Two electrolytes, one pump

Every cell in the body holds potassium inside and pushes sodium out. The gradient this creates is the battery that nerve cells discharge to send a signal and muscle cells discharge to contract — including the heart. The machine that maintains it is the sodium-potassium ATPase, usually just called the sodium-potassium pump, and it accounts for a large share of the energy your body burns at rest.

The pump is powered by ATP. ATP, however, is not biologically active on its own: the form enzymes actually use is Mg-ATP, with a magnesium ion bound to the phosphate tail. No magnesium, no usable ATP, no pump. A comprehensive review in Physiological Reviews describes magnesium as a required cofactor for essentially every ATP-dependent reaction in the cell.

de Baaij, Hoenderop & Bindels, 2015, Physiological Reviews — PubMed 25540137

What happens when magnesium is low

Two things go wrong at once. Inside cells, the pump slows and potassium leaks out through channels that magnesium would normally keep partly closed. In the kidney, the same channels in the tubule cells open up, and potassium that should be reabsorbed is lost in urine. The result is low blood potassium that does not respond to potassium supplements alone — the potassium goes in and straight back out again.

This is a well-described clinical picture. A review in the Journal of the American Society of Nephrology set out the mechanism and concluded that hypokalaemia caused by magnesium deficiency is refractory to potassium replacement until the magnesium is corrected.

Huang & Kuo, 2007, Journal of the American Society of Nephrology — PubMed 17804670

Why both are short in a modern diet

The two minerals come from the same foods: green vegetables, beans, nuts, seeds, potatoes, fruit. Processing strips them and replaces them with sodium. The UK recommendation for potassium is 3,500mg a day and for magnesium 300mg for men and 270mg for women, and national diet survey data has consistently shown a large share of adults, particularly younger adults, falling below both. An analysis in Advances in Nutrition makes the same point about potassium across Western diets generally.

Weaver, 2013, Advances in Nutrition — PubMed 23674806

Both are also lost in sweat, and both are depleted by loop and thiazide diuretics. So the groups who are short of one are, more often than not, short of the other.

Potassium and blood pressure — what is authorised and what isn't

Potassium helps the kidneys excrete sodium and relaxes the smooth muscle in blood-vessel walls. The largest analysis of controlled trials, published in the BMJ, found that higher potassium intake was associated with lower blood pressure in people with hypertension and no adverse effect on blood lipids, hormones or kidney function in healthy adults.

Aburto et al., 2013, BMJ — PubMed 23558164

On a UK product, that evidence translates into one authorised claim and no more: potassium contributes to the maintenance of normal blood pressure. "Lowers blood pressure" and "for high blood pressure" are not authorised, and a supplement that used them would be making a medicinal claim. If you take blood pressure medication, potassium is something to discuss with your GP before starting, because several of those medicines raise potassium themselves.

The thirteen authorised claims

These are the health claims authorised for the two minerals on the GB Nutrition and Health Claims Register, in the register's wording. A product may only carry them if it supplies at least 15% of the NRV per daily serving.

Notice what is not there: cramps, sleep, anxiety, heart health, restless legs. Those are the things people search for, and there is research on several of them, but none has been authorised as a claim. A UK seller who tells you a magnesium-potassium capsule fixes cramps is guessing on your behalf.

Choosing the forms

Potassium: citrate vs chloride

Potassium chloride is what most prescription potassium is, and it is cheap, but it is acidifying and often irritating to the stomach. Potassium citrate is the form found in fruit and vegetables; citrate is metabolised to bicarbonate, so it is mildly alkalising, and it is much better tolerated with food. For a supplement, citrate is the sensible choice.

Magnesium: glycinate, and what "glycinate" means on a label

Magnesium oxide alone has the most magnesium by weight but is poorly absorbed and laxative. Magnesium citrate absorbs well but is also laxative at higher amounts. Glycinate forms pair magnesium with the amino acid glycine and are the gentlest on the gut.

One thing worth knowing: not every "magnesium glycinate" is the same material. A fully reacted magnesium bisglycinate chelate is one magnesium to two glycines and contains about 14% magnesium. A buffered glycinate combines magnesium oxide with glycine at roughly one to one and contains about 18%. The buffered form fits more magnesium into a capsule; the chelate is the purer chemistry. Neither is wrong, but the label should tell you which one you are getting.

How much, and when

Take them with food. Potassium salts on an empty stomach can cause nausea, and both minerals are better absorbed with a meal. Splitting the daily amount across two meals is slightly better for potassium, because it keeps the amount reaching the gut at any one time lower. Timing beyond that does not matter much; some people prefer magnesium in the evening.

Putting it in one capsule

We formulated our Magnesium & Potassium Citrate Capsules 1000mg on exactly the logic above: two-thirds tri-potassium citrate, one-third magnesium glycinate, in a large 1,000mg capsule so the potassium reaches a useful level. Two capsules give 480mg potassium (24% NRV) and 120mg magnesium (32% NRV), which qualifies the product for every claim in the table. The magnesium is the buffered 18% glycinate, declared as such on the label. No fillers, no flow agents, filled at our own facility in Essex.

If you only want one mineral, we also sell Magnesium Glycinate 650mg and Potassium Citrate Pro 900mg on their own.

Frequently asked questions

Can I just eat more bananas?
Yes, and you should. A banana is about 400mg of potassium and 30mg of magnesium. A supplement is for the gap that is left when diet, sweat or medication has opened one.

Is it safe to take magnesium and potassium at the same time?
For a healthy adult, yes — they come together in food. The exceptions are the kidney and medication groups above.

Why does my potassium never seem to come up?
If a blood test keeps showing it low despite supplements, ask for magnesium to be checked. Low magnesium is the classic reason potassium replacement fails.

Which magnesium is best?
For tolerability, a glycinate. For the most magnesium per capsule, a buffered glycinate or citrate. Oxide alone is the one to avoid unless you want the laxative effect.

Does potassium help with cramps?
Neither cramps nor sleep is an authorised claim for either mineral in the UK. The authorised wording is normal muscle function.

This article is educational and is not medical advice. Food supplements are not medicines and are not a substitute for a varied and balanced diet and a healthy lifestyle. Only the authorised claims quoted above are claims for any product; the research described is background, not a benefit claim. Consult your GP before taking potassium if you have kidney problems or take any medication. Charge Products is the trading name of Big Idea Services Ltd, Company No. 11645389, Southend-on-Sea, Essex.