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Magnesium 5: 1-8 (1986)

Magnesium Content of the Food Supply in the Modern-Day World1

J.R. Marier

Division of Biological Sciences, National Research Council, Ottawa, Canada


Presented at the International Symposium on 'Magnesium and Its Relationship to Cardiovascular, Renal, and Metabolic Disorders', Los Angeles, Feb. 12 1985.


Key Words. Magnesium · Diet · Drinking water · Food refining · Vegetarians · Hypertension · Stress · Cardiac · RDA

Abstract. A large-scale US survey has shown that the dietary magnesium intake tends to be lower than recommended. The suboptimal intake prevalent among US adults is consistent with the pattern observed in other North American and European surveys. Several factors are discussed, including the waterborne magnesium factor, the loss of magnesium during food refining and the magnesium content of vegetarian diets, as well as various metabolic situations, e.g., hypertension, pregnancy, osteoporosis, drug therapy, alcoholism, stress and cardiac trauma. The benefits of magnesium supplementation among those with sub-RDA intakes are illustrated.

In 1977-1978, the US Department of Agriculture (USDA) conducted a 'Nationwide Food Consumption Survey' which showed that the dietary intake of magnesium in the continental US tends to be lower than recommended [38]. The pattern is illustrated in table I. Note that more than 37,000 individuals were surveyed (including all age groups from the newborn to the elderly), but that only 25% of these persons had a dietary magnesium intake that equalled or exceeded the Recommended Dietary Allowance (RDA) proposed by the US National Academy of Sciences. In contrast, 75% of the individuals did not meet the RDA criterion, and this group was about equally divided between those whose magnesium intake was from 70 to 99% of the RDA, and others with intakes that were less than 70% of the RDA, In particular, it must be emphasized that 39% of the subjects were in the latter category of lowest magnesium intakes.

Modern Table I

Another aspect of the USDA data reveals that the lowest intake group consisted mostly of teenagers and adults [38]. On this basis, it can be calculated that the dietary magnesium averaged 76% of the RDA among those aged 19 or more, and this translates into the quantitative per diem intakes shown in table II. It can be seen that the suboptimal intakes observed in the USDA survey are compatible with those reported in other surveys conducted in various regions of North America and Europe [34]. However, bear in mind that the USDA data represent an average for the entire continental US, but that much lower magnesium intakes have been reported in some geographical regions such as the Newfoundland area of Canada; and also among institutionalized elderly populations. In such cases, the average magnesium intake can be as low as 143-189 mg/day (i.e. only about 50% of the RDA), and the daily shortfall in magnesium intake can be double the amount derived from the USDA data. Here, it is important to emphasize that - on an overall basis - the shortfall in magnesium intake tends to range from 72 to 161 mg/day (see table II).

Table II

Some critics might claim that the magnesium status of modern-day populations is more adequate than indicated by the RDA criterion. But if indeed the magnesium status of modern-day populations was adequate, it would be impossible to observe the waterborne magnesium factor which provides an extra source of daily magnesium intake and has been seen to reduce the mortality/morbidity patterns on a global basis [35]. For cardiac cases in the Ontario region of Canada, it has been calculated that a daily magnesium supplement of 159 mg (in addition to the normal dietary intake) would be required to prevent efflux of myocardial magnesium [35]. This dosage is identical to the upper limit of the shortfall range shown in table II, and suggests that individuals with very low magnesium intakes may be at particular risk from cardiac-related trauma.

The question of why the modern-day world's magnesium intake is suboptimal has been addressed in previous reviews [1, 32, 33]. Basically, this situation has evolved because of an increased dependency on overly refined and/or processed cereal-crop and carbohydrate food staples in which the preponderance of the magnesium has been removed (see table III). Given this impoverished magnesium content of traditional dietary staples, it is difficult to provide additional magnesium by recourse to alternative modern-day food sources. Substances such as nuts, chocolate, beans, shrimp and bran all contain elevated concentrations of magnesium [44], but are not easily incorporated into a daily dietary regimen. In comparison, meats and eggs are a relatively poor source of magnesium. It is true that milk and cheese can con tribute a sizeable proportion of the dietary magnesium [2, 38]. However, these dairy, products provide about 10 times more calcium than magnesium (i.e. about 6 times more calcium on a molar basis), and this factor might be important if one wants to avoid calcium supplementation while replenishing a magnesium deficiency [see ref. 8, 13]. Moreover, for those who advocate a 'protective' effect of calcium in syndromes such as hypertension and heart disease, it is well to remember the epidemiological pattern in Finland where the per capita dietary intake of calcium is among the highest in the world [i.e. 1,300 mg/day; see ref. 22], but where there is an exceptionally high death rate from cardiovascular diseases [26].

