Stress fractures are one of the most common and potentially serious overuse injuries reported in athletes, both elite and recreational. With this in mind, Andrew Hamilton looks at recent evidence linking stress fractures and nutritional status… MORE
Why magnesium matters to athletes
Ask most athletes to name some key minerals for human performance nutrition and you’ll probably find calcium, iron, zinc and even chromium popping up in their lists. But they are unlikely to mention magnesium. Despite magnesium’s pivotal role in energy production, many coaches and athletes remain unaware of its critical importance in maintaining health and performance. Indeed, dietary intakes of magnesium in the West have declined to less than half of those recorded 100 years ago, and are still falling. Yet many scientists believe that the amount of magnesium required for optimum health has been underestimated in the past, and now new research suggests that even small shortfalls in magnesium intake can seriously impair athletic performance. Clearly, magnesium nutrition is an area that no serious athlete can afford to overlook!
Pure magnesium is a silvery-white metal, which burns with a dazzling brilliance – something you’ve probably seen demonstrated by your science teacher at school! It is the second most abundant mineral in cells after potassium, but the two ounces or so found in the typical human body is present not as metal but as magnesium ions (positively-charged magnesium atoms found either in solution or complexed with other tissues, such as bone). Roughly one quarter of this magnesium is found in muscle tissue and three-fifths in bone; but less than 1% of it is found in blood serum, although that is used as the commonest indicator of magnesium status. This blood serum magnesium can be further subdivided into free ionic, complex-bound and protein-bound portions, but it’s the ionic portion that’s considered most important in measuring magnesium status, because it is physiologically active.
Magnesium is well supplied in unrefined whole grain cereals, such as wholemeal bread, and also in green leafy vegetables, nuts and seeds, peas, beans and lentils (see table below). Fruit, meat and fish supply poor levels, as do refined foods. Contrary to common belief, milk and dairy products are not particularly rich sources of magnesium The magnesium content of plant foods tends to reflect soil magnesium concentrations and growing conditions, especially as magnesium is not routinely added to soils by farmers during intensive fertilisation(1).
|Food||MAGNESIUM Content (milligrams per 100g)|
|Pumpkin seeds (roasted)||532|
|Peanuts (roasted, salted)||183|
|Rice (whole grain brown)||110|
|Yoghurt (plain, low fat)||17|
|Cornflakes||6 (‘Frosties’ or ‘Honeynut’)|
Source; USDA Nutrient Database
Magnesium is a fairly soluble mineral, which is why boiling vegetables can result in significant losses; in cereals and grains, it tends to be concentrated in the germ and bran, which explains why white refined grains contain relatively little magnesium by comparison with their unrefined counterparts.
Magnesium plays a number of roles in the body, being required for more than 325 enzymatic reactions, including those involved in the synthesis of fat, protein and nucleic acids, neurological activity, muscular contraction and relaxation, cardiac activity and bone metabolism. Even more important is magnesium’s pivotal role in both anaerobic and aerobic energy production, particularly in the metabolism of adenosine triphosphate (ATP), the ‘energy currency’ of the body. The synthesis of ATP requires magnesium-dependent enzymes called ‘ATPases’. These enzymes have to work extremely hard: the average human can store no more than about 3oz of ATP, yet during strenuous exercise the rate of turnover of ATP is phenomenal, with as much as 15kgs of ATP per hour being broken down and reformed (from adenosine diphosphate and phosphate)!
In normal adults, a magnesium deficiency results in altered cardiovascular function, including electrocardiographic abnormalities(2,3), impaired carbohydrate metabolism, with insulin resistance and decreased insulin secretion(2,4), and high blood pressure (5). Disease states that have been associated with magnesium imbalances and deficiencies include coronary heart disease, neuromuscular disorders, kidney diseases, asthma(6), migraines, premenstrual syndrome, pre-eclampsia and eclampsia (both potentially serious complications of pregnancy), menopausal bone problems (3) and even obesity!
The UK recommended intake for magnesium (the daily amount deemed adequate to prevent deficiencies in 97.5% of the UK population) is set at 300mgs for men and 270mgs for women (7). The US has recently revised its figures upwards and now recommends an intake of 400mgs per day for men aged 19-30 and 420 for those over 30; the figures for women under and over 30 are 300 and 310mgs per day respectively(8). However, some investigators believe these should be set even higher at 450-500mg/day (9).
Many people go short of magnesium
Dietary intakes of magnesium in the United States have been declining over the last 100 years from about 500mg/day to 175–225mg/day (10) and a recent national survey suggested that the average magnesium intake for women is as low as 228mgs per day (11). But since this figure is derived using a one-day diet recall method, it may actually be an overestimate of actual magnesium intakes (12). Meanwhile, the UK’s Food Standards Agency estimates that the average daily intake of magnesium in Britain for both men and women is just 227mgs – only two thirds of the US recommended daily amount (RDA).
