Magnesium is one of the most important minerals for health. Around half the magnesium in your body is stored in your bones, with most of the remainder found inside your cells. As levels in the blood do not accurately reflect your magnesium status, this is measured by assessing the amount of magnesium within your red blood cells (erythrocytes). Typical normal laboratory reference values are:
- serum magnesium 0.7 – 1.1 mmol/L (1.7-2.5 mg/dl)
- red blood cell magnesium 2.4 – 2.57 mmol/l (4.04 – 6.9 mg/dl)
If your magnesium status is low, it will take around 4 months for magnesiumm supplements to normalise a low red blood cell level.
What does magnesium do?
Magnesium is needed for over 300 enzymes to work properly, and is vital for every major metabolic reaction from the synthesis of protein and genetic material to the production of energy. One of its most important roles is to maintain the integrity of ion pumps which control the flow of sodium, potassium, calcium and other electrolytes in and out of cells. This makes it essential for nerve cells to pass electrical messages and for muscle fibres to contract, including those within the heart. It is also needed for the interaction of sex hormones with the genes they switch on and off, and for the production of brain chemicals to help maintain mood.
Within Europe, the EU register on nutrition and health claims allows the following product label claims, that magnesium contributes to:
- a reduction of tiredness and fatigue
- electrolyte balance
- normal energy-yielding metabolism
- normal functioning of the nervous system
- normal muscle function
- normal protein synthesis
- normal psychological function
- maintenance of normal bones
- maintenance of normal teeth
- and/or has a role in the process of cell division.
Good dietary sources of magnesium include:
- Whole grains (wheatbran provides 520mg/100g)
- Nuts (Brazils provide 410mg magnesium/100g)
- Seeds (sunflower seeds provide 390mg/100g)
- Dark chocolate (89mg per 100g)
- Beans (especially soy beans which provide 63mg per 100g cooked beans)
- Dark green, leafy vegetables and herbs (raw baby spinach provides 54 mg/100g)
- Seafood (sardines contain 46mg/100g)
Drinking water is also a good source in hard-water areas (average 100mg per day).
How much magnesium do you need?
Adults need around 375mg per day according to the EU Nutrient Reference Value, or 400mg according to the US Daily Value. Deficiency is common, however, as the average magnesium intake from food is around 280mg per day. Drinking water can also contain high levels of magnesium in some areas, and the maximum intake from food, plus supplements, plus water has been estimated at 1400mg per day.
The amount of magnesium in your body at any one time is a balance between the amount absorbed from the small intestines and the amount excreted via the kidneys – when your status is optimal, these two amounts are similar.
The amount of magnesium you absorb is proportional to how much is present in a meal and varies between 20% and 70%. Typically, you only absorb around half the magnesium present in food. Some magnesium crosses the intestinal wall by passive diffusion and some by an active transport process, although its regulation is still not fully understood but may involve vitamin D. Within the kidneys, reabsorption of magnesium from fluid filtered out of the circulation is highly efficient (80%) and this fine-tuning ensures you don’t lose too much when dietary intakes are low.
A wealth of research findings support the importance of good magnesium intake for optimum health.
Magnesium and blood pressure
Magnesium has numerous beneficial effects on the circulation, including relaxing blood vessels to lower a raised blood pressure. Recent research suggests that, after taking other known risk factors into account, people with the highest magnesium levels were 48% less likely to develop hypertension than those with the lowest magnesium levels.
In those who already have hypertension, data from seven studies, involving 135 people on antihypertensive medication, and whose average systolic blood pressure (upper reading) was greater than 155 mmHg (ie poorly controlled), showed that adding in magnesium supplements reduced their readings by 18.7/10.9 mmHg – a large and highly significant effect.
Other studies have produced less impressive results, however, which may depend on whether or not volunteers have an underlying magnesium deficiency. Analysis of data from 22 trials, involving 1173 people who took an average dose of 410 mg magnesium, for example, showed a decrease in systolic blood pressure (upper reading) of 3 to 4 mm Hg, and a decreased in diastolic blood pressure (lower reading of 2-3 mm Hg). Some of these trials only lasted for 3 weeks, and some involved relatively low doses of 120mg magnesium. Better results were seen in those taking at least 370 mg magnesium per day for longer periods.
