Recently, Glen Matten delivered this interesting lecture to BCNH students on this controversial topic. Here is an extract from this very informative lecture:-
There is a pervading idea that we should all drink more. But how many of us question why this is so important, or even, upon what evidence it is founded?
All living things depend upon water. In the human body, a multitude of biochemical reactions are dependent on water and fluid regulation is essential to homeostasis.
Water is perhaps one of the most important nutrients in the body.
However, what should be a relatively straightforward topic is, on the contrary, a topic riddled with controversy and conflicting opinion. Not only is there a lack of consensus on how much water we should consume, there are also issues surrounding the purity of our water supply.
1. The Human Body: “A Hairy Bag of Water”!
Water is absolutely vital to the human body. In fact, it represents the largest single component of the body. The total weight of the human body is composed of 45-75% water. Adult male body weight is composed of about 60% water, whereas females have slightly less at around 55% of body weight. This variance can be explained by the fact that adipose tissue contains only a very small percentage of water. By the same logic, overweight people have a smaller proportion of water as a percentage of body weight than lean people. Infants have the highest percentage of water, with up to 75% of their body weight being water (Tortora and Grabowski 1996).
Approximately two thirds of bodily water resides inside the cells. This is known as intracellular fluid (ICF). The remaining one third is found outside of the cells. This is known as extracellular fluid (ECF). Around 80% of ECF is interstitial fluid, which is found in the tiny spaces between tissue cells. The remaining ECF, around 20%, is blood plasma (Tortora and Grabowski 1996). Extracellular and intracellular compartments are separated by cell membranes that are selectively permeable to the movement of water, thus maintaining balance between them. It is primarily through osmosis that water moves into and out of body fluid compartments. There is a relatively small amount of water in the bloodstream yet this is vital to body function and must be kept relatively constant. In contrast, the water outside of the bloodstream acts as a reservoir that can either replenish or absorb excess water from the bloodstream as required.
Water is the main component of all body fluids and predominates in most tissues. For example, water accounts for 92% of blood plasma, 75% of muscle tissue and 60% of red blood cells. Even tooth enamel and bone contain some water, albeit a very small amount (Tortora and Grabowski 1996).
2. The role and functions of water?
By looking at the properties of water, it becomes apparent why it is such an abundant component of the human body. The following properties of water are adapted from Tortora and Grabowski (1996):
- Water participates in chemical reactions. One example may be during digestion, where water can be added to a large nutrient molecule in order to break it down to a smaller molecule (hydrolysis). Similarly, in synthesis reactions, a water molecule can be removed to enable two smaller molecules to form a larger one. This type of reaction occurs in the production of proteins for example.
- Water has a high heat capacity. A property of water is that it is able to absorb heat and only show a small increasing in temperature. Likewise, the opposite is true. Water is able to give off heat with only a small decrease in temperature. This makes water an ideal component of living organisms for maintaining homeostatic temperature control.
- A large amount of heat is required to change water from a liquid to gas. The process or perspiration gives off a large amount of heat. This provides the body with an excellent cooling mechanism.
- Water acts as a lubricant. Water is a major component of lubricating fluids such as mucus. For example, lubrication in the chest and abdomen allows internal organs to slide easily over one another. Water also lubricates the joints, where bones, ligaments and joints rub against one another.
- Water is an excellent solvent and suspending medium. Water is a versatile solvent. The dissolving quality of water is essential to health as it is able to suspend many different substances. This makes it an ideal medium within which metabolic reactions can take place.
- Water dissolves waste products. Water is essential for eliminating toxins from the body, allowing them to be flushed out of the body in urine via the kidneys.
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3. Balancing water loss and water gain
Under normal circumstances, water loss equals water gain in the body.
Water gain is primarily from ingested liquids and foods where it is absorbed throughout the gastrointestinal tract, mainly in the jejunum and also in the colon. A smaller amount is gained through metabolic water. This is water produced through normal metabolism during the breakdown of substances such as carbohydrates, fats and proteins. Estimates for water need vary, but it is commoly cited that the body needs around 2.5 litres of water per day (Tortora and Grabowski 1996). The Institute of Medicine of the National Academies of Science report (2004) stated that, in the US, the average woman needs about 2.7 litres daily, whilst the average man needs 3.7 litres daily. The report used these figures to set an Adequate Intake (AI) for total water intake.
