Potassium is the main mineral inside your cells. It controls fluid balance, nerve signalling and muscle contraction, and it works against sodium to regulate blood pressure. Adults in the UK need 3 500 mg a day. Most people get it from food, and the people who need to be careful are those with reduced kidney function or on certain blood pressure medicines.
Potassium is the rare nutrient where the risk runs in both directions. Too little and your muscles cramp and your heart rhythm becomes unreliable. Too much and the same thing happens, faster and more dangerously. That is why the search results for potassium sit oddly between a food page and a blood test page, and why most guides pick one and ignore the other.
This article covers both, with the numbers that apply in Britain: the UK reference intake, what the National Diet and Nutrition Survey found about how many of us fall short, the trial evidence on blood pressure, and the specific situations where a potassium salt substitute on a British supermarket shelf becomes a genuinely bad idea.
What does potassium do for your body?
Potassium is the most abundant cation inside your cells, and sodium is the most abundant one outside them. That difference in concentration across the cell membrane is not a detail. It is a stored electrical charge, maintained continuously by a pump that consumes a large share of your resting energy, and every nerve impulse and every muscle contraction in your body is a controlled discharge of it.
Four consequences follow, and they explain every symptom in this article.
- Nerve signalling. The gradient is what allows a nerve to fire and then reset. Change potassium levels and the timing of that reset changes.
- Muscle contraction. Including the heart, where the electrical sequence has to be identical every beat.
- Fluid balance. Potassium holds water inside cells while sodium holds it outside, and the balance between them sets how much fluid sits in your circulation.
- Bone and kidney stones. A higher potassium intake reduces urinary calcium loss, which is why observational work links it to higher bone density and a lower risk of kidney stones. The trial evidence here is thinner than for blood pressure.
- Blood pressure. Higher potassium intake helps the kidneys excrete sodium and relaxes the walls of blood vessels. This is the mechanism behind the trial evidence below.
Why potassium moves blood pressure
How much potassium do you need, and how many people fall short?
The NHS figure is precise. Adults aged 19 to 64 need 3 500 mg of potassium a day. That is the UK Reference Nutrient Intake, and it sits alongside a Lower Reference Nutrient Intake of 2 000 mg, the level below which intake is considered inadequate for most people.
Britain has real data on how well that is met, and it is not flattering. A secondary analysis of the UK National Diet and Nutrition Survey by Emma Derbyshire, published in Frontiers in Nutrition in 2018, examined 3 238 adults aged 20 to 59. Nearly a quarter of women, 24.3%, had potassium intakes from food below the Lower Reference Nutrient Intake. Across nine micronutrients including potassium, women’s intakes were significantly lower than men’s as a percentage of the reference intake, and adults in their twenties were significantly lower than adults in their thirties, forties and fifties.
of British women aged 20 to 59 had potassium intakes from food below the Lower Reference Nutrient Intake
Derbyshire E., Frontiers in Nutrition, 2018, n = 3 238, National Diet and Nutrition Survey years 1 to 6. DOI: 10.3389/fnut.2018.00055
For scale, the PURE study estimated potassium excretion in 101 945 people across 17 countries and found a mean of 2.12 g per day, which is well under the UK reference intake and gives a sense of how common the shortfall is internationally.
Two important qualifications. This is intake from food, excluding supplements, so it is a measure of dietary quality rather than of anyone being ill. And falling below a reference intake is not the same as having low blood potassium: your kidneys defend the blood level hard, which is precisely why a normal potassium test does not tell you your diet is adequate. Vitamin K has a similar blind spot: the routine test reports clotting, and status in bone is read from uncarboxylated osteocalcin instead.
Which foods are high in potassium?
Potassium is not concentrated in exotic foods. It is spread across ordinary plants, dairy and fish, which is why a diet built on fruit, vegetables, pulses and unprocessed protein reaches the reference intake without anyone trying. Those are also the food groups that carry the largest modelled gain in life expectancy, which makes two pieces of advice into one.
| Food | Typical portion | Potassium, approximate |
|---|---|---|
| Baked potato, skin on | 1 medium, 180 g | about 950 mg |
| Baked beans | half a 400 g tin | about 600 mg |
| Avocado | half, 100 g | about 490 mg |
| Banana | 1 medium | about 420 mg |
| Spinach, cooked | 80 g portion | about 420 mg |
| Lentils, cooked | 100 g | about 370 mg |
| Salmon | 100 g fillet | about 360 mg |
| Dried apricots | 30 g | about 330 mg |
| Milk | 200 ml glass | about 300 mg |
Values are approximate and taken from standard food composition tables. Portion size varies more than the food does, which is the main reason published figures differ.
