Why is there not enough research on heart stent and exercise?
There is research of stents and exercise! How much is enough?
In fact there are exercise programmes, known as cardiac rehabilitation designed to help those who have had some forms of cardiac disease or procedures-- including stent insertions. Cardiac rehabilitation has evidence to suggest that it is a beneficial programme for those who have suffered eg heart attacks or who who have had stents.
There is evidence to suggest that exercise shortly following stent insertion is safe too; though perhaps inadvisable in the context of having a sore groin from the femoral access used.
Certain types of exercise, such as high-intensity interval training (HIIT) may protect stent function, though it should be noted that the study that suggested this had a relatively small sample size. It may have other beneficial effects, but again note 1) small sample size and 2) a surrogate marker (heart rate variability (HRV)).
It should be reasonable to conclude that other factors notwithstanding some forms of exercise is not harmful in and of itself following a stent. Of course, context is everything- if those 'other factors' were that a person who has had a stent is terribly unfit and so has very poor heart function, or has other significant co-morbidities, or similar it may not be safe for them to exercise!
And so to the next point:
Why isn't there research to say if me running this marathon is safe? Why wouldn't my doctor tell me it's safe to run in the LA marathon?
'Why' questions can be tricky to answer; but in brief it may be tricky to do good, conclusive research as to whether having a stent affects marathon running. Research or trial design is a very broad topic to go into (it's a science unto itself), but it's hard to do well at the best of times. In this particular case there are several potential pitfalls.
Stent or not, some people die during marathons. This may be due to cardiac arrest or in the context of sudden cardiac death.
So, say you have someone who sadly dies during a marathon, and they have a stent. How do you work out whether they died due to it not being safe to run because they have a stent, or for another reason? Stent or not, were they fit to run- are they a previously fit-and-healthy individual with a background of running before their procedure, or were they a sedentary individual who had a heart attack out the blue and leapt 'off the couch' to get fit after they had to have stents put in?
In the absence of evidence, it's a judgement call. I can't and wouldn't wish speak for your doctor, but if they were to tell you that based on you being previously fit and well, and that you are doing well following the procedure you should be fine; if you then were to have something adverse happen, you may decide they had advised you poorly.
That being said, there is a programme for those with stents (and other medical devices) to be sponsored to enter a marathon; I am not affiliated with them. It's also worth pointing out that one of the requirements is that the putative runner:
Certify they have consulted with a physician who deems the runner medically fit to participate in the race.
So there you go. Run safe.
Citations and Further Reading
Cardiac rehabilitation- NHS info and a linked British Heart Foundation guide
Impact of Cardiac Rehabilitation on Mortality and Cardiovascular Events After Percutaneous Coronary Intervention in the Community
Early exercise after coronary stenting is safe
High-intensity interval training may reduce in-stent restenosis following percutaneous coronary intervention with stent implantation : A randomized controlled trial evaluating the relationship to endothelial function and inflammation
High-intensity interval exercise training improves heart rate variability in patients following percutaneous coronary intervention for angina pectoris.
Marathon-related cardiac arrest
Cardiac Arrest during Long-Distance Running Races
Risk for sudden cardiac death associated with marathon running