Magnesium and heart health
Low magnesium levels are associated with spasm of coronary arteries and an increased risk of coronary artery calcification and unwanted blood clots. These effects are more pronounced during times of stress and can lead to abnormal heart rhythms, angina or heart attack. And among obese individuals with type 2 diabetes, heart rhythm abnormalities were twice as common in those with low magnesium levels.
Research involving over 313,000 people shows that every 0.2 mmol/l increment in circulating blood levels of magnesium is associated with a 30% lower risk of heart attack or stroke. For dietary intakes of magnesium, each 200 mg per day increment was associated with a 22% lower risk of ischemic heart disease.
The importance of magnesium in regulating calcium is demonstrated in a recent study involving 1,276 volunteers, aged 30 to 75 years, who underwent a full medical check-up, which included CT scanning of their coronary arteries. After adjusting for other known coronary heart disease risk factors (such as age, weight, exercise, smoking, alcohol, cholesterol, medication and diabetes), those with the highest magnesium levels (above 2.18 mg/dl) were 42% less likely to have calcified coronary arteries than those with the lowest magnesium level (below 1.97 mg/dl). Every 0.17 mg/dl increase in serum magnesium was associated with a corresponding 16% lower risk of coronary artery calcification.
Magnesium and diabetes
Good magnesium intakes are associated with improved glucose control. Data from 13 studies involving over 536,000 people shows that a high intake of magnesium (whether from food or supplements) can reduce the risk of developing type 2 diabetes by 22% overall, with every 100 mg per day increment in magnesium intake reducing the risk by 14%. Taking magnesium supplements may also improve glucose control and raise ‘good’ HDL cholesterol levels in people already diagnosed with type 2 diabetes, with improvements seen within 4 to 16 weeks.
Magnesium regulates the movement of calcium in and out of bone cells and is important for the prevention of osteoporosis in post-menopausal women.
Researchers have found that women with osteoporosis have significantly lower magnesium levels than similar women without osteoporosis and, in the EPIC-Norfolk study, both men and women with the lowest magnesium and potassium intakes had a greater risk of hip fracture. Taking magnesium supplements for two years has been shown to increase bone mineral density and prevent bone fractures in menopausal women.
Magnesium and asthma
Low magnesium levels are associated with increased spasm of the airways and asthma attacks. Magnesium is even used intravenously, or via nebuliser, to help treat acute asthma in emergency departments. A study investigating the effects of oral magnesium supplements in adults with asthma, who took 340mg magnesium (170 mg twice a day) or placebo for 6 months, found improved airway stability, increased peak expiratory flow rates and significantly improved quality of life scores in those taking magnesium compared with placebo.
Magnesium and fibromyalgia
Fibromyalgia is associated with long-term pain, fatigue, sleep disturbance and muscle tenderness.
A study involving 60 women with fibromyalgia found their magnesium levels were significantly lower than in similar women without fibromyalgia, and that lower levels were associated with more severe symptoms. Taking magnesium citrate supplements (300mg per day) significantly reduced the number of tender points and the intensity of pain. A combination of magnesium plus the antidepressant drug, amitriptyline, improved all symptoms except numbness.
A pilot study of transdermal magnesium recently found that applying magnesium chloride to the skin of the upper and lower limbs, via a spray, helped to improve symptoms within 2 to 4 weeks.
Magnesium and pre-menstrual syndrome
A crossover study involving 38 women found that taking 200mg magnesium for 2 months significantly improved premenstrual symptoms associated with fluid retention (weight gain, oedema, breast pain, bloating) but did not improve depression, anxiety or total symptoms compared to cycles when the women did not take magnesium supplements. Supplements may also reduce menstrual migraine and mood changes.
Magnesium and longevity
The wide-ranging health benefits of maintaining good magnesium levels may explain why people with the highest dietary intakes of magnesium appear to live longer than those with low intakes.
