A pediatrician is a medical doctor (MD or DO). All doctors diagnose and treat diseases, that's kind of what being a doctor is about. Unless you work for an HMO, then you try to keep other doctors from diagnosing and treating diseases using expensive methods .
An MD or DO takes 4 years of medical school to earn. Then you complete a residency, which is usually 3 or 4 years on top of that, depending on what specialty you choose. I think pediatrics is 3 years.
If you're interested in nutrition and nutrition only, medicine is not really the career path for you I think.
What about nutrition interests you?
Would you rather work in a clinical setting, research or even both?
If you prefer working in a clinical setting, I suggest you enroll in a dietetic intership program. Many universities offer this option as a masters degree (MS or MPH) along with the professional degree as a registered dietian (RD). Afterwards, you need to be licensed in the state where you wish to practice (LD). The MS or MPH will give you some research experience as well.
If you interested in solely research, I suggest getting a PhD in nutritional sciences.
I'd like to do research and work in a clinical setting. I'm a very good resercher also. I research every thing i buy over $50 usually. Going and asking on forums for weeks about what model is the best bang for the buck, I'm into computer hardware. I dont care if they only make 45,000 a year. Nutrition is my true calling.