Magnesium 6: 18-27 (1987)
This paper was presented at the 4th International Symposium on Magnesium, July 23-28, 1985, Blacksburg, Va, USA.
Key Words. Pregnancy · Diet · Nutrient density · Drinking water · Protein · Magnesium · Preeclampsia
Abstract. The mean dietary magnesium intake of pregnant women is 35-58% of the recommended dietary allowance of 450 mg. Low-income women consumed 97-100 mg magnesium/1,000 kcal while women with higher incomes averaged 120 mg/1,000 kcal. Diets high in fat and sugar and low in whole grains, vegetables and fruits have a lower magnesium density. Magnesium content of water can also make a significant contribution to magnesium intake. Magnesium from prenatal supplements, if present, is seldom more than 100 mg. Additional supplementation is needed for adequate magnesium nutriture during pregnancy.
There is a growing literature which provides evidence that a compromised magnesium nutritional status may be involved in several disorders that can occur during pregnancy. These include hypertension [1, 10, 11, 19], vasospasm [1, 2, 27], coagulation defects , premature delivery [12, 26], intrauterine growth retardation [10-12, 26] and muscle cramping [4, 9, 16, 31]. Magnesium nutritional status can be influenced by a number of factors, but the easiest to correct is magnesium intake. In addition, dietary factors can modulate the absorption of the amount of magnesium ingested.
Before discussing magnesium intake during pregnancy, and factors that affect it, I would like to draw attention to previous reviews of this subject by Seelig [38, 42]. Her studies have provided an indepth review of material published prior to 1980. Rather than summarizing her work, I will address work published since then and, hopefully, provide some additional insights into the subject.
There have been six studies, published since 1980, that have reported magnesium intake during pregnancy in the United States (table I). The mean magnesium intakes from these studies have ranged from 158 to 259 mg a day when no supplements were taken. These intakes are 35-58% of the recommended dietary allowance (RDA) of 450 mg .
Johnson and Philipps  estimated the magnesium intake in 47 pregnant women during the fall and winter of 1974. Each women recorded her food intake every 8th day from the 5th month until delivery. This provided a total of 568 dietary records. Magnesium intakes ranged from 103 to 333 mg/day with an average of 204 ± 54 mg (± SD). None reached the recommended allowance of 450 mg. Two cases of toxemia occurred. The daily magnesium intakes of these two women were estimated to be 178 and 186mg.
In 1981, Endres et al,  reported the magnesium intake of pregnant women participating in a supplemental food program in 1978-79. Nutrient intakes were estimated from 24-hour recalls obtained by trained interviewers from 10% of participants in the Special Supplemental Food Program for Women, Infants, and Children (WIC) in the state of Illinois. This program is sponsored by the US government. Of the 766 women, 15% had been receiving supplemental foods for at least 6 months, 85% were applicants to the program. Estimated magnesium intakes of WIC applicants 35 ± 19% of the RDA of 450 mg. Those participating in the WIC program averaged 41 ± 21% of the RDA. This translates to 158 and 184 mg magnesium per day.
Butte et al.  in 1981 reported the magnesium intake of pregnant Navajo women. These women were from a rural, semiarid part of Arizona in the southwestern part of the United States and were planning to breastfeed their infants. Typical food intakes were determined using a questionnaire and by a 24-hour dietary recall at term. The median magnesium intake was 230 mg, which is only 51% of the RDA. The mean daily magnesium intake was 241 ± 114 mg. Diets reflected few fruits and vegetables and frequent soft drinks. Prenatal supplements were prescribed which would provide an additional 100mg of magnesium, if used.
Brennan et al.  analyzed duplicate food composites collected from 22 pregnant low-income women who attended an obstetrical clinic in an urban area. Food composites were collected with a nutritionist present in the participant’s home. Even though women were instructed ‘to prepare and consume meals in the usual manner’, the presence of the nutritionist could have influenced food choices. Mean magnesium intake was 200 ± 85 mg.
Magnesium intake was determined by Lillien et al.  as part of a study evaluating diet and ethanol intake during pregnancy. Diet records were determined by a 24-hour recall type interview 1-4 days postpartum. Subjects were asked to recall food intake of an ‘average day’. The mean magnesium intake from diet alone for the 578 women was 259 ± 4 mg. When supplements were included, magnesium intake was 316 ± 5 mg.
