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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.
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).
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+.
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.
We contacted the municipal water authorities of the 25 most populous cities in North America to obtain mineral analysis reports. We requested information regarding levels of Ca2+, Mg2+, and Na+ for all of the water sources in each of these municipalities. In each case, we obtained mineral analysis reports for finished drinking water, i.e., water that is ready to be distributed through the tap water delivery system. Nineteen of the 25 cities provided us with mineral analysis reports for water samples collected between 1994 and 1997. Two additional cities provided us with reports for water samples collected between 1988 and 1991. The remaining 4 cities (Dallas, Tex; Jacksonville, Fla; San Antonio, Tex; and San Francisco, Calif) could not provide Ca2+, Mg2+, and Na+ data for each of their tap water sources. Most municipalities provided analyses summarizing data collected during a 12-month period, three provided summaries for samples collected during a single month (Baltimore, Md; Chicago, Ill; and Milwaukee, Wis), and the city of Seattle, Wash, provided a summary for samples collected during a single day. Based on 1996 estimates, the populations in the twenty-one participating cities represent approximately 10% of the total North American population.36
Our data included mineral analysis reports of tap water originating from watersheds such as lakes, rivers, and streams (surface water) or from wells (groundwater). According to U.S. Environmental Protection Agency (EPA) regulations, the treatment of surface water must include coagulation, filtration, and disinfection procedures. In contrast, groundwater receives natural treatment by traveling through the soil and does not usually require any additional processing, with the exception of disinfection.37 Because of the inherent differences between the two water types, we grouped tap water sources according to surface water or groundwater.
The EPA imposes stringent water treatment regulations under the authority of the Safe Drinking Water Act. The Act was established to protect the quality of drinking water and focuses on all waters actually or potentially designed for drinking use. In addition to maximum contaminant levels, EPA regulations include standard methods for the examination of water as well as analytical methods for compliance determinations of chemical and microbiological contaminants in drinking water. Primary maximum contaminant levels (MCLs) have been set to regulate the levels of arsenic, cyanide, mercury, chromium, and other chemicals associated with risks for public health. Secondary maximum contaminant levels (SMCLs) have also been set to regulate the aesthetics of tap water and relate to factors such as alkalinity, temperature, odor, color, pH, and water hardness. Importantly, owners or operators of public water systems are obligated to attain primary standards set by the EPA but are only encouraged to attain secondary standards. Levels of Ca2+, Mg2+, and Na+ are included in the SMCL category because their levels in tap water are not currently associated with risks for public health.
We obtained Ca2+, Mg2+, and Na+ levels for 37 commercially available North American bottled waters from a previous study and from published data regarding bottled waters.19,38–39 Mineral levels for commercially available European bottled waters were obtained from a single source, The Good Water Guide, detailing the geographical source, history, and market share of 250 bottled waters in 42 countries.39 In our study, we included the 73 European waters for which Ca2+, Mg2+, and Na+ levels were available in this publication.
Significant differences exist between North American and European standards regulating the bottled water industry. For example, the sale of distilled water (i.e., water that is deficient of all dissolved substances) is permitted according to the United States Bottled Water Regulations.40 In contrast, the European Economic Community Mineral Water Regulations prohibit the processing and treatment of any water bottled from a source.38 The Food and Drug Administration requires that “mineral waters“ contain between 500 and 1,500 mg/L of total dissolved solids, a combination of the dissolved minerals.38 In Europe, however, water with any level of mineralization is considered “mineral water.”
In our analyses we grouped bottled waters according to their level of mineralization. North American bottled waters were grouped into spring waters or mineral waters, according to their label. Because all European bottled waters are labeled “mineral waters,” they were grouped into low, moderate, or high mineralization waters. Precise definitions of mineralization levels vary from country to country.39 For the purpose of this study, low mineralization indicates less than 200 mg/L of Ca2+, Mg2+, and Na+, moderate mineralization indicates between 200 and 700 mg/L of these minerals, and high mineralization indicates more than 700 mg/L.
Over the past five decades, nutritional experts have established recommended dietary allowances (RDAs) for various minerals and nutrients. Recently, a cooperative effort between the United States and Canada revised previous recommendations and created dietary reference intakes (DRIs).41 Compared to the old RDAs, the new DRIs incorporate the concept of preventing nutrient deficiencies as well as risk reduction for chronic conditions such as heart disease, diabetes, hypertension, and osteoporosis. In our analyses, we compared mineral levels in tap and bottled waters to DRIs in order to examine the clinical significance of mineral intake from drinking water.
