For older people in the Netherlands, how much potassium they pee out doesn’t seem to predict whether they’ll have a heart problem or die sooner—even though very low levels might hint at a slightly higher risk. This means a single urine test might not tell us how much potassium they’ve been eating over time.

For people who already have heart disease, the more potassium their body gets rid of in urine, the more likely they are to have a heart attack, stroke, or die — which is the opposite of what most people think potassium does.

For people who already have heart disease, having just the right balance of salt and potassium in their urine — not too much, not too little — seems to be linked to the lowest chance of heart problems or dying from any cause. Too little or too much of this balance is linked to higher risks.

For people with heart disease, the more potassium their body gets rid of in urine, the higher their blood pressure tends to be — every extra gram of potassium excreted per day raises systolic pressure by about 1 point and diastolic by about 0.6 points, which is the opposite of what doctors usually expect.

People who pee out more potassium tend to have lower blood pressure, at least in adults between 20 and 59—but this isn’t true for everyone everywhere.

For older adults who are overweight but don’t have heart disease or high blood pressure, eating more salt than potassium might raise their chance of dying within 5 years by about 19%—so it’s not just how much salt or potassium you eat, but how they balance each other.