"...production of NO declines steadily with increasing age..." (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3390088/)
It could also explain why heart disease, diabetes, and high blood pressure are comorbid for Covid 19 as they are all associated with lower N.O.
Just look at zinc and heart disease. I am just posting one study but the evidence is plentiful. https://care.diabetesjournals.org/content/30/3/523.short
So if one has heart disease they may also be low in zinc. So zinc might be the common denominator to both CVD and COVID19. It is not that heart disease is a risk factor for COVID19, but that zinc deficiency is a risk factor for both.
And they find some odd effects in NO synthesis from zinc deficiency. It lowers NOS activity but has other bad effects as well:
https://journals.lww.com/co-clinicalnutrition/Abstract/2000/...
On the other hand, total nitric oxide synthase activity in the intestine of zinc-deficient animals is significantly lower than that in controls, indicating that zinc deficiency may induce a potential vulnerability to nitric oxide rather than an absolute increase of nitric oxide synthase activities.