Background: Formal nutrition training in medical schools and residencies is lacking and needed. Registered dietitians (RDs) are formally trained in nutrition support and considered experts in the nutrition field. Our purpose was to examine prescribing and recommending discrepancies of parenteral nutrition macronutrients between medical residents (MRs) and RDs and compare results with the ASPEN clinical care guidelines. We also looked at discrepancies among obese patients, due to their increased risk of mortality. Materials and Methods: The primary end point of this retrospective review was discrepancies in nonprotein calories (NPCs) and grams of protein (PRO) between MRs and RDs. The secondary end point was discrepancies in NPCs and PRO between MRs and RDs among patients stratified by body mass index category. Results: MRs prescribed 300 NPCs more versus RDs (P < .001). When compared with RDs, MRs prescribed fewer NPCs for underweight patients and more for obese patients (P < .001). The same analysis found that the PRO discrepancies significantly varied by body mass index classification as well (P = .022). When these results were compared with the ASPEN clinical care guidelines, RDs adhered closer to the guidelines than did MRs in terms of permissive underfeeding of obese patients. Conclusion: It is widely accepted that MRs are in need of increased formal training, and the results of our study confirm this need and suggest a short-term solution of increasing order-writing privileges for the RD. RDs with this privilege may adhere more closely to clinical care guidelines and therefore increase patient safety.