Beet greens, potatoes, lentils, sweet potatoes and clams have the highest levels of potassium per serving size, according to WebMD. High-potassium foods contain 201-300 milligrams per serving. Foods with very high levels of potassium have over 300 milligrams per serving.
One cup of cooked beet greens has 1,309 milligrams of potassium, while one baked potato, flesh only, has 1,081 milligrams, notes WebMD. One cup of cooked lentils contains 731 milligrams of potassium, and one cooked sweet potato in the skin has 694 milligrams. Three ounces of drained, canned clam solids contain 534 milligrams of potassium.
Potassium is an essential macromineral found in cells and is vital to a steady heart rate, explains WebMD. It is also required for nerve transmission, muscle contraction, and maintaining of proper fluid balance by the kidneys as they filter blood. A study cited by WebMD showed that people with high blood pressure who took potassium supplements lowered their systolic blood pressure.
Potassium levels must be in balance for optimal health. Levels that are both too high and too low are dangerous, cautions WebMD. The U.S. Department of Agriculture recommends 4,700 milligrams per day for healthy people. Because some medications can raise potassium levels and others deplete it, patients on medications should check their levels regularly and consult with their physician before supplementing their diet with potassium.