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FLUID REPLACEMENT AND HEAT STRESS
Renal activity is inhibited neither by profuse sweating at room temperature nor with intake of a saline solution. When one drinks pure water, renal secretion is markedly reduced because of sweating.
Drinking of saline solution inhibits sweat excretion in people in a steam bath, while drinking of saline solution increases renal excretion in hot as well as at normal temperatures.
The best fluid for quenching thirst is 0.7% to 0.9% saline (isotonic solution), for the body retains it to a higher degree than it does any other fluid. In addition, the ingested salt is able to inhibit excessive sweating and thus prevent additional water loss and further development of thirst. Optimal drinks for workers in hot environments are slightly hypotonic solutions.
To be suitable for the quenching of thirst, a fluid must not only have the proper ingredients but it should also taste good. The taste of drinking water is determined not only by its temperature and its hardness (mineral content) but also by its acidity (carbonic acid content).
Acidified isotonic salt solutions can be used effectively as tasty liquids for the quenching of thirst.
The so-called soda waters (artificial seltzer waters) are not suitable for the quenching of thirst for they are rich in carbonic acid, which stimulates diuresis and sweating.
The only reasonably complete study of voluntary beverage selection (Sohar et al., 1962) was conducted on 19 fit young men (18-21 years old) who marched 370 miles from Eilat in the south of Israel to Metulla in the north of Israel in 24 days; this included 3 days of rest. They marched 17 miles/day, and each person carried a load of 16 kg. The various drinks investigated were warm tap water (20°-30°C), cold tap water (10°-16°C), cold sweetened lemon tea, water with sweetened citrus syrup, pasteurized bottled milk, soda water, a bottled citrus drink, carbonated lemonade, Malton (a cola), and beer. All drinks except tap water were kept between 10°C and 16°C. One 650-ml bottle of beer or citrus juice was given to each man at lunch, regardless of any other drinking. Tea and coffee were provided ad libitum at dinner.
The drink preferences were taken 11 times during the 24-day march and were divided into four groups: the most preferred drinks to the least preferred drinks.