The practical point is that no single food gets you there. Two bananas supply roughly 840 mg, under a quarter of the daily reference intake. A jacket potato, a portion of beans and a glass of milk supply nearly twice that between them. Cooking matters too: potassium is water soluble, so boiling vegetables and pouring the water away removes a meaningful share of it. This is also, deliberately, how a renal dietitian reduces potassium for someone with kidney disease.
Potassium and blood pressure: what the trials show
This is the outcome with the most evidence, and the evidence is unusually clean because potassium can be given as a supplement in a blinded trial.
Aburto and colleagues, in the BMJ in 2013, pooled 22 randomised controlled trials covering 1 606 participants for blood pressure, alongside 11 cohort studies covering 127 038 adults for disease outcomes. Increased potassium intake reduced systolic blood pressure by 3.49 mmHg, with a confidence interval of 1.82 to 5.15, and diastolic pressure by 1.96 mmHg. The effect was concentrated in people with raised blood pressure, which is the pattern you would expect and the pattern that makes it useful.
Dietary pattern trials point the same way. The DASH trial enrolled 459 adults and fed them for eight weeks. The combination diet, rich in fruit, vegetables and low-fat dairy, reduced systolic pressure by 5.5 mmHg and diastolic by 3.0 mmHg more than the control diet, with sodium intake and body weight held constant. Among the 133 participants who had hypertension, the reductions were 11.4 mmHg systolic and 5.5 mmHg diastolic. Potassium is not the only thing that changed in that diet, but it is one of the largest changes.
Now the finding nobody repeats, and the one that should stop anyone reaching for a high-dose supplement. Filippini and colleagues, in the Journal of the American Heart Association in 2020, modelled the dose-response relationship across 32 randomised trials using doses from 30 to 140 mmol per day. The relationship is U-shaped, not linear. The blood pressure benefit weakens above a difference of about 30 mmol per day in potassium excretion, and above roughly 80 mmol per day blood pressure starts to rise again. That rise appeared in participants on antihypertensive medication and not in their untreated counterparts.
Systolic blood pressure change, and stroke rate with a potassium salt substitute
Salt substitutes: the biggest potassium decision on a British shelf
Reduced-sodium salts sold in UK supermarkets replace part of the sodium chloride with potassium chloride. They are the most consequential potassium product most people will ever buy, and almost nobody reads them as a potassium decision.
The evidence for them is strong. SSaSS, published by Bruce Neal and colleagues in the New England Journal of Medicine in 2021, was an open-label cluster-randomised trial across 600 villages in rural China, enrolling 20 995 people who had either had a stroke or were 60 or over with high blood pressure. Villages were randomised to a substitute of 75% sodium chloride and 25% potassium chloride, or to ordinary salt. Over a mean 4.74 years, stroke fell from 33.65 to 29.14 events per 1 000 person-years, a rate ratio of 0.86. Major cardiovascular events and death from any cause both fell as well.
The safety result matters just as much. Serious adverse events attributed to hyperkalaemia were not significantly more common with the substitute, at 3.35 against 3.30 events per 1 000 person-years.
So why the caution? Because that trial excluded the people most at risk. The UK Scientific Advisory Committee on Nutrition has advised that potassium-based sodium replacers are not suitable for people at risk of hyperkalaemia, naming reduced kidney function and medicines including ACE inhibitors and potassium-sparing diuretics. In a population where ramipril and spironolactone are among the most prescribed drugs, that exclusion is not a small group.
The practical rule is simple. If you have healthy kidneys and are not on any of those medicines, a potassium salt substitute is one of the better-evidenced swaps available in a supermarket. If you have chronic kidney disease, heart failure, or a prescription that includes an ACE inhibitor, an angiotensin receptor blocker or a potassium-sparing diuretic, ask your GP or pharmacist before you switch, and do not assume that a product labelled as a healthier salt is healthier for you.
What happens when your potassium is low?
Low blood potassium is called hypokalaemia, spelled hypokalemia in American sources. Most UK laboratories report a reference range of roughly 3.5 to 5.3 mmol/L, though ranges vary between labs, and it is the lab’s own range printed on your result that counts.