The DRI of Ca2+ is highest for adolescents (1,300 mg) and for the elderly (1,200 mg). Adult men and women 19 to 50 years of age require 1,000 mg of Ca2+ per day. A 250 ml glass of milk typically contains 300 mg of Ca2+, one cup of cottage cheese contains approximately 100 mg of Ca2+, and two tablespoons of cream cheese contain approximately 30 mg.42 For Mg2+, the DRI has been set at 6 mg/kg/day in industrialized countries.28 A 70-kg North American male, for instance, requires 420 mg of Mg2+ daily. Dietary reference intakes of Mg2+ are generally higher for males than for females but also depend on age. A 30-g serving of almonds or half a cup of spinach contain approximately 80 mg of Mg2+, and one third of a cup of bran cereal contains approximately 50 mg.42 Currently established DRIs do not yet include estimates for Na+. Previously established RDA estimates, however, indicate that healthy adults require at least 500 mg of Na+ per day,43 and nutritional experts have set a maximum recommended intake of 2,400 to 3,000 mg of Na+ per day.23 A hamburger typically contains more than 500 mg of Na+, 1 cup of macaroni and cheese contains more than 700 mg of Na+, and 2 slices of pizza may contain more than 1,000 mg.42
Published data on water consumption are limited, and the few available studies have reported an important variability in tap water intakes in North America.14 The amount of water consumed daily varies from individual to individual and largely depends on other sources of fluids.11 Nutritional experts have recommended that consumption of 30 ml/kg/day of water is sufficient for the elderly and that a provision of 150 ml/kg/day is recommended for infants.45 To examine the clinical significance of mineral intake from drinking water, we made assumptions regarding the consumption of tap water and bottled water in North America. We assumed that adults drink 2 liters of tap water per day, equivalent to eight 250 ml glasses. Because bottled water is more expensive and less readily available than tap water, we also assumed that adults only drink 1 liter of bottled water per day, equivalent to approximately three (commonly sold) 355 ml bottles. In Table 1, we provide the gender and age-specific DRIs of Ca2+ and Mg2+. The reader may therefore compare recommended intakes with actual intakes according to varying quantities and sources of water.
Levels of Ca2+, Mg2+, and Na+ varied within each type of tap or bottled water in our study. In addition, sample sizes were small for groundwater sources (n = 8), for North American mineral waters (n = 9), and for high mineralization European bottled waters (n = 7). Mean levels can be skewed by extreme values in small samples. Consequently, we report the mean, standard deviation, median, and range of Ca2+, Mg2+, and Na+ levels for the different tap and bottled waters in our study. We also report correlation coefficients (r) to examine the association between Ca2+, Mg2+, and Na+ levels within the same type of drinking water.
Important variations exist in the mineral content of tap water among the North American cities investigated (Table 1 and Table 2). In general, levels of Ca2+, Mg2+, and Na+ were higher among groundwater sources than among surface water sources (Table 2). Tap water sources that contained high levels of Ca2+ generally contained high levels of Mg2+ (r = 0.86) but not necessarily high levels of Na+ (r = 0.36). Of the twelve states and three provinces in our study, mineral levels were highest in Arizona, California, Indiana, and Texas. Variations were also found in mineral content of different water sources within the same North American city. Calcium levels, for example, varied from 9 to 60 mg/L among the three water sources in San Jose. In Los Angeles, Mg2+ levels varied from 5 to 29 mg/L (4 sources), and in Columbus, Na+ levels varied from 10 to 51 mg/L (2 sources). Variations therefore exist in the levels of Ca2+, Mg2+, and Na+ among the tap water sources of North American cities and even among different water sources within the same city.
When compared to the recommended daily intakes of Ca2+, Mg2+, and Na+, mineral intake from tap water is generally low but may be important when drinking from mineral-rich sources. For half of the tap water sources, adults may fulfill between 8% and 16% of their Ca2+ DRI by drinking 2 liters per day. Similarly, in every other water source, adult men may fulfill between 6% and 23% of their Mg2+ DRI, and adult women may fulfill between 8% and 31% of their Mg2+ DRI by drinking 2 liters per day. In most tap water sources, however, 2 liters contain less than 5% of the maximum recommended daily intake of Na+. Thus, in some North American cities, drinking 2 liters of tap water per day from mineral-rich tap water sources may fulfill clinically significant portions of the Ca2+ and Mg2+ DRIs in adult men and women.
Mineral levels varied among commercially available North American and European bottled waters (Table 1,Table 3,and Table 4). North American spring waters contained very low mineral levels. North American mineral waters generally contained high levels of Na+ and some contained important levels of Ca2+ and Mg2+. The only strong correlation found was between Ca2+ and Mg2+ levels in mineral waters (r = 0.71). Among European bottled waters, moderate mineralization waters contained the highest levels of Ca2+ and Mg2+, and high mineralization waters contained the highest levels of Na+ (Table 1 and Table 4). Among moderate mineralization waters, higher Ca2+ levels corresponded to lower Na+ levels (r = −0.61), and among high mineralization waters, higher Na+ levels corresponded to lower levels of Ca2+ (r = −0.75) and Mg2+ (r = −0.76).
When compared to the recommended intakes of Ca2+, Mg2+, and Na+, mineral intake from bottled water depends on the type of water that is being consumed. Adults fulfill very little (<3%) of their DRIs when drinking most spring waters. Drinking North American mineral waters, however, may fulfill an important proportion of the Ca2+ and Mg2+ DRIs as well as the maximum recommended intake for Na+. For instance, one liter of Mendocino mineral water contains more than 30% of the Ca2+ and Mg2+ DRIs in adult women, and 1 liter of Vichy Springs contains more than one third of the maximum recommended Na+ intake. On the other hand, drinking 1 liter of most moderate mineralization European waters may help North Americans fulfill between 20% and 58% of their Ca2+ DRI and between 16% and 41% of their Mg2+ DRI. High mineralization European waters are rich in Na2+ and 1 liter may contain up to 47% of the maximum recommended daily intake of this mineral. Thus, Ca2+, Mg2+, and Na+ intake from selected commercially available bottled waters may be appreciably higher than from most tap water sources, even when drinking only 1 liter of bottled water per day.
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.
Dr. Eisenberg is a research scholar of the Heart and Stroke Foundation of Canada.