Many athletes feel that they need to enhance their diets with vitamin and mineral supplements because they are losing more of these minerals and vitamins as a result of their strenuous work out routines. Is this actually true? Rather than discuss minerals in general, this column will focus on the mineral magnesium.
Magnesium is the second most abundant intracellular divalent cation (i.e., it has two positive charges and is located mostly within cells). It also has been established as a cofactor (or "helper") for more than 300 metabolic reactions in the body (1,2). Magnesium is involved in things like protein synthesis, nerve conduction, muscle contraction, and glucose and insulin metabolism (3-8). The Dietary Reference Intake (DRI) for magnesium for adults is 310 to 420 mg/day; however, magnesium intake often is below these recommendations, especially as people age (9). Decreased magnesium intake has been related to an increased risk of metabolic syndrome and type 2 diabetes mellitus (10,11). In addition, stressors such as exercise and type 2 diabetes mellitus may deplete magnesium, which, together with a suboptimal dietary magnesium intake, may negatively impact normal metabolism and possibly impact athletic performance.
MAGNESIUM INTAKE OF ATHLETES
Clark et al. (12) assessed the dietary intake of a Division I women's soccer team at preseason and then during postseason. They found that preseason and postseason intakes of magnesium, among other vitamins and minerals, were marginal (i.e., they were <75% of the DRI). Although research to date has not established if athletes need more magnesium than the average population, not meeting the DRI could have a negative impact on athletic performance.
With respect to ultraendurance adventure racers, although most of their vitamin and mineral intake was adequate, their magnesium intake was not (13). The researchers suggested a need for better education of these athletes for overall nutritional intake (because other vitamin and mineral intake was not adequate either).
MAGNESIUM STATUS IN ATHLETES
Several researchers have evaluated magnesium status in athletes. Lukaski and colleagues (14) conducted a well-controlled pilot study in three male endurance cyclists. These cyclists consumed controlled diets that provided the exact number of kilocalories (energy) and nitrogen (protein) for 28-day periods. These researchers focused on the carbohydrate and fat intake of these athletes and reported that diets high in polyunsaturated fats (especially linoleic acid, which is one of our essential fats) may impair absorption of some minerals, including magnesium, and may decrease endurance performance. Now, despite the well-controlled nature of this study, it is important to note that there were only three cyclists. In addition, a measurement of the athletes' magnesium status while on their typical diets would have provided more typical information (but, remember, these researchers wanted to ascertain if minerals were impacted by changes in fat intake).
Endurance cycling may affect magnesium status as well. Dressendorfer and colleagues (15) studied the effects of intense supervised endurance training on several blood mineral levels, including magnesium, in nine male competitive cyclists. They reported that magnesium status remained unchanged in these male athletes.
MAGNESIUM SUPPLEMENTATION AND EXERCISE PERFORMANCE IN HUMANS
In a study of 32 physically active women, aged approximately 21 years, Finstad et al. (16) provided 16 of the subjects with 212 mg/day of magnesium oxide, and the other 16 received a placebo in a double-blinded fashion. Some of these women were marginally magnesium deficient, based on a sensitive technique of measuring magnesium (ionized magnesium assessment). The participants were measured at baseline and after 4 weeks of supplementation. After that, all subjects were given a "washout" period (i.e., they took no supplement or placebo for 6 weeks), then the placebo group received the magnesium supplement, whereas the supplement group received the placebo (this is called a "crossover design" and is a very good way to study supplementation because researchers can assess both within- and between-participant changes even more closely). Finstad and colleagues (16) did find that ionized magnesium increased with supplementation, indicating that the supplement was well absorbed; however, the supplementation did not enhance exercise performance or recovery. These results are interesting, especially because some of these physically active women were marginally magnesium deficient (see Figure for percent magnesium content in different magnesium supplements).
MAGNESIUM AND STRENGTH PERFORMANCE
In a study in humans, Selsby and colleagues (17) evaluated whether a group given both magnesium and creatine would fair better in a strength test, compared with a creatine-only group and a placebo group. Their subjects were 31 weight-trained men, who were randomly, and in a double-blinded manner (meaning that the researchers and the participants did not know the group to which they were assigned), assigned to one of the three aforementioned groups. Their main outcome measure was a 1-repetition maximum (1-RM) and 70% of 1-RM for the bench press. After 10 days of supplementation, they found that the magnesium-creatine group and creatine-only group had significantly greater increases in 1-RM and 70% 1-RM bench press than the control group. This research was mostly conducted to evaluate the effect of low-dose creatine; however, it does bring a question to mind: does magnesium also have an impact on strength gains? Clearly, additional research is needed in this area.
SUMMARY AND KEY POINTS
Based on this short article, there is some support that a marginal magnesium deficiency could impair exercise performance and intensify the harmful consequences of intense exercise (18). Intense exercise has been shown to increase sweat and urinary magnesium losses, which could possibly increase magnesium requirements by 10% to 20% in athletes who participate in heavy workout routines (18). In addition, inadequate dietary magnesium intake in athletes has been reported by several researchers. Thus, many athletes may have suboptimal magnesium status, which, over time, could impair performance. Although the research on overall mineral requirements for athletes is still lacking, it would behoove athletes to obtain sound advice from a registered dietitian (R.D.) to ascertain his/her overall diet, so that the R.D. could evaluate his/her specific dietary needs. Supplementation may be warranted for some athletes; however, supplementation studies have shown no effect. Thus, increased intake of foods high in magnesium would be a good first start (see Table for foods high in magnesium). Healthy eating is always a part of any good training program, regardless of individual goals (e.g., competitive athlete, recreational athlete, etc.).
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