November 30, 2010

Higher magnesium levels linked with lower risk of sudden cardiac death

An article published online on November 24, 2010 in the American Journal of Clinical Nutrition reports a protective effect for higher plasma and dietary magnesium against the risk of sudden cardiac death in women. Up to 68 percent of women and 55 percent of men who undergo sudden cardiac death have no clinically recognized cardiovascular disease prior to the events, which take 184,000 to 462,000 people's lives each year.

Researchers at Brigham and Women's Hospital and Harvard University analyzed data from 88,375 women enrolled in the Nurses' Health Study who were free of disease in 1980. Questionnaires completed in 1980, 1984, 1986 and every four years through 2002 provided information on magnesium intake from food and supplements. Blood samples drawn from 32,826 participants between 1989 and 1990 were analyzed for plasma magnesium, lipids and other factors. Sudden cardiac deaths were defined as those that occurred within 1 hour of symptom onset or involved arrhythmia.

Over the 26 year follow-up period, there were 295 definite and 210 probable sudden cardiac deaths among the participants. Women whose magnesium intake was among the highest 25 percent of the subjects at greater than 345 milligrams per day had a 34 percent lower adjusted risk of sudden cardiac death compared to those whose intake was lowest at less than 261 milligrams. A similar reduction in risk was observed in those whose intake was in the second quartile.

Having high plasma magnesium also appeared to be protective against sudden cardiac death, with those whose levels were among the highest top 25 percent at over 2.1 milligrams per deciliter experiencing a 77 percent lower adjusted risk compared to those in the lowest 25 percent.

In their introduction to the article, the authors remark that magnesium plays a vital role in cardiac electrophysiology, and that studies have found that the mineral has an antiarrhythmic property. Epidemiologic studies have uncovered a protective effect for increased plasma magnesium levels against cardiovascular disease and for fatal coronary artery disease events, which the authors suggest could be explained if magnesium was protective against fatal ventricular arrhythmias. "Given that most Americans do not meet the RDA [recommended daily allowance] for magnesium, increasing intake of magnesium presents a potential opportunity for sudden cardiac death prevention in the general population," they write. "If further studies replicate these findings, this hypothesis may warrant testing in randomized trials."

Arrhythmias

The four-chambered heart is made from special cardiac muscle that conducts electricity. Beats are generated by electrical impulses in the atria (top chambers of the heart) and are then conducted to the ventricles, where they produce the powerful muscle contraction that pumps blood. The generation and conduction of these electrical signals, as well as the muscle contraction itself, depend on the flow of sodium, potassium, calcium, and magnesium through cardiac cells. Imbalances in the levels of these minerals, as well as damage to cells (e.g., from surgery, oxidants, toxins, or drugs), can prevent impulses from forming, cause them to form prematurely, or prevent their proper conduction through the heart. The result is an arrhythmia.

Ventricular fibrillation is a life-threatening arrhythmia responsible for more than 50 percent of deaths from cardiovascular disease in the United States each year (Heart Rhythm Society 2004). It begins abruptly, causing the ventricles to quiver rather than contract, which causes immediate cessation of blood flow.

Both magnesium and potassium are intricately involved in the heart’s electrical stability (Cybulski J et al 2004); consequently, maintaining normal functional blood levels and ratios of each is important. Potassium is found in every cell of the body, and magnesium, the second-most-abundant intracellular mineral, is involved in many chemical processes (Swain R et al 1999). Magnesium deficiency may result in irregular heartbeats, muscle weakness, and irritability. Conversely, an excessive amount may cause a very slow heartbeat (bradycardia), dizziness, blurred vision, or breathing difficulty.

Magnesium deficiency is usually due to inadequate dietary intake or depletion. Most present-day diets include inadequate amounts of magnesium, and aging is a risk factor for deficiency. Insufficient magnesium may contribute to the symptoms routinely associated with aging (Durlach J et al 1998). Medications such as diuretics, used to treat chronic diseases, may be responsible for more loss of magnesium. Magnesium deficiency is more likely among older people who are institutionalized (Durlach J et al 1998).

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