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From water to whiskey: the importance of fluids in the diet

Sarah Schenker
BSc SRD PhD
Nutrition Scientist British Nutrition Foundation
London
E:S.schenker@nutrition.org.uk

Water, as the principal constituent of the body, comprises 50-70% of total body weight, depending on factors such as age, gender, physiological state and proportion of body fat. Thus an average 70kg man is estimated to contain about 42 litres of water, but the body of an average woman would contain less because of the larger fat stores.
To replace the average daily obligatory fluid losses through the kidneys, gut, skin and lungs, it is estimated that approximately 1.1-1.5 litres of liquid (8-10 cups) must be drunk in addition to the water derived from food and metabolism.(1) It is not just water that can be drunk to replace fluid losses: most other types of drinks can count, including fruit juice, squash, milk, soft drinks and even moderate amounts of caffeine-containing drinks such as tea, coffee and cola.
Generally, it is assumed that fluid intake takes care of itself because people respond to their thirst, but this is not always the case, especially among older people, in whom the sensation of thirst may diminish. In fact, the elderly are probably most at risk of fluid depletion. Although severe dehydration with its characteristic physical signs may be rare, a mild degree of depletion in the elderly may contribute to constipation, and the reduced urine volume will increase the risk of urinary tract infections. Mild fluid depletion reduces the ability to concentrate and may also cause lethargy and headaches, and low urine volume contributes to renal calculi in susceptible individuals.
Drinks also act as vehicles for important nutrients: for example, minerals such as calcium in milk, fluoride in tea, potassium in fruit juices and coffee, and vitamins such as vitamin C in fruit juices, and riboflavin and vitamin B(12) in milk.

Fluid intake in the elderly
The National diet and nutrition survey: people aged 65 years and over looked at the diets of 1,275 free-living people and 412 people living in institutions.(2) Of the free-living participants, the survey found that 95% drank tea (with milk), 60% drank coffee and only 2% drank herbal teas. The average amount of tea drunk per week was much greater than the average amount of coffee (24 cups versus 7 cups). The survey found that participants also consumed nondiet soft drinks (28% - mostly squash), fruit juice (26%), soup (31%), diet soft drinks (8%) and bottled water (3%). Just under 20% drank alcoholic drinks. The average fluid consumption of free-living participants was 9.7 litres per week and was higher for men (11.0 litres) than for women (8.7 litres). In both men and women, fluid consumption decreased with age.
Of the participants living in institutions, patterns of fluid consumption were similar to free-living participants. Nearly all institution participants drank tea, fewer drank coffee, with more tea being drunk than coffee (25 cups versus 4 cups per week). Forty-nine per cent drank nondiet soft drinks, and 40% consumed soup; however, only 14% drank fruit juices and diet drinks. Few consumed alcoholic drinks. The average fluid consumption of those living in institutions was 9.4 litres per week and was again higher for men (10.1 litres) than for women (9.2 litres).
The lower average fluid intake of those living in institutions may reflect a small proportion of people who are at risk of dehydration and may be due to reduced availability and accessibility of drinks. The inability to be able to drink ad libitum can be a potential problem for those living in institutions. However, the new national minimum standards that have been developed for care homes for older people in England and Wales address this by stating that hot and cold drinks and snacks must be available at all times and must be offered regularly.(3)
Older people are often reluctant to drink for fear of urinary incontinence. This may be secondary to an existing condition that hinders their mobility, such as suffering a stroke or severe arthritis. The person may worry about not being able to reach the toilet in time, difficulty in using the toilet, or that they will need to pass urine more often than usual and may wake several times at night. Stress incontinence may cause leakage when coughing or sneezing, or even during gentle exercise like walking. It is most usual in older women and is caused by a weak bladder outlet and pelvic floor muscles. After the menopause the body stops producing the hormones that help keep the vagina and bladder outlet healthy. Being overweight can put added strain on the muscles. Men may develop stress incontinence after a prostate operation.
Additionally, some medicines disturb the bladder. For example, diuretics (often prescribed to lower blood pressure) make it fill more often. People who fear incontinence should be reassured that not drinking enough and being dehydrated will only make the problem worse. Not drinking enough makes urine very strong and concentrated, the bladder then becomes used to holding very little and there is a greater risk of developing a urine infection, which may cause pain or a burning feeling and a frequent need to pass water. Encourage the person to drink normally and include a variety of fluids in their diet. However, caffeine-containing drinks and alcoholic drinks should be limited or avoided until the person feels more confident.