The symptoms follow directly from the electrical role: muscle weakness, cramp, fatigue, constipation as the gut muscle slows, and in more serious cases an irregular heartbeat. The question that gets asked in a strikingly specific way is what potassium does to your legs, and the answer is that legs are where you notice it first, because the largest muscles under the heaviest load complain earliest. Night cramps and heavy, weak legs are the classic presentation.
What causes it is rarely diet. Your kidneys are efficient at conserving potassium, so a poor diet on its own seldom drives the blood level down. The usual causes are losses and drugs: prolonged vomiting or diarrhoea, and above all diuretics. Thiazide and loop diuretics such as bendroflumethiazide and furosemide, both heavily prescribed in the UK for blood pressure and heart failure, push potassium out in the urine. That is why anyone on them has their urea and electrolytes checked periodically.
Low magnesium is worth mentioning because it is routinely missed. Potassium will not correct while magnesium is low, no matter how much potassium is given, which is why a clinician treating stubborn hypokalaemia checks magnesium as well.
What happens if potassium is high?
High potassium in the blood is hyperkalaemia, hyperkalemia in American sources, and it is the more dangerous direction. It is also quieter. Mild hyperkalaemia often produces no symptoms at all, and when symptoms do appear they are vague: muscle weakness, tingling, fatigue, sometimes nausea. Untreated, a high potassium level can lead to a dangerous heart rhythm. The first real sign can be a change on an ECG, because the cardiac conduction system is the tissue least tolerant of the change.
Diet is almost never the cause in someone with working kidneys. Healthy kidneys excrete a large potassium load without difficulty. Hyperkalaemia is a signal that something else is wrong, and the list is short.
| Cause | Why potassium rises | Common in the UK |
|---|---|---|
| Chronic kidney disease | Reduced ability to excrete potassium in the urine | The single most common reason |
| ACE inhibitors and ARBs | Reduced aldosterone signalling, so less potassium is excreted | Ramipril, lisinopril, losartan, candesartan |
| Potassium-sparing diuretics | They act specifically to retain potassium | Spironolactone, amiloride, eplerenone |
| NSAIDs | Reduced renal blood flow and altered potassium handling | Ibuprofen, naproxen, including over the counter |
| Potassium supplements and salt substitutes | A direct load on top of any of the above | Reduced-sodium salts, effervescent potassium |
| A haemolysed sample | Potassium leaks from damaged red cells in the tube, not in the patient | A common reason a result is repeated |
That last row is worth knowing before you panic at a result. A difficult blood draw can damage red cells in the sample, which releases their potassium and produces a high reading in a person whose potassium is normal. A repeat test is the standard response, not a treatment.
Supplements, tests, and when to speak to your GP
The NHS position on supplements is specific and it is stricter than most product labels suggest. Taking too much potassium can cause stomach pain, nausea and diarrhoea. Taking 3 700 mg or less a day in supplement form is unlikely to have obvious harmful effects in a healthy adult. And older people are advised not to take potassium supplements at all unless a doctor has told them to, because kidney function declines with age and the ability to clear a potassium load declines with it.
Set that against the Filippini dose-response finding and a clear picture emerges. Potassium is a nutrient where food and supplement behave differently. Food delivers it slowly, packaged with water and fibre, alongside magnesium and nitrate and everything else in a plant. A supplement delivers a bolus. The trials that found benefit mostly used modest doses, and the trials that found blood pressure rising again used large ones in people already on medication.
A potassium test is a routine part of the urea and electrolytes panel your GP orders. If you take a diuretic, an ACE inhibitor, an ARB or spironolactone, or if you have chronic kidney disease, you are almost certainly already being monitored. If you are not on any of those and you have no kidney problem, there is no reason to seek out a potassium test because you read about cramp.
See your GP promptly rather than adjusting your diet if you have persistent muscle weakness, palpitations or an irregular heartbeat, if you have a kidney condition and are thinking about a salt substitute, or if you are on any of the medicines listed above and have been offered a potassium supplement by someone who is not your prescriber.
One clarification worth making, because it is a common wrong turn. If aching legs and stiff joints are what sent you looking at minerals in the first place, an electrolyte is rarely the answer. Persistent joint pain is usually a low-grade inflammatory process rather than a deficiency, and it responds to different things: a well-absorbed anti-inflammatory formula works on that terrain, and targeted support for ageing joints is built around it. Neither has anything to do with potassium, and neither replaces a GP appointment for pain that does not settle.
