Abstract
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.
Keywords: tap water, bottled water, calcium, magnesium, sodium
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 to almost 11 gallons in .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.
METHODS
Tap Water
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 and . Two additional cities provided us with reports for water samples collected between and . 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 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.
Bottled Waters
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.
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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.
Dietary Reference Intakes
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.
Table 1.
Mineral Content of North American Tap Water, North American Bottled Waters, and European Bottled Waters (mg/L)
Ca2+
Mg2+
Na+
Males and Females
Males
Females*
Males and Females
Dietary reference intake mg/day
'1 ' 3 years
500
80
80
Maximum recommended intake of 2,400 to 3,000 mg per day
'4 ' 8 years
800
130
130
'9 ' 18 years
240'410
240'360
'19 ' 50 years
400'420
310'320
'>50 years
420
320
North American tap water
'Surface water sources (n = 36)
''Mean ±SD
34 ± 21
10 ± 8
35 ± 41
''Median
36
8
18
''Range
2 ' 83
0 ' 29
0 ' 169
'Ground water sources (n = 8)
''Mean ±SD
52 ± 24
20 ± 13
91 ± 67
''Median
48
12
83
''Range
26 ' 85
2 ' 48
8 ' 195
North American bottled waters
'Spring Waters (n = 28)
''Mean ±SD
18 ± 22
8 ± 18
4 ± 4
''Median
6
3
4
''Range
0 ' 76
0 ' 95
0 ' 15
'Mineral Waters (n = 9)
''Mean ±SD
100 ± 125
24 ± 42
371 ± 335
''Median
8
7
240
''Range
3 ' 310
1 ' 130
36 '
European bottled waters
'Low mineralization waters (n = 40)
''Mean ±SD
60 ± 40
16 ± 19
13 ± 13
''Median
54
14
9
''Range
4 ' 145
1 ' 110
1 ' 56
'Moderate mineralization waters (n = 26)
''Mean ±SD
262 ± 139
64 ± 37
157 ± 197
''Median
217
56
49
''Range
78 ' 575
9 ' 128
2 ' 660
'High mineralization waters (n = 7)
''Mean ±SD
60 ± 59
16 ± 20
1,151 ± 153
''Median
33
9
1,133
''Range
5 ' 176
4 ' 60
900 ' 1,419
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Statistical Analysis
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.
RESULTS
North American Tap 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.
Table 2.
Mineral Content of Tap Water in Major North American Cities (mg/L)
City
Water Source
Ca2+
Mg2+
Na+
Surface water
'Baltimore, Md
Montebello
21
6
11
Ashburton
20
4
9
'Boston, Mass
Winsor Dam
2
1
3
Wachusett
4
1
7
Norumbega
4
1
10
Weston
4
1
12
Spot Pond
5
1
16
'Chicago, Ill
North
37
1
8
South
36
12
7
'Cincinnati, Ohio
Single Source
38
6
17
'Columbus, Ohio
Dublin Road
36
8
51
Hap Cremean
27
10
10
'Denver, Colo
Marston
31
7
18
Foothills
28
7
21
Moffat
18
3
8
'Detroit, Mich
Moffat
26
7
5
'El Paso, Tex
Central
43
15
132
East
56
13
160
'Houston, Tex*
Single Source
21
2
38
'Indianapolis, Ind
White River
83
28
47
Fall Creek
64
25
20
TW Moses
51
18
18
White River North
78
29
41
'Kansas, Mo
Single Source
51
8
57
'Los Angeles, Calif
Los Angeles Aqueduct
21
5
37
River Conduit
58
13
48
Jensen
39
16
57
Weymouth
68
29
98
'Milwaukee, Wis*
Weymouth
36
12
8
'Montreal, Quebec
Single source
34
8
11
'New York, NY
Catskill-Delaware
6
1
6
Croton
21
4
18
'Philadelphia, Pa
Baxter
28
5
14
Queen Lane
39
13
33
Belmont
42
12
24
'Phoenix, Ariz
Croton
51
20
169
'San Diego, Calif
Skinner, Winchester
66
27
92
'San Jose, Calif
Santa Clara Valley
56
14
57
Hetch Hetchy
9
3
9
'Toronto, Ontario
Tolt
40
9
12
'Vancouver, British Colunbia
Single source
2
0
0
'Ground water
'Columbus, Ohio
Parsons Ave.
32
10
62
'El Paso, Tex
West
26
2
145
Northeast
44
11
104
East
52
12
195
Airport
34
8
134
'Indianapolis, Ind
Geist
83
26
8
Harding
85
40
32
'San Jose, Calif
Hetch Hetchy
60
48
48
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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.
Commercially Available Bottled Waters
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).
Table 3.
