Potassium is an essential mineral in the human diet that is found in relatively high amounts in fruits, vegetables, and legumes. It is commonly seen as the counter-part to sodium when it comes to blood pressure and regulation of water in the body with high potassium diets being associated with reduced blood pressure.
Potassium is relatively unique when it comes to the topic of dietary supplements since it cannot legally be sold in levels high enough to provide much benefit due to safety. As an easily absorbed mineral that influences blood pressure, large doses taken in a powder form can be associated with cardiac arrythmia and in a few cases hospitalization. This has largely prevented wide-scale potassium fortification of foods leading this mineral to be a relatively common deficiency. This risk does not seem to apply to when large amounts of potassium are consumed from the fibrous food it is found in as the foods slow the rate of potassium absorption leading to less of a 'spike' in the blood, known as hyperkalemia.
When it comes to potassium, benefits are seen when it comes to circulatory disorders such as cardiovascular disease and particularly stroke when potassium intake is increased. This benefit is seen when potassium is consumed in minor increases (additional 500 mg a day) or large increases of a few grams over the course of the day, has similar protective effect regardless of the form of potassium consumed (food or supplemental), and shows benefit even if sodium is held constant.