Potassium, magnesium and the balance in body water

Water, salts and electrical voltage in tissue

About 60 percent of an adult human's body weight consists of water. This water is not present as a pure liquid but as an aqueous solution of charged particles: sodium, potassium, chloride, magnesium, calcium, phosphate and bicarbonate. In physiology these particles are called electrolytes, because in dissolved form they conduct electric current.

Their distribution is anything but random. Around two thirds of body water lies inside the cells, one third outside — in the interstitial space and in blood plasma. Inside and outside differ markedly in composition: sodium dominates the space outside the cell, potassium the space inside. This gradient is actively maintained by the cell and costs energy continuously.

The sodium-potassium pump and the resting membrane potential

A transport protein sits in the membrane of almost every body cell: sodium-potassium ATPase. Per working cycle it moves three sodium ions out and two potassium ions in, consuming one molecule of ATP in the process. Because more positive charge goes out than comes in, the inside of the cell remains negatively charged relative to the outside. In muscle and nerve cells this voltage difference typically lies between −70 and −90 millivolts and is called the resting membrane potential.

An EU-authorised claim refers to this relationship: “Potassium contributes to normal functioning of the nervous system” — Under Regulation (EU) No 432/2012.

According to estimates, 20 to 30 percent of total ATP turnover in a resting organism is accounted for by this pumping work alone. In nerve and muscle tissue the share is even higher, because there the original ion distribution has to be restored after every signal.

Potassium in depolarisation and repolarisation of the muscle cell

When a nerve signal reaches the muscle fibre, voltage-dependent sodium channels open. Sodium flows in, the voltage briefly tips into the positive — this is depolarisation. Immediately afterwards potassium channels open, potassium leaves the cell and the voltage falls back to its initial value. This repolarisation lasts only a few milliseconds and is the precondition for the fibre being responsive a second time at all.

For this function there are two separate claims, each assigned to one mineral. Potassium contributes to normal muscle function. Magnesium also contributes to normal muscle function. Both claims are listed individually in Regulation (EU) No 432/2012 and apply in each case to the mineral named alone.

Magnesium as the counter-ion of ATP and at the calcium channel pore

Magnesium rarely appears in biochemistry as a free ion. Inside the cell, by far the greatest part is bound to ATP: only the complex of magnesium and ATP is recognised as a substrate by most enzymes. Magnesium is thus indirectly involved in every reaction that splits ATP — including the work of the sodium-potassium pump from the previous section.

A second site of action lies at the pore of certain calcium channels, where magnesium ions can attach and dampen the influx of calcium. In the muscle fibre, incoming calcium is the signal to contract; the interplay of the two ions therefore influences how strongly and how long a fibre stays tensed.

For magnesium, a separate claim on salt balance is also authorised: “Magnesium contributes to electrolyte balance” — Under Regulation (EU) No 432/2012.

Where potassium and magnesium in the daily diet come from

Both minerals occur in ordinary foods, but at very different densities. Potassium is widely distributed: plant cells store it in large amounts, which is why vegetables, fruit and pulses are consistently good sources. Magnesium, by contrast, is concentrated in particular product groups, above all oilseeds, nuts and wholegrain cereals.

Processing changes these values. When cereals are milled, the germ and outer layers go into the bran — and with them most of the magnesium. A type 405 white flour therefore contains only a fraction of what was in the whole grain. Potassium, in turn, is water-soluble: if vegetables are boiled in plenty of water and the cooking water is poured away, a measurable share is lost. Steaming or cooking in their own juices keeps more of it in the food.

On the consumer side, a single reference figure applies across the EU. Nutrition tables and nutrient declarations are calculated against the reference intakes in Annex XIII of Regulation (EU) No 1169/2011 — for potassium that is 2,000 mg per day, for magnesium 375 mg. A food may only be described as a source of a mineral if it supplies at least 15 percent of this reference intake per 100 grams.

What this page does and what it does not

Serpentinhain is an editorial information resource. We reproduce the authorised wording of the EU claims, name the source and describe the biochemical background as precisely as is possible without technical vocabulary. We make no dosage recommendations, assess no preparations and make no diagnoses.

Anyone wanting to know how their own diet is composed will find in our guide tables with content values per 100 grams, a glossary of the technical terms from salt and water balance, and the exact sources of the four claims in the Official Journal of the European Union.