Magnesium – is it more important for athletes than we thought?

Think of the ‘big hitters’ in minerals for sports nutrition and the chances are you’ll come up with iron, calcium and perhaps zinc. Yet despite magnesium’s pivotal role in energy production, many coaches and athletes remain unaware of its critical importance in maintaining health and performance. To make matters worse, magnesium is a mineral that is often poorly supplied in the diet; dietary intakes of magnesium in the West have declined to less than a half of those recorded at the end of the 19th century and are still falling⁽¹⁾. Moreover, many nutritionists believe that the amount of magnesium required for optimum health has been underestimated in the past, and research suggests that even small shortfalls in magnesium intake can seriously impair athletic performance. This evidence includes the following :

• A study on women put on a magnesium restricted diet, which showed for a given cycling workload, peak oxygen uptake, total and cumulative net oxygen utilisation and heart rate all increased significantly during the period of magnesium restriction, with the amount of the increase directly correlated with the extent of magnesium depletion (ie the magnesium deficiency reduced metabolic efficiency, increasing the oxygen consumption and heart rate required to perform a given workload)⁽²⁾;
• A study of male athletes supplemented with 390mg of magnesium per day for 25 days, which resulted in an increased peak oxygen uptake and total work output during work capacity tests⁽³⁾;
• A sub-maximal work study, which showed that magnesium supplementation reduced heart rate, ventilation rate, oxygen uptake and carbon dioxide production for a given workload⁽⁴⁾;
• A study on physically active students, which showed that supplementing with 8mg of magnesium per kilo of body weight per day produced significant increases in endurance performance and decreased oxygen consumption during sub-maximal exercise⁽⁵⁾.

The likely explanation for these findings lies in the fact that that magnesium is required for the activation of crucial enzymes known as ATPases, which are required for the generation of ATP, the body’s ‘energy currency’ used for all muscular contraction (see box below). A magnesium shortfall also appears to reduce the efficiency of muscle relaxation, which accounts for an important fraction of total energy needs during exercise.

A study carried out using rats earlier this year provides further evidence of the magnesium/ lactate connection⁽⁷⁾. Taiwanese researchers investigated the effects of administering pre-exercise magnesium (17mg per kg of body weight) on rats forced to swim for 15 minutes. In particular, they wanted to observe the effect of the supplemented magnesium on blood lactate, glucose and pyruvate (an important intermediate compound at the ‘crossroads’ of aerobic metabolism).

Prior to exercise, the blood levels of lactate, glucose and pyruvate were no different in magnesium-supplemented rats when compared with rats given no magnesium (control group). However, following the forced swimming, the lactate levels in the magnesium-supplemented rats rose to only 130% above pre-exercise levels compared with a 160% rise in the control group. Moreover, swimming caused brain glucose and pyruvate levels in the control group to decrease to 50-60% of the pre-exercise level; in the magnesium-supplemented rats, brain glucose levels increased to 140% of the pre-exercise level, and increased pyruvate levels to 150% of the basal level during forced swimming!

The researchers concluded that not only did supplemental magnesium help suppress lactate production, but that it also somehow increased glucose availability and metabolism in the brain during exercise. This is important because scientists now believe that the brain and central nervous system play a large role in determining the degree of muscular fatigue we feel⁽⁸⁾; higher brain glucose availability could in theory translate into lower levels of perceived fatigue.

An antioxidant role for magnesium?

Until recently, magnesium has had something of a Cinderella status among sports nutritionists, many of whom have not appreciated just how important optimum magnesium status is for athletic performance. However, it now seems that magnesium has another surprise to reveal, as new research indicates it may play a vital role as an antioxidant, helping to protect the body from the potential ravages caused by oxidative stress (cellular damage occurring as a result of oxygen generated free radicals within the body.

Although other minerals such as copper, zinc and selenium are known to be involved in activating enzymes that deactivate free radicals and thus protect the body, the possible role of magnesium as an antioxidant nutrient is extremely surprising to say the least. That’s because unlike other antioxidants, magnesium is not ‘chemically speaking’ considered adept at accepting and passing on electrons (something that characterises all other antioxidant molecules). However, despite this fact, a growing body of recent evidence suggests that adequate dietary magnesium is essential for the control of oxidative stress.