Water is lost from the body through a number of different channels. Principally, the kidneys excrete water from the body in the form of urine. Other avenues of water loss include the skin, the lungs and the gastrointestinal tract. Menstruation is a further route for loss of fluid from the body.
It is important to note that many of these channels of fluid loss are highly variable – for example, additional fluid might be lost through excessive sweating during strenuous exercise or in a hot climate, or lost in faeces during a bout of diarrhoea.
Certain substances, such as alcohol, dehydrate the body, and increase requirements for water.
4. Dehydration
When water loss is greater than water gain, dehydration results. Dehydration can be defined as a deficiency of body water. When dehydration occurs, the thirst sensation is stimulated in at least three ways:
· Decreased production of saliva (leading to a dry mouth and pharynx)
· Increased blood osmotic pressure (stimulates osmoreceptors in the hypothalamus)
· Decreased blood volume (increases angiotensin II)
These factors work together to stimulate the thirst centre in the hypothalamus. This serves to increase the sensation of thirst, stimulating increased fluid intake, thus restoring normal fluid balance.
However, this may not always be a sufficiently efficient response to prevent dehydration. In certain situations, such as heavy sweating or fluid loss from diarrhoea or vomiting, it is prudent to start replacing fluids before the thirst sensation is activated. It should also be noted that the thirst mechanism might not always be reliable in young children, the elderly, or those in a confused mental state.
Aside from such specific situations, the idea that activation of the thirst sensation automatically means that dehydration has already set-in, may be no more than a nutritional myth. Phillips et al (1984) demonstrated that a group of men, left to their own devices, became thirsty and drank fluids before their hydration levels showed signs of dipping. The authors concluded, “The results indicate that during free access to water humans become thirsty and drink before body fluid deficits develop, perhaps in response to subtle oropharyngeal cues, and so provide evidence for anticipatory thirst and drinking in man.” This tends to suggest that the thirst mechanism is a very sensitive mechanism for regulating fluid intake.
Certain abnormal conditions may greatly influence water loss. Hyperventilation increases the loss of water vapour via the lungs. Vomiting and diarrhoea lead to fluid loss form the GI tract. Fever, heavy perspiration, and burns that cause destruction to extensive areas of the skin, can bring about extensive water loss through the skin. Diseases such as diabetes mellitus, diabetes insipidus and Addison’s disease can lead to excessive water loss and dehydration, as can the use of diuretics.
If water consumption does not meet fluid loss, dehydration becomes more severe. Sweating is decreased and less urine is produced. A large amount of the body’s water reserve is inside cells. This reservoir is drawn upon and water is moved into the bloodstream.
Common causes of dehydration (such as excessive sweating, vomiting, diarrhoea) also lead to the loss of electrolytes (especially sodium and potassium). Dehydration can thus also be accompanied by a deficiency of electrolytes. In this situation, water doesn’t move as efficiently from the reservoir inside the cells into the blood, further reducing blood volume. Blood pressure can drop, causing light-headedness, or the sensation of impending blackout, notably upon standing. Should water and electrolyte losses continue unabated, blood pressure can fall to a dangerously low level. This results in shock and severe damage to internal organs.
5. Water Intake: How much?
Quantifying an ideal or optimal total water intake is not as straightforward as it may seem.
There appears to be a pervading opinion that we are all dehydrated to varying degrees and that we would all benefit from drinking more water. But to what extent is this actually based upon sound reasoning or scientific evidence?
The Food Standard’s Agency suggest, "In climates such as the UK, we should drink approximately 1.2 litres (6 to 8 glasses) of fluid every day to stop us getting dehydrated. In hotter climates the body needs more than this. We also get some fluid from the food we eat." Note that these recommendations relate to ‘average’ conditions, not accounting for variance according to diet, activity levels, climate, body size, stage of life or other factors.
A more definitive assessment of fluid needs was presented in a report by the Institute of Medicine of the National Academies of Science (2004). This report set out an Adequate Intake of water for the average US women of around 2.7 litres daily, and the average US man of around 3.7 litres daily. Those who are very physically active or who live in hot climates may need to consume more water. The report concluded that the vast majority of people can meet their need for water by drinking when they are thirsty.