The figures above suggest that many people fall short of optimum magnesium intakes, and this has been confirmed in a number of studies. For example, American researchers found that more than 60% of US adults were failing to meet even the previous (lower) RDA for magnesium(13). Even athletes, who might be expected to take greater care with their diets, are not immune from magnesium deficiency; for example, studies carried out in 1986/87 revealed that gymnasts, footballers and basketball players were consuming only around 70% of the RDA(14), while female runners fared even worse, with reported intakes as low as 59% of the RDA(15).
Given magnesium’s vital role in energy production, two key questions emerge:
- Can these all-too-common sub-optimum dietary magnesium intakes impair athletic performance?
- Could extra magnesium intake, over and above RDA levels, enhance performance?
While there is plenty of evidence that oral magnesium therapy improves cardiac function and exercise tolerance in coronary heart disease patients(16, 17), until recently, there has been little hard evidence about the effects of sub-optimum magnesium intakes in healthy exercising adults.
However, in a very tightly controlled three-month US study, the effects of magnesium depletion on exercise performance in 10 women were observed – and the results make fascinating reading (18). In the first month, the women received a magnesium-deficient diet (112mgs per day), which was supplemented with 200mgs per day of magnesium to bring the total magnesium content up to the RDA of 310mgs per day. In the second month, the supplement was withdrawn to make the diet magnesium-deficient, but in the third month it was reintroduced to replenish magnesium levels.
At the end of each month, the women were asked to cycle at increasing intensities until they reached 80% of their maximum heart rate, at which time a large number of measurements were taken, including blood tests, ECG and respiratory gas analysis. The researchers found that, for a given 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 related to the extent of magnesium depletion. In plain English, a magnesium deficiency reduced metabolic efficiency, increasing the oxygen consumption and heart rate required to perform work – exactly what an athlete doesn’t want!
The researchers concluded: ‘This report provides the first evidence that low dietary magnesium, in amounts consumed by some groups of physically active individuals, impairs function during exercise.’ The mechanisms behind this effect are unclear, but it seems likely that a magnesium shortfall can cause a partial uncoupling of the respiratory chain, increasing the amount of oxygen required to maintain ATP production. There is also evidence that a magnesium shortfall boosts the energy cost, and hence oxygen use, of exercise because it reduces the efficiency during exercise of muscle relaxation, which accounts for an important fraction of total energy needs during an activity like cycling (19).
While many studies on magnesium supplementation and exercise have been carried out, the results have been inconsistent and may indicate that there is nothing to be gained by supplementing an already magnesium-sufficient diet.
One study of male athletes supplemented with 390mgs of magnesium per day for 25 days resulted in an increased peak oxygen uptake and total work output during work capacity tests (20); in another, on sub-maximal work, supplemental magnesium elicited reductions in heart rate, ventilation, oxygen uptake and carbon dioxide production(21); in a third, physically active students, supplemented with 8mgs of magnesium per kilo of body weight per day, experienced significant increases in endurance performance and decreased oxygen consumption during standardised, sub-maximal exercise (22).
However, other studies carried out on physically active people with ‘normal’ serum magnesium and muscle magnesium concentrations have found no functional or performance improvements associated with supplementation (23, 24).
On the evidence available so far, the scientific consensus is that extra magnesium can enhance performance when (as is all too often the case) magnesium intakes fall below optimum levels. But in subjects already consuming magnesium at or above this optimum level, there is little hard evidence to suggest that taking more confers extra benefits.
Given the growing body of evidence pointing to the need for optimum magnesium nutrition in athletes, what tests are available to coaches for determining magnesium status? Muscle magnesium (obtained through a needle biopsy) is one of the most accurate methods of assessment, but it is time-consuming, very invasive and can cause discomfort. Magnesium status can also be measured by means of a ‘magnesium load’ test, followed up with measurement of urinary excretion. However research suggests that urinary magnesium is too variable to accurately evaluate magnesium status (6).
Testing for magnesium status
Total blood magnesium (TMg) is the most widely used assay, but this has the disadvantage of including complex and protein-bound magnesium, whereas it’s the ionic portion that’s physiologically active. This test is also insensitive to the movements of magnesium that occur within the body as a result of exercise.