One of the earliest studies to indicate a possible connection between magnesium and oxidative stress was conducted at the Military Medical Academy in Belgrade involving young military recruits exposed to chronic stress(9). The researchers monitored markers of oxidative stress such as increased superoxide anion (free radical) concentration and malondialdehyde (a marker of cell lipid damage) in each subject as well as magnesium status. They discovered that a low magnesium status was correlated with increased levels of oxidative stress and that the poorer the magnesium status, the higher the recorded oxidative stress.

Correlation of course isn’t the same as cause, but further evidence of a link between magnesium and oxidative stress surfaced three years later in an Indian study carried out on rats that were given an injection to make them diabetic(10). Compared to non-treated rats (controls), the diabetic rats showed a significant decrease in blood magnesium levels and an increased urinary excretion of magnesium. In addition, there was a marked increase in markers of cell damage and a corresponding decrease in the antioxidant vitamins C and E, and other protective compounds called thiols.

Interestingly however, giving the diabetic rats magnesium supplementation for four weeks restored blood magnesium levels to near normal levels and reduced markers of cell damage. Moreover, supplementing magnesium also boosted vitamin C and thiols, and increased antioxidant enzyme activity generally, suggesting a strong causal link.

Another very recent animal study examined the effect of a magnesium deficiency on free radical damage in cultured cells from chick embryos⁽¹¹⁾. In particular the researchers wanted to investigate whether magnesium deficiency enhanced the oxidative damage caused by a naturally produced pro-oxidant (a substance that enhances oxidative stress) in animal cells called hydrogen peroxide. They found that incubating the cells in a magnesium deficient environment doubled the amount of hydrogen peroxide produced and significantly enhanced cell damage caused by this compound. This effect was probably because the magnesium deficiency reduced the activity of an enzyme called catalase, which helps to break down and render harmless any hydrogen peroxide produced in the body. Other recent animal studies have also confirmed that low magnesium intake is strongly correlated with increased oxidative stress^(12-15).

Antioxidant and anti-inflammatory activity of magnesium in humans

Animal studies are all very well, but can optimising magnesium status help protect the human body? Very few studies have been conducted in this area so far, but the evidence so far suggests this is quite likely. There’s certainly a growing body of evidence that low magnesium intakes are correlated with increased inflammation, which is itself strongly associated with oxidative stress.

For example, an Italian study carried out last year of over 1,600 adults showed that low intakes of dietary magnesium were correlated to increased levels of an inflammatory marker known as C-reactive protein(16); although this study looked at middle-aged sedentary adults, an increased tendency towards inflammation is undesirable in all populations, especially athletes, where it is generally associated with increased post-exercise muscle soreness and joint stiffness.

Another study looked at lung function and in particular whether dietary antioxidants might protect lung tissue against reactive oxygen species-induced injury, adverse respiratory effects and reduced pulmonary function⁽¹⁷⁾. Healthy, non-smoking freshmen students who were lifetime residents in the Los Angeles or the San Francisco Bay areas of California completed comprehensive residential history, health history and food frequency questionnaires. Blood samples were also collected and forced expiratory volume (lung power) measurements were obtained. Using a statistical technique called multivariable regression, the researchers showed that the higher the intake of dietary magnesium, the more positive the lung function (indicating healthier more elastic lung tissue).

A third study published just a few months ago examined the effect of magnesium supplementation on inflammatory markers in patients with chronic heart disease⁽¹⁸⁾. The study, conducted by Israeli researchers, compared the levels of the inflammatory marker C-reactive protein in patients given 300mg a day of magnesium citrate with a control group given no magnesium.

The result showed unequivocally that the extra magnesium produced a significant drop in C-reactive protein levels, indicating reduced inflammation, so much so that the researchers commented that ‘targeting the inflammatory cascade by magnesium administration might prove a useful tool for improving the prognosis in heart failure.’

Implications for athletes

What does this all mean for athletes? The simple message is that a growing body of evidence suggests that maintaining an optimum magnesium status is probably even more important than we’ve previously realised (see box, right). The latest research on magnesium and lactate adds further weight to the evidence indicating that a healthy magnesium intake is vital for both endurance and anaerobic performance. In the longer term (and perhaps more surprisingly), it appears that an optimal magnesium intake may also be essential for antioxidant protection and for the correct regulation of inflammation, both of which are desirable for athletes, young and old. Although more research is needed to discover the underlying mechanism behind these effects, the take home message is that you should ignore the importance of magnesium at your peril!

Andrew Hamilton BSc Hons, MRSC, ACSM is a member of the Royal Society of Chemistry, the American College of Sports Medicine and a consultant to the fitness industry, specialising in sport and performance nutrition

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