In the study of Moser et al. , 3-day dietary records were kept by 35 middle-class women during the 37th week of pregnancy. Those women who were planning to nurse their infants had a mean magnesium intake of 239 ± 11, while those not planning to nurse had a mean intake of 198 ± 15 mg. When compared on the basis of nutrient density, the magnesium content of the diets were the same. This shows that those women who were not going to nurse their infants were eating less total food, which was reflected in a decreased magnesium intake.
Since none of the mean intakes of the previous studies reached the RDA, it is possible that the RDA is too high. This possibility was indirectly tested in an extensive magnesium balance study with 10 pregnant women . The goal was to perform two 7-day balance studies on all 10 women during each trimester of pregnancy, but this was not achieved in all cases. In all, 47 7-day balance periods were obtained. Magnesium intake averaged 269 ± 55 mg/day. Negligible amounts were provided by prenatal supplements. Only 3 of the 47 balances were positive for magnesium. Magnesium balance averaged -40 ± 50 mg/day. If it were assumed that these negative balances occurred for 200 of the 280 days of pregnancy, the women would have lost 8 g of magnesium during pregnancy. Since it has been estimated that a 70-kg individual contains 20-25 g of magnesium in the entire body, an 8-gram loss is significant.
The results of this balance study suggest that average magnesium intakes are not adequate for pregnant women. No justification for lowering the RDA below 450 mg can be made.
When the diets previously described were analyzed on the basis of magnesium nutrient density (table I), low-income women had magnesium intakes of 97-100 mg/1,000 kcal. As the income increased, magnesium nutrient density increased. Those women from middle-class families had magnesium intakes of 119-121 mg/1,000 kcal. This suggests that women with more money are able to buy food that has greater magnesium nutrient density.
The magnesium density of common foods consumed in the United States is illustrated in table II. Note that foods with nutrient densities of less than 50 mg/1,000 kcal are high in fat or sugar. As the nutrient density increases, foods contain less fat and sugar. Whole grains and legumes appear. Vegetables and fruits gradually dominate the foods with the highest nutrient density, because these foods are inherently low in calories.
Dietary magnesium intake reflects the nutrient density of individual food choices. Diets low in magnesium density would contain refined cereal grains, fatty meats, high amounts of fats and sugars, few fruits and vegetables and sugar-containing soft drinks. Diets high in magnesium density would contain whole grains, lean meats, low amounts of fats and sugars, abundant fruits and vegetables, and low-fat milk. It is possible to have diets which contain more than 200 mg/1,000 kcal if wise choices are made.
Chadhuri  observed that toxemic women preferred more refined cereal grains, fewer leafy green vegetables and more fat than women who did not develop toxemia. Chung et al.  found that toxemic women chose diets higher in fat and cholesterol than women without toxemia.
It has been estimated that magnesium intake from 2 liters of water can range from 2 to more than 50 mg/day depending upon the hardness of the water . Johnson and Philipps  estimated that the pregnant women in their study in Wisconsin ingested about 35 mg of magnesium a day from the water.
Water hardness in the United States varies from very soft to very hard . Water hardness is expressed in terms of calcium content but magnesium can make a substantial contribution to its hardness. Those states with soft water are found in the northwest, New England, and in the southeastern parts of the country.
Maternal deaths attributed to toxemia of pregnancy from 1961 to 1965 in the United States were 6.2/100,000 live births . This is the national average, but there was a wide range deviating from it, depending upon where the woman lived. Some states had half the national average while others exceeded the national average. Toxemic deaths in Mississippi were nearly five times the national average. The highest incidence of toxemic deaths occurred in the southeastern part of the United States. These are the states with the softest water.
It has been noted that per capita income of states is inversely related to deaths from toxemia . If states were divided into three groups based on their per capita income from 1961 to 1965, those which would be in the upper third of the income groups had a toxemic death rate of 61% of the national average. Those states in the middle third of the per capita income groups had a toxemic death rate of 95% of the national average, while those states in the lowest third of the income groups had a toxemic death rate of 192% of the national average. The poorest state is Mississippi which had a toxemic death rate of 487% of the national average.
