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OBJECTIVES: Because of growing concern that constituents of drinking water may have adverse health effects, consumption of tap water in North America has decreased and consumption of bottled water has increased. Our objectives were to 1) determine whether North American tap water contains clinically important levels of calcium (Ca2+), magnesium (Mg2+), and sodium (Na+) and 2) determine whether differences in mineral content of tap water and commercially available bottled waters are clinically important.
DESIGN: We obtained mineral analysis reports from municipal water authorities of 21 major North American cities. Mineral content of tap water was compared with published data regarding commercially available bottled waters and with dietary reference intakes (DRIs).
MEASUREMENTS AND MAIN RESULTS: Mineral levels varied among tap water sources in North America and among bottled waters. European bottled waters generally contained higher mineral levels than North American tap water sources and North American bottled waters. For half of the tap water sources we examined, adults may fulfill between 8% and 16% of their Ca2+ DRI and between 6% and 31% of their Mg2+ DRI by drinking 2 liters per day. One liter of most moderate mineralization European bottled waters contained between 20% and 58% of the Ca2+ DRI and between 16% and 41% of the Mg2+ DRI in adults. High mineralization bottled waters often contained up to half of the maximum recommended daily intake of Na+.
CONCLUSION: Drinking water sources available to North Americans may contain high levels of Ca2+, Mg2+, and Na+ and may provide clinically important portions of the recommended dietary intake of these minerals. Physicians should encourage patients to check the mineral content of their drinking water, whether tap or bottled, and choose water most appropriate for their needs.
Certain constituents of drinking water may have adverse health effects. Epidemiological studies have examined the relation between exposure to trace elements (e.g., copper, zinc, arsenic) and minerals (e.g., magnesium) and the occurrence of disease, including reproductive outcomes,1 certain forms of cancer,2 rare congenital malformations of the central nervous system,3–6 cardiovascular disease,7–11 and sudden death.12–13 Because waterborne minerals are in ionic form and are easily absorbed by the gastrointestinal tract, it has been suggested that drinking water may be an important source of mineral intake.14–16 In this study, we examined calcium (Ca2+), magnesium (Mg2+), and sodium (Na+) levels because these minerals may be abundant in drinking water. In addition, Ca2+, Mg2+, and Na+ have important physiological functions, and an unsuitable intake of these minerals may increase the likelihood of disease.
Calcium intake is important at all ages,17–18 but the need for Ca2+ is higher during childhood, fetal growth, pregnancy, and lactation.19 Epidemiological, animal, and clinical studies support the existence of an inverse relation between Ca2+ intake and the occurrence of osteoporosis.20–21 A diet that is fortified in Ca2+ may reduce the rate of age-related bone loss and hip fractures, especially among adult women.22 In spite of this knowledge, nutritional surveys indicate that more than 50% of North Americans consume inadequate levels of Ca2+ and, on average, adult women consume only 60% of the required daily Ca2+ intake.23 Although many foods are now fortified with calcium (e.g., orange juice), naturally bioavailable Ca2+ is found almost exclusively in milk, milk products, and water. Drinking water may be a significant source of Ca2+, and Ca2+-rich mineral water may provide over one-third of the recommended dietary intake of this mineral in adults.15
Epidemiological studies suggest that an inverse relation exists between Mg2+ intake and the occurrence of ischemic heart disease, cardiac arrhythmias, and sudden death.12–13 Studies also suggest that an inverse relation exists between Mg2+ levels in drinking water and the occurrence of cardiac disease.24 Nonetheless, a majority of the U.S. population consumes less than the daily Mg2+ requirement, and many individuals ingest less than 80% of the recommended level.24 The major portion of Mg2+ intake is via food25 such as nuts, green leafy vegetables, cereals, and seafood.19 However, Mg2+ in water is highly bioavailable, and waterborne Mg2+ is absorbed approximately 30% faster and better than Mg2+ from food.26–27 Consequently, Mg2+ supplementation may be best achieved using a high Mg2+ nutrient with the best bioavailability such as drinking water.28
Unlike the low Ca2+ and Mg2+ intakes in the North American diet, Na+ intake generally surpasses the recommended limits and has been estimated to be in the range of 4,000 to 6,000 mg per day.23 Numerous studies have shown that a high Na+ intake is associated with the occurrence of hypertension20,22,29–32 and that dietary Na+ restriction, achieved by not adding salt and avoiding Na+-rich foods, may effectively reduce blood pressure.19 Cheese, bread, cereals, and processed and preserved foods have a high Na+ content.23,33 However, drinking certain waters may unnecessarily increase Na+ intake to a level that may be detrimental for health, especially for individuals on a Na+-restricted diet.
Over the past decade, consumption of tap water in North America has declined as sales of commercially available bottled waters have risen. One in 5 North American households now uses bottled drinking water and, in the United States, annual per capita consumption of bottled water increased from less than 8 gallons in 1991 to almost 11 gallons in 1996.34–35 Because drinking water may be an important source of mineral intake, the shift in consumption from tap water to bottled water may have important implications for health and disease. Thus, the objectives of this study were 1) to determine whether North American tap water contains clinically important levels of Ca2+, Mg2+, and Na+, and 2) to determine whether differences in the mineral content of tap water and commercially available bottled waters are clinically important.
