Pretty easy at my program and not uncommon, we have students do a PhD both in house and at other institutions.
Also, I think a good number of MD/PhD programs would support you doing a biostats or exercise phys. thesis. They may not have standalone tracks, but could support you and get you the appropriate courses/mentors as needed. Feel free to pm me for more details.
Thank you, and will do! And thanks for the additional background - I only have one med school in my state so my knowledge of how other MD/PhD programs work is limited to my online research and what my state does.
In my conversations with my state school, I did ask them if I could do exercise science if I were lucky enough to matriculate into their MD/PhD program and they told me that I would not be able to do that. Maybe it is because it is not an MSTP, but a still fully-funded MD/PhD? They do have a Population Health Sciences PhD, though, so I'd 100% do that (if I had the choice between the two, I'd probably still pick PHS).
We had at least one student who started in the regular MD program, and then during the first year of medical school successfully applied into the MD/PhD track. This was a pretty exceptional student, and I guess they lucked out that there was room/ funding for them to join the MSTP cohort. I wouldn't bother asking too much about this possibility before enrolling somewhere (places probably won't know until there MSTP cohort shakes out whether they could accommodate you) but might be an argument for trying to go somewhere that does have funded MSTP positions (these also tend to be stronger research institutions).
In terms of doing a PhD during a LOA, there is the issues of paying for the MD (as others have pointed out) but there's also sometimes time limits for low long someone can take to complete the degree. 5 years is no problem (lots of students take some extra time for research) but I believe once you start getting into the more than 6 year territory you have to have received an additional degree though a sanctioned pathway (which may apply to you). Just a heads up that it may not be super easy to take out a 4 year chunk of time (and also if your not officially in an MSTP no guarantees that a program will get you through that quickly given that the average biomedical PhD in the US currently takes 6.5 years). Nothings impossible though, so best of luck as you pursue your training interests.
My state school was the one who told me about the possibility of applying as an MS1 or even MS2, saying that due to my research experience but because of my non-competitive uGPA that I'd have a tough time getting accepted as a pre-med applicant but a much stronger chance once in medical school. So my current plan of attack is as follows:
a) apply MD/PhD and if not accepted but I matriculate into an MD,
b) apply MD/PhD as MS1 and if not accepted,
c) apply MD/PhD as MS2 and if not accepted,
d) request to do a PhD as a LoA between MS2 and MS3, and if that is rejected,
e) apply for a PhD granting residency, and if I don't match,
f) apply for a PhD program toward the end of residency
As for the timeline to complete one, if I do a LoA then (since I would not have yet graduated from the MD program) I am under the understanding that some of my MD courses could be applied toward the extracurricular credits needed to complete the PhD. Further, the two specific Population Health Science PhDs I have looked at that are attached to MD/PhD programs have completion times averaging three years (they lower the amount of credits needed for those who already have masters degrees so that most of the time is working on research and being a TA). If I go the exercise science route, though, I can see the danger of approaching the 5 year threshold for certain.
Thank you so much for the great information and well-wishes! *crosses fingers*