A large meta-analysis that crunched all the data from 19 studies, involving almost 533,000 people, found that the risk of cardiovascular events (heart attack, stroke) was 15% lower in those with the highest dietary magnesium intakes, and 33% lower in those with the highest magnesium red blood cell levels.
Other studies have found that people with good magnesium intakes (average 454mg/day) were 37% less likely to die from any medical cause, over a 5 year follow-up period, than those with the lowest intakes (average 318mg per day) even where they were at high risk of a heart attack or stroke through having type 2 diabetes, or at least three major risk factors such as a strong family history, smoking, being overweight, high blood pressure or a raised cholesterol.
Magnesium deficiency is common
Food processing removes much of the magnesium naturally present in foods such as seeds and grains, so deficiency is widespread. Adults need around 375mg per day according to the EU Nutrient Reference Value or 400mg according to the US Daily Value. However, average adult intakes in Europe and the US are around 323 mg for males and 228mg for females. At least 65% of adults have low intakes, rising to over 80% by the age of 70 plus. Almost half (48%) of the US population overall have intakes that are below the recommended levels.
Symptoms that can result from magnesium deficiency include:
- loss of appetite
- muscle weakness, trembling or cramps
- numbness and tingling
- poor co-ordination
Magnesium supplements are increasing in popularity, with doses varying from 150 mg to 375 mg a day. Take them with food to optimize secretion of hydrochloric acid which aids their absorption. The upper safe level for long-term use from supplements is suggested as 400mg per day – above this you are likely to get laxative effects, as discussed more fully below.
While magnesium citrate is most readily absorbed, magnesium gluconate is less likely to cause intestinal side effects.
Magnesium oxide has a low bioavailability but provides more magnesium per gram to help offset this and is therefore more cost-effective, which is not often made clear:
- 1 gram magnesium oxide contains 600 mg elemental magnesium
- 1 gram magnesium citrate contains 113 mg elemental magnesium
- 1 gram magnesium gluconate only contains 58 mg elemental magnesium because the ‘gluconate’ part is relatively heavy.
If you are magnesium deficient, you will absorb good amounts of magnesium from supplements regardless of the type of salt they provide. Many ‘experts’ dismiss magnesium oxide by claiming its magnesium is less bioavailable, but other sources such as magnesium gluconate would have to be ten times more bioavailable than magnesium oxide to compensate for the lower load of active magnesium per gram, which they are not. In fact, research suggests that in the presence of magnesium deficiency, between 50% and 67% of magnesium is absorbed from whatever magnesium salt is used. If you can afford the more expensive, more bioavailable forms, great. If not, then don’t feel that cheaper versions supplying magnesium oxide are ‘useless’ as they are not.
Magnesium sulphate is the main component of Epsom salts, which was a popular laxative in Victorian times, and still used today. Magnesium in high doses is also prescribed medically to clear the bowel before procedures such as colonoscopy and surgery (eg 3.5 grams magnesium oxide in a preparation called Picolax – ten times the dose found in many supplements).
The UK Expert Group on Vitamins and Minerals carried out a risk assessment on magnesium in 2003. They stated that ‘A few studies reported mild and reversible diarrhoea in a small percentage of patients and healthy volunteers at levels of 384mg to 470mg per day, but these symptoms were not observed in the majority of studies using similar or higher doses. For guidance purposes only, 400mg/day supplemental magnesium would not be expected to result in any significant adverse effects.’
The laxative effect can be beneficial for people with constipation-predominant irritable bowel syndrome, especially as magnesium also has a muscle relaxing effect to soothe bowel spasms. Supplements are best taken at night, to promote a looser bowel movement next morning, but seek medical advice before trying this to confirm that your symptoms are, in fact, due to IBS and do not require further investigation or treatment.
If you are sensitive to the laxative effects of oral magnesium supplements, another way to take it is to absorb it via the skin. Rub magnesium oil spray into your arms or legs, or add magnesium salts (eg Dead Sea Salts, Epsom Salts) to your bath water. If done at night, this will aid muscle relaxation and a good night’s sleep.
NB If taking magnesium supplements, it is important to ensure a good intake of calcium as these two work together in the body.
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