Hey Feyona,
I'm really interested in your story because I'm considering moving in exactly in the opposite direction. I had a similar undergrad major (Nutrition and Exercise), but was also pre-med and ready to apply to schools at the end of 4 years. Now, I've got cold feet and can't bring myself to make such a permanent decision about the rest of my life. I like the idea of seeing patients, helping them understand their problems and find solutions to improve their lives. I don't mind studying and I can work hard, but I just can't get over the idea of 80-hour workweeks or how I could stay balanced when so much of my time goes to my career.
What is it about the medical field that's calling your name? What will you be doing with your MD that you can't do as an RD or PhD in Nutrition?
Just curious because I was thinking about going back to do an RD and focusing on my efforts on the preventative side of wellness and healthy living. Even though there are vast differences between the two fields, I have never been interested in many of the medical opportunities (like surgery or hospital rounds). I just want to talk (and listen!) to people and help them make healthier decisions about nutrition, exercise, drugs, stress, sleep, etc.
I have to agree with the advice about shadowing, reading and looking into the field very carefully before committing yourself. There's nothing wrong with taking some classes you won't end up using, or even studying for an MCAT that you aren't going to use (even though it may be frustrating); it's medical school tuition that creates the point-of-no-return decision.