Medical Nutrition Therapy Act of 2021 This bill expands Medicare coverage of medical nutrition therapy services. Currently, Medicare covers such services for individuals with diabetes or kidney disease under certain circumstances; such services must also be provided by a registered dietitian or nutrition professional pursuant to a physician referral. The bill extends coverage to individuals with other diseases and conditions, including obesity, eating disorders, cancer, and HIV/AIDS; such services may also be referred by a physician assistant, nurse practitioner, clinical nurse specialist, or (for eating disorders) a clinical psychologist.