There are eleven states with soft water where deaths from toxemia were less than 85% of the national average. Nine of these states had a high per capita income. These states are Connecticut, Delaware, New York, Massachusetts, Maryland, Washington, Rhode Island, Oregon, and New Jersey. Only Vermont and Maine did not have high per capita incomes. Conversely, there were nine states with toxemic deaths of more than 150% of the national average. Seven of these states are in soft-water areas. Of these seven, six rank in the lowest third of the per capita income groups and one is in the middle third. The six states, Georgia, North Carolina, Alabama, Arkansas, South Carolina and Mississippi, are in the southeastern part of the country. The seventh state, New Hampshire, is in New England. The remaining two states with high rates of toxemic deaths are Tennessee and Texas. Tennessee has moderately soft water and is in the lowest third of the per capita income groups. Texas has hard water and is in the middle third of the income groups, but it has a large low-income population.
The average temperature of the state may also be associated with toxemic deaths. The lowest incidence of toxemic deaths occurs in the northern states. The further south the states are located, the more the toxemic death rate increases. The highest toxemic death rates generally occur in the most southern states at every latitude. Since the average temperature rises as the location of the state is further south, sweat losses of magnesium may be an important factor. The highest average temperatures would be in the southern states: the same states where the toxemic death rate was the highest. This may be a factor in Texas which contributes to the increased maternal death rate related to toxemia even though the state has hard water.
By putting water hardness, per capita in come, mean temperature and toxemic deaths together, the environment where toxemia occurs becomes evident. Toxemic deaths would have the highest incidence where (1) per capita income is low, (2) the mean temperature is elevated, and (3) magnesium content of the water is low.
A low per capita income would limit the ability of the woman to buy foods with a high magnesium density. I have previously shown that low-income women have a lower nutrient density of magnesium in their diet. With an elevated mean temperature, there would be increased sweat losses. A low-income mother would not be able to afford housing with air conditioning, which would help to minimize sweat losses. Sweat losses of magnesium may be considerable depending upon the mean temperature [13, 38]. Magnesium content of drinking water would only be important if the mother is in a marginal magnesium state. This may occur with a low per capita income and a high mean environmental temperature. Now, the magnesium content of the water could be crucial. If the water were soft with essentially no magnesium, the mother would develop toxemia. If the water were hard with a high level of magnesium, it could be protective against development of toxemia.
In Great Britain, it has been observed that there is less preeclampsia and eclampsia in London than in other parts of the country . London also has hard water .
Not all prenatal supplements used in the United States contain magnesium. Within the last few years, more include magnesium than previously. Magnesium is usually included in the form of magnesium oxide. Magnesium oxide is 60% magnesium by weight.
If magnesium is included in a prenatal supplement, it is normally added in amounts of 100mg of elemental magnesium. There is no information about the bioavailability of magnesium contained in these supplements. Prenatal supplements do not provide enough magnesium to meet the RDA since most dietary intakes are less than 300 mg. Additional magnesium supplementation for pregnant women is recommended.
Several factors have been implicated in modulating magnesium absorption from the intestine. These have been reviewed extensively by Seelig [38-41], Lindeman , and Tansy and Kendall .
It has been quoted for a long time that increasing protein in the diet will increase magnesium absorption. This is based on the work of McCance et al.  in 1942. They fed diets which contained 0.7-0.9 g of protein/kg body weight and found a magnesium absorption of 32%. When the diets contained 2.3-2.6 g of protein/kg body weight, magnesium absorption was 41%. These diets also contained fiber and phytic acid from vegetables and 92% extraction wheatmeal bread.
Hunt and Schofield  fed women diets in which the protein intake varied from 20 to 48 g. Magnesium absorption varied from 28 to 53% with the highest absorption occurring in diets with 48 g of protein. When this study is analyzed on the basis of grams of protein per kilogram of body weight, protein intake with a mean of 0.36 g/kg body weight resulted in a 28% absorption of magnesium. Mean protein intakes of 0.46, 0.59 and 0.76 g/kg body weight resulted in magnesium absorptions of 46, 42 and 53%, respectively. Essentially, protein intakes of more than 0.4 g/kg body weight resulted in magnesium absorptions of more than 40%. These diets were low in phytic acid but fiber was present from fruits and vegetables.
