What do plants naturally do with magnesium and what is the bioavailability in food compared to supplements?

Q:

In what form is magnesium naturally found in food such as sunflower seeds? There are so many different types of magnesium supplements. What I would like to know is what plants naturally do with magnesium and the bioavailability of the magnesium in food compared to supplements. Can you tell the difference in foods that have magnesium and the same foods that don’t?

A:

This is a rather difficult question as the absorption of magnesium from different magnesium-containing compounds by humans is not only biochemically complex but still under investigation as there are many factors, apart from the type of magnesium supplement, that will play a role in this.

In plants, magnesium is typically found as part of the chlorophyll molecule which is why deep green leafy vegetables such as spinach have high levels of magnesium. However, this magnesium is highly water-soluble and can be leached out into water during cooking or processing, thus reducing the magnesium content of the food.

Magnesium is also found in high levels in unrefined whole grains, seeds, and nuts where it plays a role in carbohydrate storage and energy release. In mung beans, for example, about 90 per cent of the total cytoplasmic magnesium concentration is complexed with adenosine triphosphate (ATP). Adenosine triphosphate is the main source of energy in cells but must bind to the magnesium ion in order to be biologically active, thus magnesium is typically present in every cell type.

In some plants, the concentration of magnesium differs depending on soil levels — thus the magnesium content of foods can vary slightly from source to source. The highest sources of plant-based magnesium are in seeds such as pumpkin kernels (307 mg per quarter cup), sunflower seeds (129 mg per quarter cup), and spinach (83 mg per half cup) with an adult requiring 300-400 mg per day. Surprisingly, humans often obtain some magnesium from water supplies, along with calcium, which are common elements found in hard water and many mineral or bottled waters.

With supplements there are no “different forms” of magnesium as magnesium is a chemical element, but there are different complexes to which the magnesium may be part of or bounded to. For example, supplements may be supplied as magnesium citrate, chloride, sulphate, carbonate, gluconate, or oxide as amino acid complexes and many others.

Some of these magnesium supplement forms are designed to be better tolerated by the digestive system, thus improving uptake and reducing side effects, while others provide a more sustained release of the magnesium over time and prevent the occurrence of side effects. Any excess of magnesium taken as supplements is usually just excreted by healthy humans.

Bioavailability is simply a term used to describe how much of the magnesium is typically absorbed by humans and how much is excreted by the urinary system; some magnesium-containing compounds are easier for the magnesium to be absorbed than others. Studies have found that magnesium from food sources such as almonds are just as bioavailable as from soluble magnesium supplements. However, while it is unlikely to obtain excessive levels of magnesium from food sources, this can occur with supplements, which in turn reduces the uptake of calcium in the digestive system.

The only way to tell the difference in foods that have magnesium and those that don’t is to check the food labelling — magnesium is often listed on packaging, particularly in foods where magnesium has been added in either during processing or as nutritional enhancement.

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Written by Lynette Morgan
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Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. Lynette is a partner with Suntec International Hydroponic Consultants and has authored several hydroponic technical books. Visit suntec.co.nz for more information.  Full Bio