Diabetes or insulin resistance (may be called "prediabetes")
- High Cholesterol or Heart Disease
- Kidney disease
- Liver disease
- Food Allergies
- Any Gastrointestinal-related disorder, including GERD, gastritis, ulcerative colitis, Crohn's disease, irritable bowel syndrome and many more.
- Pregnancy (before, during and after).
- Weight Loss. Sure, you might know generally what you need to do, but meeting with a dietitian, having s/he perform a diet analysis can be very helpful. Continuing to meet occasionally will help keep you on the right track. This is true of any medical condition requiring nutrition and lifestyle changes.
- Family Nutrition. There are dietitians who specialize in pediatric and family nutrition. Everyone's heard the stories about kids who only eat beige foods...
- Disordered Eating. Ideally, treatment of eating disorders should take a multidisciplinary approach and include - at a minimum - a physician, a dietitian and a mental health professional (psychiatrist or psychologist).
- Sports and Fitness. A sports dietitian will help you maximize the benefits from your fitness training. Ensure that your physical efforts are supported by the appropriate nutrition strategies.
- Supplement Confusion. Unlike drugs, there is little regulation of supplements. What is safe? What really works? What the heck do those claims mean, like "supports healthy cholesterol levels"? A dietitian has the knowledge and resources to help you sort it through.
Hope this is of some help, and thanks for the question!
P.S. Either spelling (dietitian or dietician) is correct, but dietitian is most commonly used, and is the choice of the American Dietetic Association.