Archives of Environmental Health, January/February 1981 (Vol. 36, No.1)
ABSTRACT. During the last decade many epidemiologists have found an inverse relationship between water hardness and cardiovascular mortality. It seems unlikely that this relationship can only be attributed to a deficiency of calcium and magnesium in drinking water, because only 10-20% of the total daily intake of calcium and magnesium is derived from drinking water. The purpose of this study was to investigate changes in the mineral composition of food when cooking with waters of different hardness. The most significant differences were found for calcium; the concentration of this element in potatoes and vegetables usually increased when cooking with hard-water types, while a decrease was noted when soft water was used for cooking.
THE TERM “hardness of water” is derived from the characteristics of cations such as calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), aluminum (Al), and strontium (Sr) to form insoluble compounds with soap. Calcium and Mg are the major constituents in drinking water that contribute to its hardness. Hardness is usually expressed in terms of the equivalent concentration of calcium carbonate (CaCO3). Drinking water hardness exceeding 150 mg/L (ppm) as CaCO3 is considered to be “hard.” Statistical evidence indicates that death rates from cardiovascular diseases are inversely correlated with the hardness of drinking water.1-4 Three possible explanations for the observed statistical associations follow.
1) Calcium and/or Mg intake by consumption of hard drinking water helps meet the total dietary requirements for these essential elements, and may in turn, suppress the toxic effect of some heavy metals.1,2
2) Heavy metals in soft drinking water [e.g., cadmium (Cd) and lead (Pb)] released from piping, resulting in supposedly higher corrosiveness of the softer waters during distribution, have toxic effects.5-9
3) While cooking food, “soft” water extracts more essential elements (e.g., Ca and Mg) from vegetables and potatoes than “hard” water.10
The second above listed explanation seems unlikely, particularly in The Netherlands, because significant positive correlations were found between the hardness of drinking water and the concentration of metals—Cu and Pb—released from lead and copper water distribution pipes.14
The purpose of this study is to investigate changes in the mineral content of foods when boiled in waters of different hardness. This type of study was recommended by a World Health Organization (WHO) working group on health effects of the removal of substances naturally occurring in drinking water.11 The total daily intake of Ca and Mg from an average diet is mainly derived from milk, bread, and vegetables. The estimated intake of Ca and Mg from 2 L drinking water per day is usually less than 10-20% of the total daily intake.1 Although Ca and Mg are readily absorbed from water, the intake of Ca and Mg from food, considering the effect of cooking in waters of different hardness, may contribute a plausible explanation for the observed inverse statistical association between cardiovascular disease mortality and hardness of drinking water.10
Water for the cooking experiments was sampled at the tap at five different localities in each of six selected water supply areas: 1) The Hague, 2) Wageningen, 3) Arnhem, 4) Zutphen, 5) Maastricht, and 6) Kerkrade. In this way a representative sample of drinking water for each city was obtained. Data on hardness, conductivity, HCO3- and pH of these water samples are given in Table 1. For the cooking experiments the following foods were chosen: potatoes, cauliflower, carrots, and endive. These foods can contribute considerably to the average daily intake of Ca and Mg12, 13 (Table 2).
All cooking experiments were conducted in duplicate. One-hundred-fifty grams of all examined foodstuffs were cooked in beakers with 250 ml water and 1.25 g NaCl. After draining, the cooked potatoes and vegetables were freeze-dried and ground. Weighed portions of the freeze-dry samples were ashed with sulfuric acid (H2SO4) at 500°C; the resulting white ash was dissolved in hydrochloric acid (HCl).
The water that remained after cooking was adjusted to the original 250 ml volume with double distilled water to correct for losses by evaporation. After centrifugation, the solution was refluxed with nitric acid (HNO3) and hydrogen perioxide (H2O2) to destruct organic compounds which might interfere with the elemental analysis by atomic absorption spectroscopy. A Perkin Elmer model 373 flame atomic absorption spectrometer was used for determination of Fe, Mn, Ca, Mg, sodium (Na), potassium (K), and zinc (Zn) in the destructed samples of food and water. Trace elements, i.e., Cd, Pb, were determined by anodic stripping voltammetry using a PAR model 374 polarographic analyzer.
The concentrations of Ca, Mg, Fe, Na, K, Zn, Cd, and Pb were determined in both food and water samples before and after cooking. A selection of data on Ca, Mg, and Pb is presented in Figure 1 and Table 2. The calcium concentration in potatoes and vegetables usually increased when cooking with hard water types, while in most cases it decreased when soft water was used for cooking. The results of the Mg analyses in food and water show that in all cases Mg is extracted from the food. This effect was slightly pronounced when the food was prepared with soft water types. The Pb concentrations in food were found to be higher after cooking with hard water and lower when soft water was used for cooking. These differences, however, were probably caused by the higher Pb concentrations in the hard water types (Table 1). The concentration in the examined foodstuffs of elements such as Fe, Mn, K, and Zn were all lower after cooking, regardless of the water hardness. A few cooking experiments were repeated under the same conditions as described above except for the addition of table salt. The results of these tests were approximately the same as for the other experiments.
The results of the cooking tests, with special reference to the changes in Ca and Mg concentration when food is cooked in hard and soft water, support the theory that the statistical inverse relation found between hardness of drinking water and cardiovascular disease mortality is possibly caused by a deficiency of these elements in soft drinking water. Such a deficiency will be further increased by the extraction of Ca and Mg from food by soft waters. A more extensive study of the factors affecting changes in the mineral composition of food during cooking is recommended before final conclusions can be drawn.
This study was conducted under contract no. 273-77-1 ENV-N of the European Communities Environmental Research Programme.
Submitted for publication July 10,1980; accepted for publication August 11, 1980.
Requests for reprints should be sent to: Dr. B. J. A. Haring, National Institute for Water Supply, P. 0. Box 150, 2260 A.D. Leidschendan, The Netherlands.
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11. World Health Organization. 1979. Health effects of the removal of substances occurring naturally in drinking water, with special reference to demineralized and desalinated water. Report on a Working Group, Brussels, 20-23 March 1978. Euro Reports and Studies 16. Copenhagen: World Health Organization.
12. United States Department of Agriculture. 1975. Handbook of the Nutritional Contents of Foods. New York: Dover.
13. Nederlandse Voedingsmiddelen Tabel. 1979. ‘s-Gravenhage: Voorlichtingsbureau voor de Voeding.
14. Haring, B.J.A., and Zoeteman, B.C.J. 1980. Corrosiveness of drinking water and cardiovascular disease mortality. Bull Environ Contam Toxicol 25: 658-62.
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