I took a nutrition class through an accredited correspondence school for a science credit. If you look around you should be able to find a school near you teaching the classes. Most of them suggest or require high school biology and chemistry and give you a nice background without marketing bs tossed in.
Valid advice if you've got serious persistent problems with food, like overeating, junk food addictions, etc. But, if you already have a good diet and have no food-related health issues, then a nutritionist will likely be of limited utility--certainly less than a few hours of research on the Internet (careful research...citations needed).
Certainly there are some nutritionists who are on the cutting edge of the field and follow the latest research (and parse it with knowledge that most popular science coverage of food is bullshit), but I've come upon a few who mingled current trends with "ancient wisdom" a little too much and end up being purveyors of "alternative medicine". A friend of mine went on a huge soy kick for a year or two, because her nutritionist had bought into the "soy is a magical cancer preventative" hype. We now know with confidence that soy in many forms is actually kinda hard on the body (hormone producing chemicals in food are something that should be treated with caution, and not consumed in large quantities without careful study), and the research that indicated cancer prevention was probably overblown and certainly not specific enough.
So, see a nutritionist if you want, but take all advice with a grain of salt. Nobody knows everything, and moderation is probably the best choice when in doubt. (Though anyone who tells that you should eat more fruits and vegetables is almost certainly right. Moderation is probably not a concern with broccoli and carrots and blueberries.)
Quite honestly I've had my eyes opened. Good nutritionists aren't that hard to find and their expertise involves not just "persistent problems with food", but can also help you to avoid diets that cause health problems in the long-term. Even more amazing to me is that those health problems can be controlled through foods. The nutritionist can go through your family history and help you to understand your relationship with food and how it affects you - now and into the future.
"Though anyone who tells that you should eat more fruits and vegetables is almost certainly right."
That I can agree with. I should have also mentioned Michael Pollan's books. His most recent is an analysis of what to eat - and the subtitle says it all: Eat food^. Not too much. Mostly plants.
