I must not be articulating myself very well, although I'm not so sure how I can do better. I'll give it one last go and then if it doesn't work take the L to save everyone some time.

I understand the concept of food deserts. Now, what evidence is there that specifically institutional racism is causing it to occur more often [if it does] for black people, or to go back to the broader question, what evidence is there that institutional racism is causing obesity in the black community?

Put another way, if I were a statistician (and I am not, so give me some leeway in the terminology please), the null hypothesis should be that we don't know what's causing black obesity (or maybe that it's something much more trivial such as personal choice), and the alternative hypothesis should be that institutional racism is causing black obesity. But the null hypothesis is that we don't know, so we should cross a good enough evidentiary threshold to reject the null hypothesis to think about "accepting" our alternative hypothesis (I know we don't rigorously accept the alternative hypothesis in statistics - I'm just trying to pass a barrier here, to say that something should at least go in the direction of convincing us it's specifically institutional racism).