What are the signs of potassium deficiency?

Mild low potassium causes constipation, fatigue, muscle weakness and malaise. Severe cases cause paralysis, poor respiration and cardiac arrhythmias.

Who is most likely to run low

In people whose kidneys cannot clear potassium, even intakes below the recommended amount can cause dangerous hyperkalemia.

What this means for you

Mild low potassium causes constipation, fatigue, muscle weakness and a general flatness. Severe cases go somewhere else entirely: paralysis, poor breathing and dangerous heart rhythms. The gap between those two descriptions is why this is a blood test question rather than a self-diagnosis one.

Here is the part that surprises people. Most people in the United States take in less potassium than recommended, 3,016 mg a day in men and 2,320 in women against targets of 3,400 and 2,600, and almost none of them have low blood potassium. Diet alone rarely causes it in a healthy person, because the kidneys hold on to potassium well.

Unsettled. Which target you are being counted against changes the size of that shortfall. The World Health Organization suggests at least 3,510 mg a day for adults, against the United States adequate intake of 3,400 for men and 2,600 for women. Two bodies, two numbers, both current. WHO also says in the same recommendation that it does not mean potassium from supplements in tablet form or from salt substitutes, which is a useful thing to know before anyone sells you either.

Established. If a low reading is the reason you are here, a supplement off the shelf is not the answer to it. Manufacturers hold potassium supplements at 99 mg a unit, about 2 per cent of the Daily Value, because of concerns tied to potassium-containing drugs. Real low blood potassium is treated by finding what is draining it, and by prescription doses under supervision when it needs them.

Established. Actual low blood potassium usually comes from losing it rather than from not eating it: vomiting, diarrhoea, diuretics, laxative overuse, or some kidney conditions. That is why the fix is to find the cause rather than to eat more bananas.

Diuretics are the commonest of those causes, and the dose decides it. Twenty-five milligrams of hydrochlorothiazide added about 2.5 episodes of low potassium per hundred people over eight weeks, and half that dose added none that could be measured. That is a conversation with whoever prescribed it rather than a reason to buy supplements.

What a blood test actually tells you

The comfortable range is narrow. Blood potassium between 4 and 4.5 millimoles per litre carries the lowest risk of dying, and a low reading is worse than a high one. Under 3.5 the heart starts to show it. In people wearing an implanted monitor, atrial fibrillation was more than twice as likely to appear on the days their potassium had dipped below that line.

Unsettled. The blood test and the dinner plate tell opposite stories, and that catches people out. In the same review, higher potassium in the blood went with 23 per cent higher mortality, while higher potassium in the urine, which is the better mirror of what somebody ate, went with 16 per cent lower. A high reading is usually a sign that clearance has gone wrong rather than a sign of a good diet. Nobody has untangled the two well enough for you to read your own result as a report card on your eating.

The other direction matters more for some people

Not for everyone. If your kidneys cannot clear potassium, even intakes below the recommended amount can push blood levels into a dangerous range. ACE inhibitors, angiotensin receptor blockers and potassium-sparing diuretics all reduce how much potassium leaves in urine. If you take any of those, or have kidney disease, salt substitutes and potassium supplements are a question for your doctor before they are a health habit.

For everyone else, raising potassium from food is worth doing and easy. Potatoes with the skin, beans and lentils, spinach and chard, yogurt, tomato products, avocado, dried apricots and, yes, bananas. Whole food is also the form that comes packaged with the fibre and magnesium that push blood pressure the same direction.

Most people arrive at potassium through blood pressure rather than through deficiency, so the size of that effect is worth knowing. In a 2017 meta-analysis of 25 randomised trials in people with high blood pressure, potassium supplementation lowered systolic pressure by 4.48 mmHg. Food is the sensible way to get there. It arrives with the fibre and the magnesium that push the same direction, and without the risk that a concentrated dose carries for anyone whose kidneys are struggling.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Where to get it from food

• Carrot, dehydrated — 1879.6 mg per 1 cup (40% DV)
• Apricots, dried, sulfured, uncooked — 1510.6 mg per 1 cup, halves (32% DV)
• Bananas, dehydrated, or banana powder — 1491 mg per 1 cup (32% DV)
• Beet greens, cooked, boiled, drained — 1309 mg per 1 cup (1" pieces) (28% DV)
• Cabbage, japanese style, fresh, pickled — 1279.5 mg per 1 cup (27% DV)

All potassium sources ranked →

Full guide: Potassium: the mineral most people under-eat, and the one some people must limit