Magnesium is essential for the development and maintenance of healthy bones and for healthy nerve and muscle function. It is also an essential component of numerous key enzyme systems, many of which are involved in energy utilisation, protein synthesis and cell division. Hence magnesium plays a pivotal role in the metabolism of carbohydrates, fats and proteins, and in regulating cell growth and membrane structure.
Requirements, Sources and intake
The Department of Health has set a Reference Nutrient Intake (RNI) for magnesium of 300mg/day for men over 18. Requirements are increased by diets high in calcium and phosphate, by certain prescribed drugs, by the consumption of alcohol, by stress and by diseases such as diabetes.
The main sources of magnesium in the diet are wholegrain cereals and green vegetables. Although animal products contain magnesium it is less well absorbed than that from plant sources.
According to The Dietary and Nutritional Survey of British Adults, based on over 1,000 men, the average intake of magnesium for all ages (16-64 yrs) is 323mg/day, with the lowest average intake being 304mg/day in the 16-24 age range. Because these represent average data it follows that many men have intakes below the RNI. Possible causes of low intakes of magnesium are food preferences for low-magnesium foods (particularly refined cereals) and infrequent consumption of high-magnesium foods (e.g., nuts, spinach, kale, etc.).
The body contains about 25g of magnesium, of which about 60% is in the bones.
The daily turnover of magnesium is high and since it is poorly stored, a regular intake is needed to avoid deficiency.
Signs and Symptoms of Magnesium Deficiency
Because of the complexity of the methodology involved, evidence of magnesium deficiency is not often sought among healthy people. However, some indication of its prevalence can be gleaned from data obtained from those under medical treatment, which suggests low magnesium status may be a common condition. In addition to often being associated with diabetes, a USA study found low magnesium status in 65% of intensive care patients and in 12% of all hospital patients.
Since magnesium is essential for proper contraction of voluntary and involuntary muscle, as well as heart muscle, when the magnesium status of the body is poor, increased magnesium intake tends to have a relaxing effect on all muscle tissues.
Severe deficiency of magnesium causes hyperactivity of nerves and muscles, psychiatric disturbances, abnormalities in calcium and potassium balance and disturbances in heart beat rhythm. Marginal deficiency has been associated with a wide range of conditions many of which have an element of muscle over-contraction.
Heart Disease
The possibility that low magnesium intake may increase the risk of coronary heart disease was first suggested by the finding that people living in hard water areas have lower rates of heart and vascular disease than those living in soft water areas. (Hard water is characterised by having high levels of magnesium as well as calcium.)
High blood presssure
A common cause of high blood pressure is over-contraction of the muscles of the small arterioles in the body tissues; this results in increased resistance to blood flow and reduced blood flow to the capillaries.
Severe high blood pressure, including that associated with alcoholism and diabetes, has been related to low magnesium status, and injections of magnesium are a well-established treatment for high blood pressure in pregnancy.
The strongest evidence linking low magnesium intake with increased risk of high blood pressure comes from the Honolulu Heart Studies. This showed that total magnesium intake was the nutrient with the strongest inverse relationship with blood pressure.
Similarly, a more recent study of Canadian men showed that a low concentration of urinary magnesium (indicative of low magnesium intake) was linked with high blood pressure.
Most of the early studies of the effects of dietary supplementation with magnesium on raised blood pressure were with small numbers of subjects and so there is need for further research. Even so, although some of these studies showed no effect, the majority had a positive outcome.
Although only modest reductions in blood pressure are brought about by magnesium supplementation, the effect is likely to be enhanced by a combination of this approach with reduction of salt intake, increased potassium and fish oil intake, together with more exercise and weight loss (if necessary).
Spasm of the coronary arteries and abnormal heart rhythm
Whilst clinical experience indicates that magnesium supplementation is helpful for the relief of heart palpitations, most of the evidence for the role of magnesium in heart function is derived from its association with more serious heart conditions (e.g. increased risk of coronary artery disease, sudden death due to heart attack and seriously abnormal heart beat or arrhythmia).
Under some circumstances magnesium deficiency causes constriction of the coronary arteries, leading to poor blood supply to parts of the heart. This may damage the muscular wall of the heart and can lead to arrhythmia. Hence, following a heart attack, a large dose of magnesium (by injection) is a routine procedure to reduce excessive heart muscle excitability, muscle spasm and the likelihood of arrhythmia.
Diabetes
Low magnesium status is widespread in diabetics of all types. It often results from low magnesium intakes (as with normal people) coupled with increased urinary losses (which are greater with diabetics).
Magnesium is required for both the secretion of insulin and the insulin-response of the body cells. Italian workers showed that ‘insulin resistance’ (poor cell response to insulin) of men with maturity-onset diabetes was improved with magnesium supplementation.
Osteoporosis (or brittle bone disease) is commonly associated with diabetes and because of the importance of magnesium in calcium metabolism, even mild magnesium deficiency may play an important part in enhancing the risk of this condition among diabetics.
Asthma
When magnesium status is low, the administration of magnesium may help in the relaxation of the muscular wall of the bronchi of the lungs, which constrict during an asthma attack. Most of the support for the role of magnesium in asthma treatment comes form the use of large doses of magnesium (by injection) during asthma attacks, benefit having been reported in some but not all studies.
Additional evidence comes from a recent study by researchers at the University of Nottingham. This involved 2,633 people selected at random and it showed those with low intakes of magnesium had a greater risk of developing asthma.
Conclusions
Intakes of magnesium are low for those eating a diet that is low in wholegrain cereals and vegetables, as often happens in developed societies. The problems associated with poor intake may be compounded by stress and by over-indulgence in alcoholic beverages, which increase urinary loss of magnesium.
Although the evidence available is not complete, the indications are that low levels of magnesium increase the risk of heart disease and asthma and contribute to poor glucose control in diabetes. Hence it is reasonable to postulate that many individuals would benefit from an increased intake of dietary magnesium.
Levels of dietary magnesium are best improved by changes in diet in line with Government recommendations for healthy eating. If this advice proves difficult to follow, an alternative approach would be to take a magnesium supplement daily.
