2.12.2 Fluid gain
In a normal diet, fluid is gained via food as well as in drinks. The amount of water in various foods is shown in Table 6. As well as plain water, most drinks, such as tea, coffee, juices and milk drinks, hydrate the body, but alcoholic drinks may not. Alcohol is a diuretic, a substance that increases the output of urine by the body. Calculations indicate that for each unit of alcohol taken in (1 unit=about 8 g alcohol), about 80 ml of extra water is lost from the body. If the unit of alcohol is taken in as a half pint of beer, then more fluid would have been taken in than was lost, so dehydration would not result. However, if the alcohol is taken in as wine or spirits, in a much smaller volume, then dehydration can result. Advice to alternate alcoholic and soft drinks, and to drink extra water at bedtime, is designed to offset the dehydration effect and go some way towards preventing a ‘hangover’ the next day. Caffeine, found in coffee and tea, is also a diuretic but over 300 mg a day is needed to have a diuretic effect and surveys in the UK find daily intakes well below this value. Individual fluid requirements vary but intakes of about 1 litre per day in food and 1.5 litres in drinks (nonalcoholic) are typical.
Table 6 The water content of some foods.
|grilled oily fish||65|
Water is also produced in the body. When proteins are synthesised by linking together amino acids, a water molecule is produced for every peptide bond made. When fatty acids are joined to glycerol to make a fat (triacylglycerol), water molecules are also generated. And finally, linking monosaccharides together to make carbohydrates also generates water. Overall, about a quarter of a litre (0.25 litre) of water per day is gained by the body from such metabolic processes.
Many people now drink mineral water, often carrying a bottle with them. Mineral water is thought to be ‘better’ in some way than drinking tapwater. In fact, tapwater contains adequate minerals too. Currently in the UK, water companies must satisfy the requirements of the Water Supply (Water Quality) Regulations 1989, which give prescribed concentration values (upper limits) for 57 different parameters. The limits for a few of them are given in Table 7.
Table 7 The legal upper limits of some dissolved mineral ions in drinking water in the UK.
|Upper limits/mg per litre|
|calcium (Ca 2+ )||250|
|magnesium (Mg 2+ )||50|
|potassium (K + )||12|
|sodium (Na + )||150|
|chloride (Cl − )||400|
|sulphate (SO42− )||250|
|nitrate (NO3− )||50|
The regulations also specify a range for the pH of 5.5–5.9 for tapwater. pH is a measure of the acidity or alkalinity of the water, with 7 being neutral, values below 7 being acidic and those above 7 being alkaline. The dry residue is the amount of material left when a sample of the water is boiled to dryness.
The values for mineral water are all far below the limits given and, in fact, mineral water appears to be rather low in minerals, compared with the limits permissible in drinking water. The bicarbonate in the mineral water is produced by the dissolved carbon dioxide and this also lowers the pH value.
Tapwater and mineral water may, of course, come from exactly the same source. However, some tapwater is obtained from sources that are at risk of contamination from microbes, and in the UK, tapwater is therefore filtered and pretreated with chlorine and other chemicals to make it safe to drink. Bottled water that is labelled as ‘natural mineral water’ is extracted from the ground, and is bottled at source without any treatment. If the water is sparkling when it comes from the ground it is labelled as ‘naturally carbonated natural mineral water’. If the carbon dioxide is added at the bottling plant, it must be labelled ‘carbonated (or sparkling) natural mineral water’. Water labelled as ‘spring water’ must be obtained from an underground source, be bottled at source and be micro-biologically safe without any treatment. However, certain other treatments, such as the chemical removal of minerals whose levels are too high, are permitted. ‘Table water’, on the other hand, need comply only with regulations on water quality for tapwater and can be bottled anywhere.