Caffeine and alcohol
There is quite a lot of controversy surrounding the effects of caffeine, and its effect on health has been the subject of much research. The bulk of the evidence demonstrates that moderate consumption of coffee and other caffeine- containing beverages, such as tea and cola drinks, does not pose a risk to health in normal, healthy individuals, with the exception of pregnant women.
However, there is confusion about the word moderate. The amount of caffeine in beverages will depend on the variety and the strength of the brew for tea and coffee. As a rough guideline, an average sized (150ml) cup of instant coffee contains approximately 60mg of caffeine, compared with about 80mg for filter coffee, and 40mg for a cup of tea or a can of cola. The amount of caffeine that people can tolerate varies, depending on individual sensitivities, but on the basis of existing research, a daily intake of about 250-300mg (four or five cups of coffee) is considered to represent a moderate intake.
Caffeine and alcohol exert a diuretic effect to varying degrees. The dehydrating effect of these drinks under normal circumstances is much less than commonly perceived. Various studies on caffeine have concluded that doses of more than about 300mg have a mild diuretic effect, while doses of less than 250mg do not show any effect. Regular coffee drinkers can develop habituation, which in turn raises the minimum dose needed to cause mild diuresis.
Alcohol has a more potent diuretic effect: 1g is sufficient to increase urine output by 10ml. However, a significant effect on hydration status has been observed only with strong alcoholic drinks such as spirits. The crucial factor is alcohol concentration, ie, the higher concentration, the greater the net fluid loss. So, for example, whiskey produces a greater negative fluid balance compared with beer; in fact, small quantities of beer may reinstate fluid balance or lead to positive fluid balance in subjects who are already dehydrated.

Fluid intake in sport and exercise
During exercise, water (and electrolytes) may be lost. This is because metabolic heat produced during physical activity is dissipated by radiation, conduction and convection and by vaporisation of water. In hot, dry environments, evaporation accounts for more than 80% of metabolic heat loss. Sweat rates will vary depending on variables such as body size, exercise intensity, ambient temperature, humidity and acclimation, but can exceed 1.8kg/hour.(4) In addition to water, sweat also contains substantial amounts of sodium (approximately 1g/litre), modest amounts of potassium and small amounts of minerals such as calcium and iron. It is crucial for athletes to maintain fluid balance during exercise, because performance is impaired with progressive dehydration,(5) and dehydration increases the risk of potentially life-threatening heat injury such as heat stroke. Therefore athletes should attempt to remain well hydrated before and during exercise. However, in most cases, athletes do not consume enough fluids during exercise to balance fluid losses, and are dehydrated at the end of the exercise session. Consuming up to 150% of the weight lost during an exercise session may be necessary to cover losses in sweat plus obligatory urine production.(6) Including some sodium reduces the diuresis that occurs when only plain water is ingested.(7) Sodium also helps the rehydration process by maintaining plasma osmolality and the desire to drink.

Water intoxication
The effects of water intoxication have been described in a number of conditions, such as specific medical conditions, extreme exercise or drug abuse. Water intoxication can lead to hyponatraemia, often defined as a sodium concentration of below 130mmol/l. Hyponatraemia can be life-threatening and requires urgent medical attention.

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References

  1. Wrong O. Water and monovalent electrolytes. In: Garrow JS, James WPT, Ralph A, editors. Human ­nutrition and dietetics. 10th ed. Edinburgh: Churchill Livingstone; 2000.
  2. Finch S, Doyle W, Lowe C, et al. National diet and nutrition survey: people aged 65 years and over. London: The Stationery Office; 1998.
  3. Department of Health. Care homes for older people. National minimum standards. London: The Stationery Office; 2002.
  4. American College of Sports Medicine. Position stand: heat and cold illnesses during distance running. Med Sci Sports Exerc 1996;28:i-vii.
  5. Barr SI, Costill DL, Fink WJ. Fluid replacement during prolonged exercise: effects of water, saline or no fluid. Med Sci Sports Exerc 1991;23:811-7.
  6. Shirreffs SM, Taylor AJ, Leiper JB, Maughan RJ. Post-exercise rehydration in man: effects of volume consumed and drink sodium content. Med Sci Sports Exerc 1996;28:1260-71.
  7. Maughan RJ, Leiper JB. Sodium intake and post-exercise rehydration in man. Eur J Appl Physiol 1995;73:317-25.

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