Marine magnesium: why should you avoid it?
Neil Goffart
Neil Goffart
More easily absorbed by the body, effective, completely safe… You have no doubt read countless articles praising the benefits of marine magnesium. But did you know that most of those claims are in fact misleading? Marine magnesium is far from as perfect as people believe, and we will explain why right away, through 4 common misconceptions.
You have probably already worked it out: marine magnesium is simply another form of magnesium, but one that comes from sea water. What matters for us is therefore to understand what makes it special, and we will answer that through two questions:
At first glance, given its high elemental magnesium content, it may seem useful to suggest it to correct a magnesium shortfall. Many people, however, struggle to tolerate it. This nutrient is difficult to absorb when you are prone to digestive problems, as it is often responsible for stomach ache and diarrhoea. This is why marine magnesium is used in cases of constipation. However, if you have a magnesium deficiency, taking a marine magnesium supplement is not advisable, as it will not be the right nutrient. Its poor bioavailability does not favour cellular absorption.
Marine magnesium is only better absorbed, at least better than magnesium on its own, provided it is taken in combination with B-group vitamins, particularly vitamin B6 or B9. What is vitamin B6 for? It helps the body absorb magnesium. Which clearly shows that marine magnesium is no better than any other. You may as well opt for another bioavailable second- or third-generation magnesium salt, with better digestive tolerance.
This is of course untrue: the side effects of marine magnesium are very real. The very reasons that wrongly earn it the label "a nutrient rapidly absorbed by the body" also make it a magnesium that loses its effects just as quickly. Out of necessity, you therefore have to take it regularly for it to work sufficiently. The inevitable consequence is an overdose of a mixture of inorganic salts, which brings on side effects (digestive irritation, bloating, diarrhoea, etc.)
Taking too large a quantity can create a risk of muscle weakness, nausea and vomiting or diarrhoea. Some people also experience intolerance to marine magnesium, which is why it is advisable to consult a specialist before any supplementation. The adverse effects usually fade quickly, but should the digestive symptoms persist, you must see a doctor immediately.
Following these side effects, the danger for the body is dehydration and a significant loss of minerals. Indeed, if you have kidney failure, too high a dose of marine magnesium can lead to arrhythmia. Finally, let us not forget that the quality of marine magnesium depends on the quality of the sea water, which declines as water pollution increases.
Although it is often used for fatigue, taking marine magnesium must not become a reflex. Because as with certain other mineral intakes (iron, copper, etc.), marine magnesium should only be used when specific symptoms are present. You also need to make sure the body is not short of calcium, in order to maintain the magnesium/potassium/calcium balance. An overdose of marine magnesium could be dangerous for anyone already on diuretic treatment, in other words treatment for high blood pressure.
Marine magnesium has both the advantage and the drawback of being natural. You can always hope to find it in certain seafood products, such as seaweed or shellfish, which are the richest in magnesium, but certainly not in sufficient quantities to meet your needs. Nor is it synthesised by the body. So there is no other choice than to turn to pharmacies, which do not always stock it, or to online food supplement retailers. For all the reasons set out above, however, we recommend avoiding marine magnesium supplementation as far as possible and asking a specialist for advice.
What should you take away from all this false information? In short, marine magnesium is a pure nutrient, but one overrated by the media: it is not as easily absorbed as you might think and should not be recommended long term without medical supervision. Watch out for unwanted effects, particularly if you have blood pressure or digestive problems. If you notice the symptoms of a magnesium shortfall, ask yourself instead which one to choose.
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FREQUENTLY ASKED QUESTIONS
Marine magnesium generally refers to magnesium obtained from raw materials of marine origin, but that name does not correspond to a single chemical form. A supplement may contain different magnesium salts, including the oxide, the hydroxide or the chloride, depending on its manufacturing process. European regulations authorise many forms of magnesium and do not define a separate nutritional category called “marine magnesium”. To assess a product, you therefore need to look at the chemical form actually stated in its composition.
That depends mainly on the magnesium salts present in the product. Several human trials show that magnesium citrate has a higher bioavailability than the oxide. In one randomised trial, the citrate led to higher urinary excretion and higher plasma magnesium concentrations than those observed with the oxide. This does not, however, allow us to state that all magnesium of marine origin is poorly absorbed: its exact composition must be known before any comparison.
Yes, high doses of magnesium from supplements can lead to loose stools or osmotic diarrhoea, whether of marine origin or not. The former European Scientific Committee set an upper level of 250 mg/day for magnesium from supplements and other non-food sources, with diarrhoea as the critical effect retained. That value does not apply to the magnesium naturally present in foods. Tolerance also varies with the form, the dose and individual sensitivity.
It is better to look at the chemical form, the amount of elemental magnesium provided and digestive tolerance than to rely on expressions such as “marine” or “natural”. In several human trials, the citrate showed a higher bioavailability than the oxide. This does not, however, allow a universal ranking to be established between all the available forms, since few studies compare them directly under identical conditions. The origin of the raw material is therefore not, on its own, an indicator of bioavailability.
Caution applies above all to high doses, people with impaired kidney function and combinations with certain treatments. The kidneys play a major role in maintaining magnesium balance; when their function is severely reduced, eliminating an excess becomes less effective. In Belgium, the current regulatory maximum is 450 mg of magnesium per recommended daily portion, but this regulatory value must not be confused with the European UL of 250 mg/day established on the basis of the digestive effects of supplements.
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