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The mystery at the core of magnesium deficiency

QUESTION: Taking magnesium supplements seems to be common for a variety of reasons, but are many people actually deficient in this mineral?

ANSWER: Magnesium plays a part in more than 300 important human enzyme systems, is abundant in the body and is found in many everyday foods. Yet, despite its crucial role in such processes as making proteins, muscle and nerve function, blood glucose control and blood pressure regulation, we know little about the population’s magnesium status.

The body needs this multitasking mineral for energy production, including the breakdown of glycogen into glucose molecules, the structural development of bones, and synthesis of DNA, RNA and certain antioxidants. It is also involved in the active transport of calcium and potassium ions across our cell membranes, a process that is critical to nerve-signal conduction, muscle contraction and even ensuring our heart beats with a normal rhythm.

With so much at stake, having a good handle on magnesium status would seem a no-brainer. However, the difficulty is most of the available magnesium in the body is stored inside cells or bone. Of the 25g of magnesium in an adult’s body, more than half – 50-60% – is in the bones and most of the rest in soft tissues. Less than 1% is estimated to be circulating in the blood.

Generally, magnesium status is measured by checking the levels of the mineral in the blood serum, even though those levels bear little relation to the body’s stores. Researchers do use other methods to assess magnesium status but none of these is very satisfactory.

So, are many of us deficient in magnesium? We have no real idea. Nutrition scientists such as Australian Tim Crowe seem to think it’s unlikely.

Symptoms of magnesium deficiency are uncommon because in otherwise healthy adults with low dietary intake, the kidneys would limit urinary excretion of the mineral. The picture is different among hospital patients, however, 9-65% of whom are reportedly low in magnesium, with those in intensive care particularly affected.

A 2018 UK study found that about one in five British adults in their twenties had magnesium intakes below recommended levels. But whether New Zealanders are consuming enough magnesium remains a mystery.

The last National Nutrition Survey, more than a decade ago, didn’t assess magnesium intake. And there’s no indication that the Ministry of Health plans to conduct another survey to revise our outdated national nutrition information.

Low levels of magnesium have been associated with a greater risk of various chronic diseases including migraine headaches, Alzheimer’s disease, stroke, hypertension, cardiovascular disease and type 2 diabetes. But “association” isn’t proof of cause and effect.

Clinical trials suggest that magnesium supplementation may be beneficial in treating, among other things, pre-eclampsia, migraines, depression, coronary artery disease, and asthma, Dutch researchers noted in a 2015 review.

New Zealand guidelines recommend a dietary intake (RDI) level of 420mg a day of magnesium for men aged 31 and over and 320mg a day for women of the same age.

Magnesium is widely distributed in plant and animal foods and even in beverages such as water. Drinking water is estimated to provide about 10% of daily magnesium needs. Green leafy vegetables such as spinach, legumes, nuts, seeds and whole grains are good sources.

Foods containing dietary fibre typically contain magnesium, whereas processed foods, such as refined grains that have lost the nutrient-rich germ and bran layer, have smaller amounts of the mineral.

So, a diet that emphasises whole foods over processed products will give you the best chance of getting the important nutrients, including magnesium, that the body needs.

This article was first published in the February 8, 2020 issue of the New Zealand Listener.

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