Mahalko et al.  arrived at similar findings. They fed men diets that contained 0.86 and 1.24 g of protein per kilogram body weight and obtained magnesium absorptions of 51.5 and 52.7%, respectively. These diets were also low in phytic acid. With such levels of protein intake, an increase in magnesium absorption would not be expected, especially if magnesium absorption was already maximal.
Fat in the diet may act in two different ways. A high-fat intake can dilute the magnesium nutrient density of the diet and has been shown to reduce the absorption of magnesium [29, 38-4l]. More recent studies showed that, while fat reduced magnesium absorption in individuals with malabsorption syndromes  varying the fat intake from 22 to 42% of the energy intake in normal men had no influence on magnesium absorption . The reason for the apparent conflict in the literature is not evident.
Fiber can reduce magnesium absorption [22, 25, 36, 47]. Phytic acid has also been implicated in reducing magnesium absorption [36, 37, 41].
Diets high in fiber and phytic acid may result in decreased absorption of magnesium. Fortunately, foods which contain fiber and phytic acid usually also contain higher amounts of magnesium.
A paradox occurs in discussing the influence of calcium and phosphorus on magnesium absorption. Calcium and phosphorus have been reported to decrease magnesium absorption [29, 38-41, 46], but Spencer et al. have shown no influence of calcium [43, 45] or phosphorus  on magnesium absorption in men with magnesium intakes typical of American diets.
Among a group of pregnant women (24 years of age and older) we were studying, we found that those women with the most muscle cramping during pregnancy had lower magnesium intakes and higher calcium and phosphorus intakes than women with less cramping. Ratios of Ca/Mg and P/Mg intakes were 4.51 ± 0.79 and 4.71 ± 0.63, respectively, in women with most muscle cramping and 3.70 ± 0.79 and 4.06 ± 0.54, respectively, in the women with less cramping. These Ca/Mg and P/Mg ratios were significantly different between the two groups of women (p = 0.025; p = 0.011; n = 25).
These data make me question the common recommendation of a quart of milk daily for pregnant women. A quart of skim milk would provide 1,200 mg of calcium, 980 mg of phosphorus, 120 mg of magnesium, and has Ca/Mg and P/Mg ratios of 10.0 and 8.7, respectively. These are in excess of similar ratios (2.7) derived from the RDA and based on magnesium at 450 mg and calcium and phosphorus at 1,200 mg each . In addition, prenatal supplements commonly provide 200 mg of calcium. With a quart of milk, a prenatal supplement and other foods in the diet, calcium and phosphorus intakes would be excessive for the magnesium content. A recommendation of 2 - 2 1/2 cups of milk when combined with the prenatal supplement and other foods in a balanced diet will meet the RDA for calcium and phosphorus for pregnancy and provide better Ca/Mg and P/Mg ratios.
Since calcium and phosphorus are highly correlated in diets (p <0.001), it is not completely clear whether it is calcium or phosphorus that influences magnesium absorption most. Since the P/Mg ratio of the food intakes of the pregnant women in our study was more significant that the Ca/Mg ratio, phosphorus may be more important than calcium. Further study is needed in this area.
Magnesium intake of pregnant women needs to be increased by either dietary means or supplements, especially among low-income women in soft-water states. Excessive intakes of calcium and phosphorus may be detrimental to magnesium absorption.
L’absorption moyenne de magnesium avec le régime chez les femmes enceintes est de 35-58% de la ration alimentaire recommandée de 450 mg. Les femmes avec un faible revenu ant consommé 97-100 mg/1000 kcal, alors que les femmes avec des revenus plus élevés ont consommé en moyenne 120 mg/ 1000 kcal. Les régimes riches en graisses et en sucres et pauvres en céréales completes, en végetaux et en fruits présentent une densité plus faible du Mg. La teneur en Mg de l’eau peut aussi contribuer significativement à la consommation de Mg. Le Mg des produits supplémentaires prénataux, s’il est present, est rarement supérieur à 100 mg. Un apport supplémentaire est nécessaire pour une nutrition adequate en magnesium au cours de la grossesse.
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Received: September 28, 1985
Accepted: January 20, 1986
Kay B. Franz, PhD,
2218 Smith Family Living Center,
Department of Food Science and Nutrition,
Brigham Young University,
Provo, UT 84602 (USA)
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