Not true... One of my MD-only mentees earned a NRSA F32, and during the process also earned her PhD. She was a general surgery resident with 2 years of protected time, and she now has switched to radiology after 4 years (hybrid with 2 residency + 2 research), took an extra year of leave of absence to complete her PhD and apply for her new residency; she will likely become an interventional radiologists.