Table III

Recent surveys in Sweden [1, 2] have indicated that vegetarian diets provide a more than adequate intake of magnesium, as illustrated in table IV. Note that the magnesium intake of men and women on vegetarian diets was considerably greater than the RDA requirement; this was attributed to the high intake of vegetables and whole-grain cereals by these individuals. In comparison, table IV also shows that the magnesium supplied by 'normal' omnivorous diets was only 58-83% of the RDA. Although these vegetarian diets tended to be particularly low in vitamin B it is nevertheless of interest to note that the vegetarians had lower levels of blood lipids (i.e., cholesterol, triglycerides with low LDL/HDL ratios) than subjects on 'normal' diets. Also, the vegetarians' blood pressure averaged 120/80, which is low for their age group of 49-55 years.

Table IV

Similar results have been reported from Australia [42a], where the prevalence of mild hypertension was found to be 10% among those ingesting an omnivorous diet, but only 1.5% among vegetarians. The significant fall in blood pressure associated with a vegetarian diet averaged 7 mm Hg systolic and 3 mm Hg diastolic. Vegetarians also had lower serum cholesterol levels. In comparison with the omnivorous regimen, the lacto-vegetarian diets supplied 34% more magnesium [42a], and this is compatible with the Swedish data presented in table IV.

The researchers in both Sweden and Australia wondered if the antihypertensive effect of vegetarian diets might be due to a higher intake of poly-unsaturated fatty acids, or fiber, or magnesium or other minerals. There is, however, compelling evidence in favor of a crucial role for magnesium.

In Hungary, a daily magnesium supplement of 80-160 mg resulted in a decreased blood pressure in 74 industrial workers during a 6-week period; the fall in blood pressure averaged 15 mm Hg systolic and 10 mm Hg diastolic [see ref. 29]. In Sweden, clinical supplementation of magnesium (i.e. 360 mg/day, equivalent to the RDA requirement) reduced both the systolic and diastolic blood pressure by an average of 12/8 mm Hg during a 6-month period [see ref. 14]. A collaborative study involving US and German researchers has provided experimental evidence demonstrating that increasing the severity of magnesium deficiency produces graded elevations of arterial blood pressure [4]. A clinical study in the US has demonstrated that there is significant depletion of intracellular ionic magnesium in erythrocytes of humans with essential hypertension, with an inverse linear relationship between intracellular Mg and diastolic blood pressure [40]. In Finland, the use of a magnesium-supplemented table salt has resulted in a fall in blood pressure, averaging about 7 mm Hg systolic and 2 mm Hg diastolic [see ref. 27]. This latter result is almost identical with the effect of a lacto-vegetarian diet in Australia [42].

Mention has been made of the fact that institutionalized elderly populations can have low magnesium intakes that account for only about 50% of the RDA. This same situation applies to pregnant women [cf. ref. 34] and also to postpartum women, whether or not they are breast-feeding [36]. At this point, it is pertinent to mention that a magnesium deficiency status has been reported in postmenopausal women who have osteoporosis [10 see also ref. 12].

Another 'low magnesium' subgroup of the population consists of women who are engaged in athletics. Thus, the magnesium intake of young adult female athletes in Indiana was only 68% of the RDA [23] and less than 66% of the RDA among female collegiate gymnasts in Salt Lake City [31] and among female students in Fairbanks, Alaska [46]. The concern about low magnesium intakes among people engaged in sports activities was the subject of a recent discussion in France [37], and the basis for this concern can be traced back to Selye's work on stress. Recent biochemical studies with experimental animals in Canada [47] and the US [5] have shown that magnesium deficiency decreases the capacity of an animal to withstand prolonged exercise, but that all activities involving endurance stress were enhanced by magnesium supplementation. In view of the generally low magnesium intakes prevalent in modern-day populations, along with the current emphasis on 'jogging' and other forms of exercise, one can only wonder about the long-term outcome.

