Should I take a potassium supplement?

Ask the doctor

potassium melon
A cup of cantaloupe contains about 400 mg of potassium.
Image: Arisara_Tongdonnoi / Thinkstock

Q. I have high blood pressure, and a friend recommended that I take a potassium supplement. Is that a good idea, and if so, how much should I take?

A. This is a great question that comes up all the time—and with good reason, because potassium can be tricky. The short answer is no, you should not take potassium supplements unless your doctor prescribes them. Let me outline why below.

To start with, you're much better off getting potassium from foods instead of pills. Many fruits and vegetables are rich in potassium, including spinach, sweet potatoes, cantaloupe, bananas, and avocado. (For a chart of foods high in potassium, see www.health.harvard.edu/100.) Potassium-rich diets help control blood pressure and have been linked to a lower risk of stroke. But such diets also tend to be lower in sodium and contain other healthful nutrients, which may contribute to the observed blood pressure benefit.

Here's where it gets a little confusing. Many blood pressure medications—especially the commonly prescribed class known as diuretics—can affect your potassium level. But while some diuretics tend to lower potassium levels, others have the opposite effect. And certain ACE inhibitors, such as lisinopril (Prinvil, Zestril) or ramipril (Altace), may also raise potassium levels. So can common painkillers such as ibuprofen (Advil, Motrin)or naproxen (Aleve).

Keeping your blood potassium level in the correct range is important, because this mineral also plays a key role in the function of nerves and muscles, including heart muscle. Your kidneys help regulate potassium levels in your blood. But age, diabetes, heart failure, and certain other conditions may impair kidney function. As a result, potassium levels can rise to high levels, leading to dangerous heart rhythm problems and even cardiac arrest.

Because of this potential danger, the FDA limits over-the-counter potassium supplements (including multivitamin-mineral pills) to less than 100 milligrams (mg). That's just 2% of the 4,700 mg recommended dietary intake for potassium. You'd have to take lots of pills to get close to that amount—another reason to get the nutrient from your diet.

However, grocery stores carry salt substitutes that may contain much higher amounts of potassium. People trying to curb their sodium intake may try these products. A mere one-quarter teaspoon of one brand contains about 800 mg of potassium. If you take a potassium-sparing diuretic, such as spironolactone, you should avoid salt substitutes and limit high-potassium foods.

However, if you take a diuretic that depletes potassium levels, such as hydrochlorothiazide or furosemide, your doctor may prescribe extended-release potassium tablets, which contain 600 to 750 mg of the mineral. And if you take any diuretic or ACE inhibitor, ask your doctor whether you need periodic testing of your potassium and kidney function, to be on the safe side.

-- Deepak Bhatt, MD, MPH
Editor in Chief, Harvard Heart Letter