However, the recent introduction of ion-selective electrode (ISE) technology now enables scientists to measure ionic magnesium directly, and this is considered one of the best methods. But even then it’s not all plain-sailing, since ionic magnesium levels tend to fluctuate significantly according to the time of day, with higher values recorded in the morning and lower values in the evening. This ‘circadian magnesium rhythm’ is believed to be linked to changes in physical activity levels through the day, but the whole subject of ‘intra-body’ magnesium fluctuations remains poorly understood. Nevertheless, the best results seem to be obtained when ionic magnesium is sampled from fasting, non-exercised subjects first thing in the morning (25).
So what’s the take-home message for athletes? First, it’s all too easy to go short of magnesium, especially if your diet is light on foods like whole grains and cereals, green leafy vegetables, pulses (peas/beans/lentils), nuts and seeds. To make matters worse, excessive sugar intake, alcohol consumption and diets high in fats, protein and calcium have all been shown to impair magnesium absorption and/or increase excretion. And even when the quality of food is good and the diet carefully balanced, diets containing fewer than 2,000 calories per day often struggle to meet magnesium needs, placing those on weight loss or maintenance régimes at added risk.
The box below summarises the kinds of dietary habits that can lead to low magnesium intakes and also some of the sub-clinical symptoms that can be signs of a sub-optimum intake (although clinical tests such as muscle magnesium or ionic magnesium are better at establishing actual magnesium status).
Risk factors and signs of low magnesium intake
Eating habits associated with low magnesium intake
- You tend to eat white flour products instead of wholemeal
- You have a low intake of green leafy vegetables
- You don’t eat much in the way of nuts and seeds or beans and lentils
- You regularly consume sugar or sugary products
- You drink alcohol regularly
- You follow a calorie-restricted or high-protein, low-carbohydrate diet
Possible symptoms of sub-optimal magnesium intake
- Muscle cramps, twitches or tremors
- Regular or excessive fatigue
- Feelings of irritability and/or lethargy
- Frequent mood swings, including depression
- Pre-menstrual bloating
- Restless legs at night
Given the potential for impaired performance on a sub-optimum magnesium intake, any athlete not already doing so should make a conscious effort to increase the proportion of magnesium-rich foods in his or her diet. Even a simple change like eating more whole grain products and boosting your intake of vegetables, nuts and seeds can make a big impact.
Magnesium intakes above the RDA are unlikely to boost performance further, but supplements are cheap and non-toxic, so can safely be used as an insurance policy. Most forms of supplemental magnesium are well-tolerated but it is inadvisable to supplement more than 400mgs per day. Some forms, such as magnesium oxide, are quite alkaline forming and can have the side effect of neutralising stomach acid and interfering with digestion. These should not be taken with meals. Finally, magnesium is best absorbed in small, frequent doses; so, for example, it is better to take 100mgs three times a day than 300mgs in one go!
- National Research Council of Canada: Ottawa, ON: NRCC Publications (1979)
- Hypertension 21:1024-1029. (1993)
- Endocrinol Metab Clin N Am 22:377-395 (1993)
- Diabetologia 33:511-514 (1990)
- Magnesium and Trace Elements, 10:Measurement 1991-90 (1992)
- Scandinavian Journal of Clinical and Laboratory Investigation, 55:549-558 (1995)
- UK Food Standards Agency/COMA
- US Institute of Medicine and National Academy of Sciences
- Scandinavian Journal of Clinical and Laboratory Investigation, 56 (Supplement 224): 211-234 (1996)
- Scandinavian Journal of Clinical and Laboratory Investigation, 54(Supplement 217):5-9 (1994)
- Clinical Nutrition of the Essential Trace Elements and Minerals:49-67 (2000)
- J Am Diet Assoc 93:462-464 (1993)
- Institute of Medicine, Food and Nutrition Board, Washington, DC: National Academy Press, (1997)
- J Am Diet Assoc;86: 251–3 (1986) and Nutr Res;7:27–34 (1987)
- Med Sci Sports Exerc; 18(suppl):S55–6 (1986)
- Am J Cardiol, 91(5): 517-21 (2003)
- Cardiovasc Drugs Ther, 12 Suppl 2: 153-6 (1999)
- J Nutr 132:930-935 (2002)
- J Appl Physiol 65:1500-1505 (1988)
- Endocrinol Metab Clin N Am 22:377-395 (1993)
- Magnesium and Physical Activity:227-237 Parthenon Publishing Group London, UK (1995)
- Med Exerc Nutr Health 4:230-233 (1995)
- Sports Nutrition: Minerals and Electrolytes:179-187 CRC Press Boca Raton, FL, USA (1995)
- Int J Sports Nutr 2:154-164 (1992)
- Critical Reviews in Clinical Laboratory Sciences, 27:409-437 (1989)