What causes if you take too much sodium in your diet. Your may get High blood pressure, Heart disease, Kidney failure, etc. Miso paste is relatively high in sodium. But it is very interesting thing is that Miso paste prevent high blood pressure and cardiovascular disease.
It is not cleared yet but there is something in Miso paste lowering blood pressure in some studies.
Also if you eat Miso with potassium that helps relax blood vessels and remove sodium from the blood. There is a list that food contains high in potassium.
Spinach, Mizuna, Kumara, Potato, Sea weed. It is a great idea that you cook miso soup with these ingredients. Miso paste its self contains quite high potassium as well.
I would say that Miso paste is high in sodium. But it is too good to miss it. Miso paste has so many health benefits and taste fantastic. You can see that Japanese people are look young and healthy who have been eating Miso paste from ancient times.