Premise
Only a minority fraction of the magnesium taken through food and supplements is actually absorbed by the human intestine; to make the idea, we are talking about a percentage between 10 and 50%.
The breadth of this interval is explained by the numerous factors capable of influencing the absorption of magnesium.
Magnesium deficiencies
Magnesium deficiency seems to offer an important contribution in the onset of various pathologies: heart and vascular diseases (ischemic heart disease, cardiac arrhythmias, hypertension, atherosclerosis), osteoporosis and asthma.
Magnesium deficiencies have also been linked with dysmenorrhea and PMS.
In the picture: Some of the Foods Highest in Magnesium
According to some surveys, subclinical magnesium deficiencies involve about 70% of the Western population, both men and women; consequently, knowing the factors that can favor (or hinder) the absorption of magnesium is very important.
NOTE: A subclinical deficiency is a mild deficiency, not sufficient to trigger the classic deficiency symptoms, but still potentially implicated in the onset of various disorders.
In other words, for many people the dietary intake of magnesium is sufficient to avoid a condition of deficiency (hypomagnesemia), but it does not allow to establish a normal plasma concentration that is protective against various pathologies.
How Absorption Happens
The absorption of magnesium occurs in the intestine, from the duodenum (initial part of the small intestine) to the colon (large intestine).
- 40% of the magnesium taken is absorbed in the small intestine, especially in the intermediate and distal tracts (jejunum and ileum);
- 5% of the magnesium is absorbed in the large intestine;
- 55% is lost in faeces.
As mentioned in the introductory part, these variables are however indicative and in some cases the overall absorption percentage can be even lower than 20%.
Before being absorbed, however, magnesium must "be" separated "from the substances to which it is bound; this passage takes place in the stomach thanks to" hydrochloric acid (not surprisingly, patients who take proton pump inhibitors are exposed to a increased risk of hypomagnesaemia).
The absorption of magnesium can exploit two mechanisms:
- passive diffusion: passage of magnesium from the point of higher concentration to that of lower concentration, using specific "channels" (TJ tight junction paracellular proteins).
- facilitated diffusion: it involves a specific transport protein (abnormal channel proteins TRPM6 and TRPM7) which "catches" the magnesium in the apical portion and transports it to the basolateral portion of the enterocyte, from which it passes into the interstitial fluid and from there to the blood.
The facilitated diffusion mechanism, typical of the colon, is
- saturable (as the availability of carriers has a limit)
- inducible (the synthesis of these transporters increases in conditions of magnesium deficiency, ie when the dietary intake is scarce and the organism does not have sufficient reserves).
Once the mineral has been absorbed in the intestine, the task of regulating the body's reserves of magnesium is mainly entrusted to the kidney, which acts on the amount of the mineral excreted in the urine, using mechanisms similar to those just described for the intestine.
Magnesium homeostasis in the human body depends on the balance between intestinal absorption and renal excretion (and reabsorption).
Low plasma concentrations of Mg2 + induce greater intestinal absorption and lower excretion at the renal level, while high plasma concentrations of Mg22 + lead to the inhibition of reabsorption in the distal convoluted tubule (renal recovery mechanism).
A small amount of magnesium is lost with sweating.
Bones contain more than half of the body's magnesium and, if necessary, it is also possible to draw on this "reserve", favoring mobilization rather than deposition.
Absorption of the magnesium contained in supplements
In general, supplements containing organic magnesium salts
such as magnesium lactate, magnesium citrate, magnesium succinate, magnesium aspartate, magnesium glycinate, magnesium taurinate, magnesium diglycinate, magnesium fumarate, magnesium ascorbate etc.
are absorbed in higher percentages than supplements based on inorganic magnesium salts:
such as magnesium chloride, magnesium carbonate,, magnesium oxide,, magnesium sulfate etc; among these, magnesium chloride has shown the best bioavailability, thanks to its remarkable solubility in water. In fact, on a general level, it has been seen that as the water solubility of a magnesium salt increases, its bioavailability also increases.
Unfortunately, in the literature there are rather conflicting data regarding the absorption percentages of the various magnesium salts; it is therefore not possible for us to provide more specific and detailed information.
