Medicare will cover registered dietitian services for diabetes only if the RD is also a CDE. Here are the codes:
G0108 Diabetes outpatient self-management training services, individual, per 30 minutes
G0109 Diabetes outpatient self-management training services, group, per 30 minutes
CPT also lists codes for medical Nutritional Therapy, but the payers differe as to what conditions they cover for these services. Also, some payers will not credential a registered dietitian, (and not all nutritionists are registered dietitians), so you'll have to check with each payer regarding billing under physician supervision. Here are the codes for Medical nutritional therapy.
97802 Medical Nutrition Therapy, initial assessment and intervention, individual, face-to-face with the patient, each 15 minutes
97803 MNT, re-assessment and intervention, individual, face-to-face with the patient, each 15 minutes
97804 MNT, group, each 30 minutes
Hope this helps.