Mineral Content of Selected Commercially Available North American Bottled Waters (mg/L)
Ca2+
Mg2+
Na+
Spring waters
'Adobe Springs, Calif
3
96
5
'Alhambra, Calif
1
1
4
'Arrowhead, Calif
20
5
3
'Black Mountain, Calif
25
1
8
'Caddo Valley, Ark
36
3
2
'Canadian Spring, Canada
11
3
2
'Carolina Mountain, NC
6
0
5
'Clairval, Canada
20
7
13
'Cobb Mountain, Calif
5
2
4
'Crystal Geyser Alpine, Calif
0
6
13
Deer Park, Me
1
1
1
Georgia Mountain Water, Ga
2
0
0
'Great Bear, NY
1
1
3
'Hawaiian Springs, Hawaii
6
3
6
'La Croix, Wis
37
22
4
'Mount Olympus, Utah
8
2
3
'Mountain Valley, Ark
68
8
3
'Naya, Canada
38
20
6
'Ozarka, Tex
18
1
5
'Poland Spring, Me
0
2
3
'Pure Hawaiian, Hawaii
0
0
0
'Pure Spring Water, Ga
49
4
0
'Sierra, Calif
0
0
0
'Sparkletts, Calif
5
5
15
'Talawanda Spring, Ohio
0
0
3
'Talking Rain, Wash
2
2
0
'Utopia, Tex
76
17
8
'Zephyrhills, Fla
52
7
4
Mineral waters
'A Santé, Calif
4
1
160
'Calistoga, Calif
7
1
150
'Canada Geese, Canada
282
10
36
'Crystal Geyser, Calif
8
3
160
'Lithia Springs, Ga
120
7
680
'Mendocino, Calif
310
130
240
'Montclair, Canada
8
12
475
'Montellier, Canada
3
3
340
'Vichy Springs, Calif
157
48
1,095
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Table 4.
Mineral Content of Selected Commercially Available European Bottled Waters (mg/L)
Ca2+
Mg2+
Na+
Low mineral content*
'Abbey Well, United Kingdom
54
36
45
'Acqua di Nepi, Italy
72
26
32
'Acqua Fabia, Italy
124
5
15
'Acqua Panna, Italy
15
5
3
'Aqua-Pura, Engalnd
53
7
27
'Ballygowan, Ireland
114
16
15
'Boario, Italy
124
41
6
'Brecon Carreg, United Kingdom
48
17
6
'Bru, Belgium
23
23
10
'Buxton, United Kingdom
55
19
24
'Chiltern Hills, England
104
1
8
'Claudia, Italy
104
22
56
'Cristalp, Switzerland
115
40
20
'Crodo Lisiel, Italy
60
2
6
'Evian, France
78
24
5
'Fiuggi, Italy
15
5
6
'Font Vella, Spain
26
5
12
'Fonter, Spain
35
7
11
'Glenpatrick Spring, Ireland
112
15
12
'Henniez, Switzerland
111
19
9
'Hella, Germany
51
4
8
'Highland Spring, United Kingdom
39
15
9
'Levissima, Italy
18
1
1
'Naleczowianka
119
24
21
'Perrier, France
145
4
14
'San Benedetto, Italy
43
25
8
'San Bernardo, Italy
12
1
1
'Spa Reine, Belgium
4
1
3
'St. Michaelis, Germany
43
4
21
'Strathmore, United Kingdom
60
15
46
'Tipperary, Ireland
37
23
25
'Thorspring, Iceland
6
1
8
'Valvert, Belgium
68
2
2
'Vera, Italy
34
13
2
'Vichy Nouvelle, Finland
70
110
1
'Viladrau, Spain
16
2
9
'Vittel Bonne Source, France
91
20
7
'Volvic, France
10
6
9
'Voslauer, Austria
57
37
5
Moderate mineral content'
'Apollinaris, Germany
89
104
425
'Aproz, Switzerland
454
67
8
'Badoit, France
200
100
160
'Contrex, France
467
84
7
'Crodo Valle d'oro, Italy
510
51
2
'Fachingen, Germany
113
62
500
'Ferrarelle, Italy
408
23
50
'Franken Brunnen, Germany
198
42
52
'Gerolsteiner, Germany
364
113
129
'Hassia Sprudel, Germany
176
36
232
'Vittel Hépar, France
575
118
13
'Passugger, Switzerland
286
24
46
'Pedras Salgadas, Portugal
132
9
550
'Peterstaler, Germany
216
49
215
'Pracastello, Italy
164
46
28
'Robacher, Germany
256
128
40
'Rippoldsauer, Germany
248
37
150
'Robacher, Germany
256
128
40
'Romerquelle, Austria
146
65
13
'Radenska, Slovenia
217
97
470
'Salus Vidago, Spain
78
10
660
'San Pellegrino, Italy
204
57
47
'Sangemini, Italy
322
19
21
'Valser, Switzerland
436
54
11
'Vichy Original, Finland
100
110
220
'Vittel Grande Source, France
202
36
3
High mineral content'
'Kaiser Friedrich, Germany
5
4
'Krystynka, Poland
176
60
900
'SaintYorre, France
30
7
'San Narciso, Spain
53
9
'Uberkinger, Germany
26
17
'Vichy Celestins, France
100
9
'Vichy Catalan, Spain
33
8
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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.
DISCUSSION
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.
Acknowledgments
Dr. Eisenberg is a research scholar of the Heart and Stroke Foundation of Canada.
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