According to some studies, only 4% of the magnesium taken in the form of magnesium oxide would actually be absorbed, while for organic salts the percentages absorbed would be 2-3 times higher, in the order of 8-12%.
Factors that reduce absorption
As for food, it is known that magnesium absorption is reduced by:
- FOOD FIBERS: especially insoluble, non-fermentable ones (contained in bran and whole grains)
- OXALATES: antinutrients found in spinach, green leafy vegetables, tea, coffee and cocoa;
- PHYTATES: antinutrients present in whole grains and in the peel of legumes.
Warning: the above can be misinterpreted, given that the foods with the highest magnesium content include those foods - such as green vegetables, oily dried fruit, legumes, chocolate and whole grains - with a high fiber and / or oxalates and / or phytates.
For this reason, these foods must absolutely not be removed from the diet, both because they are a precious source of magnesium and because they bring other very important micronutrients.
The soaking of legumes and whole grains before cooking, the consumption of sprouted seeds and the cooking of foods rich in oxalic acid (such as spinach), increase the bioavailability of the magnesium they contain.
Other factors capable of decreasing the absorption of magnesium:
- CATIONS: the excess of one or more cationic minerals (such as manganese, potassium, calcium, iron, zinc, phosphorus and copper) in the same meal, can reduce the absorption of magnesium by forming insoluble aggregates.
- DAIRY PRODUCTS and COLA-like beverages: the association of foods rich in calcium (such as dairy products) and / or phosphorus (contained in carbonated drinks such as cola) with foods rich in magnesium or magnesium supplements should be avoided in particular. Therefore, regular consumption of cola-like drinks with meals exposes the subject to greater risks of magnesium deficiencies.
- EXCESS OF FATS in the same meal, due to the saponifying action on magnesium.
Other factors that can reduce absorption or increase magnesium losses are:
- SOME DRUGS: antibiotics (tetracyclines, gentamicin), proton pump inhibitors, antacids, neuroleptics, salazopyrine, diuretics (furosemide, thiazides), chemotherapy (cisplatin), laxative abuse.
- CERTAIN HEALTH CONDITIONS: reduced gastric acidity, alcohol abuse, celiac disease, Crohn's disease, kidney disease, diarrhea and vomiting, diabetes, malabsorption syndromes, steatorrhea and pancreatic insufficiency;
- NATURAL DIURETICS: tea, coffee, energy drinks, other caffeine-based drinks and herbal preparations with diuretic action, tend to increase urinary excretion of magnesium;
Factors that favor absorption
As for food, the absorption of magnesium can be favored by:
- FRUCTOSE AND COMPLEX CARBOHYDRATES
- ANIMAL PROTEIN
- MEDIUM CHAIN TRIGLYCERIDES
- SOLUBLE OR FERMENTAL FIBERS
- VITAMIN D (in association with parathyroid hormone)
Conclusions
Although it is not possible to identify, as in the case of iron, unambiguous and well-defined recommendations for improving the absorption of magnesium, we can still draw up some basic recommendations to ensure an adequate intake of magnesium:
- consume adequate quantities of fresh fruit (400-600 g / day, mainly in secondary meals) and vegetables (100-200g, both at meal and at dinner)
- better to boil the spinach before consumption
- prefer whole grains to refined ones, at least in 50% of meals
- limit (or better avoid altogether) the consumption of carbonated drinks containing phosphorus (such as cola), especially at meals
- secondary meals can be structured by combining the consumption of fresh fruit with small quantities of oily dried fruit (10-15g of almonds or walnuts or hazelnuts etc.)
- avoid binges and excessive variety of dishes consumed in the same meal;
- those who consume large quantities of dairy products, those who follow a diet low in fresh fruit and vegetables, or consume a large amount of beverages containing phosphorus, such as coca-cola, should seriously consider regulating their eating habits and / or resorting to one specific magnesium supplement. The same goes for patients who take drugs or are suffering from diseases that can reduce the levels of magnesium in the body; in such circumstances a preventive medical consultation becomes mandatory.