The summary is short. Eat the potassium, do not swallow it. Get it from potatoes, pulses, greens, fish and dairy, in ordinary quantities, every day. If you are on a blood pressure or heart medicine, treat any concentrated potassium product, supplement or salt substitute alike, as something to clear with your pharmacist first.
Frequently asked questions
What does potassium do for your body?
It is the main mineral inside your cells and it maintains the electrical gradient across the cell membrane. That gradient powers nerve signalling and muscle contraction, including the heartbeat. Potassium also balances sodium in regulating fluid and blood pressure: higher intake helps the kidneys excrete sodium and lowers systolic pressure by around 3.5 mmHg on average.
What happens when your potassium is low?
Hypokalaemia causes muscle weakness, cramp, fatigue and constipation, and in more serious cases an irregular heartbeat. Diet is rarely the cause because the kidneys conserve potassium well. The usual causes are prolonged vomiting or diarrhoea and diuretics such as bendroflumethiazide or furosemide. Low magnesium prevents potassium from correcting, so it is checked alongside.
What happens if potassium is high?
Hyperkalaemia is often silent, and when symptoms appear they are vague: weakness, tingling, fatigue, nausea. It is dangerous because the heart’s conduction system is the least tolerant tissue. In someone with working kidneys, diet is almost never the cause. Chronic kidney disease, ACE inhibitors, potassium-sparing diuretics and NSAIDs are the usual reasons, and a damaged blood sample can produce a falsely high reading.
Which food is high in potassium?
A baked potato with the skin on is the best everyday source, at roughly 950 mg per medium potato. Half a tin of baked beans supplies about 600 mg, half an avocado about 490 mg, a banana and a portion of cooked spinach about 420 mg each. Boiling and draining vegetables removes a meaningful share, because potassium is water soluble.
Is 2 bananas a day enough potassium?
No. Two medium bananas supply roughly 840 mg, which is under a quarter of the 3 500 mg adults in the UK need each day. The banana has a reputation it does not deserve: a single jacket potato beats two bananas. Reaching the reference intake takes potassium across several meals rather than one famous fruit.
What does potassium do to your legs?
Legs are usually where a potassium problem is noticed first, because the largest muscles under the heaviest load complain earliest. Low potassium typically produces night cramps, heaviness and weakness in the legs. Persistent leg cramp is worth mentioning to your GP, particularly if you take a diuretic, rather than treating with a supplement.
Should I take a potassium supplement?
Usually not. The NHS advises that older people should not take potassium supplements unless a doctor tells them to, and that up to 3 700 mg a day is unlikely to cause obvious harm in a healthy adult. A dose-response meta-analysis of 32 trials found the blood pressure benefit weakening at higher intakes and reversing at high ones in people already on antihypertensive medication. Food is the better route.
Sources
- Aburto N.J., Hanson S., Gutierrez H., Hooper L., Elliott P., Cappuccio F.P. Effect of increased potassium intake on cardiovascular risk factors and disease: systematic review and meta-analyses. BMJ, 2013. DOI: 10.1136/bmj.f1378
- Neal B., Wu Y., Feng X., et al. Effect of salt substitution on cardiovascular events and death. New England Journal of Medicine, 2021. DOI: 10.1056/NEJMoa2105675
- Filippini T., Naska A., Kasdagli M.I., et al. Potassium intake and blood pressure: a dose-response meta-analysis of randomized controlled trials. Journal of the American Heart Association, 2020. DOI: 10.1161/JAHA.119.015719
- Appel L.J., Moore T.J., Obarzanek E., et al. A clinical trial of the effects of dietary patterns on blood pressure. New England Journal of Medicine, 1997. DOI: 10.1056/NEJM199704173361601
- O’Donnell M., Mente A., Rangarajan S., et al. Urinary sodium and potassium excretion, mortality, and cardiovascular events. New England Journal of Medicine, 2014. DOI: 10.1056/NEJMoa1311889
- Derbyshire E. Micronutrient intakes of British adults across mid-life: a secondary analysis of the UK National Diet and Nutrition Survey. Frontiers in Nutrition, 2018. DOI: 10.3389/fnut.2018.00055
- Whelton P.K. Effects of oral potassium on blood pressure: meta-analysis of randomized controlled clinical trials. JAMA, 1997. DOI: 10.1001/jama.1997.03540440058033
This article is general information, not medical advice. If you have kidney disease, heart failure or high blood pressure, or take diuretics, ACE inhibitors, angiotensin receptor blockers or potassium-sparing diuretics, speak to your GP or pharmacist before changing your potassium intake or using a salt substitute.