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Mineral levels of tap water vary among North American cities and even among different water sources within the same city. Variations in mineral levels also exist among commercially available bottled waters. North American tap water and North American bottled waters generally contain low mineral levels. European bottled waters contain higher mineral levels than North American tap and bottled waters. Calcium and Mg2+ levels are highest among moderate mineralization European waters and Na+ levels are highest among high mineralization European waters.
Mineral intake from drinking water depends on the individual and on the source and quantity of the water that is being consumed. Adults who drink 2 liters of tap water that contains at least 50 mg/L of Ca2+ and 16 mg/L of Mg2+ may fulfill more than 10% of the DRIs of these minerals. This is the case for most individuals in Indianapolis, Ind; Los Angeles, Calif; San Jose, Calif; and Phoenix, Ariz; where tap water sources are generally rich in minerals. Because of their lower intake requirements, children may fulfill an important portion of their DRIs by drinking tap water. Toddlers in certain North American regions may fulfill 17% of their Ca2+ DRI and 50% of their Mg2+ DRI by drinking 4 glasses (1 L) of tap water per day.
Mineral intake from spring waters is minimal, and only some North American mineral waters contain high Ca2+ and Mg2+ levels. Drinking selected European waters may nonetheless fulfill an important portion of the Ca2+ and Mg2+ DRIs. Bottled waters such as Evian and Perrier (France) are labeled “mineral waters” but contain low mineralization levels. Mineral waters that contain moderate mineralization levels (e.g., Aproz, Contrex, Vittel Hépar), however, may best fulfill the DRIs of Ca2+ and Mg2+. Adult women may fulfill more than 20% of their Ca+ DRI and more than 17% of the Mg2+ DRI when drinking 1 liter of such bottled waters. In contrast, high mineralization bottled waters contain little Ca2+ and Mg2+ but up to 100% of the maximum recommended Na+ intake. The American Heart Association has recommended that drinking water contain a maximum of 20 mg/L of Na+ for individuals on a severely restricted Na+ diet (500 mg of Na+ per day).14 One liter of high mineralization North American or European waters may contain up to three times this maximum level.
The results of our study have several implications for the consumption of water in North America. Because of the variations in the mineral content of tap water in North American cities, North Americans do not equally consume Ca2+, Mg2+, and Na+ when drinking the same quantity of tap water. Sodium levels are generally low in tap water, but dietary intake of Ca2+ and Mg2+ can be supplemented by drinking at least 2 liters per day from mineral-rich tap water sources. This may be especially true for children and for individuals with poor dietary habits.
If North Americans prefer to drink commercially available bottled waters, they should be selective when deciding which water to drink. Individuals should choose to drink bottled water with an optimal mineral profile, i.e., high levels of Ca2+ and Mg2+ and little Na+. However, few of the bottled waters we examined have an optimal mineral profile. North Americans may also be more likely to drink mineral-deficient bottled water, such as spring waters, rather than mineral-rich bottled water. This is because mineral-rich bottled water is generally associated with an unfavorable taste. In addition, most European bottled waters are more expensive than North American waters, and many are not available to consumers in North America.
Several potential limitations of our study should be mentioned. First, although we examined the mineral content of tap water in 21 major North American cities, these cities represent only 10% of the North American population. The variation in the mineral content among all North American tap water sources may therefore be even greater than in our study. Second, the levels of Ca2+, Mg2+, and Na+ in tap water were obtained from municipal analysis reports, and levels of Ca2+, Mg2+, and Na+ in bottled waters were obtained from published data. Examining tap and bottled water samples in a single laboratory would have provided more reliable results. Finally, our study only examined levels of Ca2+, Mg2+, and Na+ in tap and bottled water. Drinking water may contain several other minerals (e.g., fluoride, potassium, zinc) and trace elements (e.g., arsenic, cyanide, lead) that are associated with benefits and risks for public health.2–11,27,44 Aesthetic factors such as taste, color, and temperature may also be important to consider when choosing drinking water.
The average North American consumes insufficient quantities of Ca2+ and Mg2+ and too much Na+. Recommended dietary intakes of Ca2+ and Mg2+ are best fulfilled via the consumption of foods in which these minerals are abundant and bioavailable. The results of our study suggest that drinking water may be an important dietary source of Ca2+, Mg2+, and Na+. This is because minerals are highly bioavailable in water and because drinking water sources available to North Americans may contain clinically important levels of these minerals. Adequate daily consumption of some tap and bottled waters may help North American children and adults supplement dietary intake of Ca2+ and Mg2+ as well as reduce Na+ intake. Physicians should therefore encourage their patients to check the mineral content of their drinking water, whether tap or bottled, and to choose the water that is most appropriate for their individual dietary needs.