Superimposed on this entire pattern is the fact that specific clinical syndromes can induce magnesium deficiency [17, 48], and that metabolic magnesium depletion can result from chronic use of digitalis or diuretics and from alcohol abuse [11, 17, 41, 48]. An alert was published on this topic in 1970 by Becking and Morrison [7] of Health and Welfare Canada:

'The occurrence of hypomagnesemia in humans, due in part to prolonged use of diuretics, alcoholism, pregnancy etc., has been shown to be more prevalent than earlier reports would have indicated.'

In this context, it is necessary to point out an additional modern-day problem; significant decreases in serum magnesium have been reported among women using oral contraceptives [18, 43].

Clinically, slow-rate infusion of magnesium (thereby doubling or tripling the serum magnesium concentration) has been used successfully in the management of premature labor seizures of pregnancy eclampsia [16, 39], and in the treatment of cardiac-related trauma [15, 30, 33, 34]. Oral magnesium supplementation has been used to inhibit formation of kidney stones [24, 33] and to prevent postsurgical cardiac arrhythmias [28].

In a recent publication [45], it was reported that patients who have a magnesium intake below the US RDA requirement respond with an improved clinical condition after magnesium supplementation. Table V presents a list of human conditions that have been improved by oral magnesium supplementation. Aside from the situations already mentioned, this tabulation includes metabolic conditions that seem to be particularly relevant to the modern-day world. The effectiveness of magnesium supplementation in all of these conditions cannot be attributed to mere coincidence, but rather suggests that the low dietary magnesium status prevalent in the modern-day world's adults increases their susceptibility to a variety of ailments, many of which involve impaired resistance to stress-induced tension.

Modern Table V

It is particularly interesting to note that, in 6 of the 9 metabolic conditions listed in table V, clinical improvement was achieved with supplemental magnesium dosages in the range of 67-160 mg/day, i.e. almost identical with the 72-161 mg/day 'shortfall' in the magnesium intake of modern-day-world's adults, as assessed by the US RDA criterion (see table II). This can be taken as an indication that the US RDA is a reasonably accurate index of metabolic magnesium requirements.

In contrast, a report issued by Health And Welfare Canada in 1983 [21] recommends daily magnesium intakes of only 240 and 190 mg by adult men and women, respectively. Both these values are 110 mg below the US RDA criteria. The only published basis for the recent Canadian recommendation is a report from 1967 [25], although close scrutiny of these data indicates a dietary magnesium requirement of 280 mg/day for a mixed population of adults [32], and graphic interpretation reveals that 360 mg/day would be required to avoid the risk of a negative magnesium balance in some individuals. At this point, it bears repeating that recent studies [45] have shown that patients who have a magnesium intake below the US RDA requirement respond with an improved clinical condition after magnesium supplementation.

Therefore, in terms of everything that has been discussed in this presentation, the modern-day world's dietary magnesium status appears bleak, unless one happens to be a vegetarian or has access to magnesium-rich drinking water (e.g., 30 to 90 mg/l [see ref. 35]). As has recently been suggested [9], it is perhaps time to return to the concept of 'subclinical magnesium deficiency'. However, one of the main problems is that many hospitals still do not perform routine magnesium analyses [see ref. 42b, 48].

La teneur en magnesium de l'apport alimentaire dans le monde d'aujourd'hui

Un examen aux USA sur une large échelle a montré que l'absorption de Mg avec la ration tend à être plus faible que celle qui est recommandée. L'absorption suboptimale, qui prévaut chez les adultes des USA, est en concordance avec les caractéristiques observées dans d'autres examens aux USA et en Europe. Nous discutons de l'importance de plusieurs facteurs, y compris le facteur du Mg véhiculé par l'eau, la perte de Mg au cours du raffinage des aliments et la teneur en Mg des regimes végétariens, ainsi que de plusieurs situations métaboliques, entre autres l'hypertension, la grossesse, l'ostéoporose, la thérapeutique médicamenteuse, l'alcoolisme, le stress et le traumatisme cardiaque. Nous présentons les bienfaits d'un apport supplémentaire de Mg chez les sujets avec un apport suboptimal de la ration.

References

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Received: March 12, 1985
Accepted: May 15, 1985

J.R. Marier,
Environmental Secretariat,
Division of Biological Sciences,
National Research Council,
Ottawa, Ontario, K1A 0R6 (Canada)


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