The report highlighted that around 80% of total water comes from drinking water and other beverages – including caffeinated beverages. Food usually provides us with around one fifth of our daily fluid intake. So for example, a significant amount of water can be obtained through the consumption of fresh fruits and vegetables, which have a very high water content. Similarly, many other foods have substantial water content, with milk at 90%, yoghurt at 80% and even foods like cooked pastas and rice containing more than 65% water.
Based upon an assumption that water obtained from drinks each day represents 81% of our fluid intake, the Institute of Medicine of the National Academies of Science report (2004) suggested that a male adult aged 19-50 years, might need to obtain 3 litres of water per day from drinks, and a female adult aged 19-50 years, might require 2.2 litres of water per day from drinks. Importantly, the types of food being consumed, other beverages, climate and level of physical activity are all important factors to consider when advising someone on water consumption. So a hot environment or strenuous physical activity will increase fluid requirements.
This approach challenges established notions of an ideal water intake that applies to everyone. Indeed the chair of the panel that wrote the report, Lawrence Appel, stated “We don't offer any rule of thumb based on how many glasses of water people should drink each day because our hydration needs can be met through a variety of sources in addition to drinking water…While drinking water is a frequent choice for hydration, people also get water from juice, milk, coffee, tea, soda, fruits, vegetables, and other foods and beverages as well.”
In fact, taken to its logical conclusion, it challenges the belief that people need to drink any plain water at all to stay hydrated, providing sufficient total water is obtained from food and other beverages. Should we therefore think twice before telling everyone that they should consume 8 glasses of water everyday? The “8-by-8 rule” appears not to have much of a scientific basis at all.
Indeed, the idea that everyone should drink 8 glasses of water daily appears to have stemmed from the advice of the Food and Nutrition Board of the National Research Council in 1945 who advised that "A suitable allowance of water for adults is 2.5 liters daily in most instances. An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods." Valtin (2002) pointed out that the last sentence may not have been fully heeded, and that the recommendation was falsely interpreted as eight glasses of water (or 2.5 litres) to be drunk each day. Valtin (2002) comments,
“No scientific studies were found in support of 8 x 8…This conclusion is supported by published studies showing that caffeinated drinks (and, to a lesser extent, mild alcoholic beverages like beer in moderation) may indeed be counted toward the daily total, as well as by the large body of published experiments that attest to the precision and effectiveness of the osmoregulatory system for maintaining water balance… It is to be emphasized that the conclusion is limited to healthy adults in a temperate climate leading a largely sedentary existence, precisely the population and conditions that the "at least" in 8 x 8 refers to. Equally to be emphasized, lest the message of this review be misconstrued, is the fact (based on published evidence) that large intakes of fluid, equal to and greater than 8 x 8, are advisable for the treatment or prevention of some diseases and certainly are called for under special circumstances, such as vigorous work and exercise, especially in hot climates.”
Whilst eight glasses of water may indeed be ideal for one person, it appears that there is not a universal rule that applies to everyone.
Another example is a recent study by Grandjean et al (2003), which demonstrated that plain water is not needed as long as sufficient fluid is consumed through food and other beverages. In healthy, sedentary subjects, two different diets were evaluated for their effect on hydration. One group were given plain water as part of the beverages served. The other group were not given any plain water from the beverages served, with food, orange juice, diet soda and coffee being consumed for fluid. Diet, physical activity and environment were controlled and monitored. The study concluded that there were no differences between the groups for body weight or other indicators of hydration status that were measured. Thus the conclusion reached was that, “Inclusion of plain drinking water compared to exclusion of plain drinking water in the diet did not affect the markers of hydration used in this study.”
If one is to rely on thirst as a barometer of hydration in the body, it is important to be aware that children, the elderly and those in a confused mental state may not be able to rely solely on thirst to regulate drinking habits and fluid consumption. These groups have a less acute sense of thirst. Likewise, in circumstances where fluids are lost through profuse sweating, diarrhoea or vomiting, it is prudent to start replacing body fluids before the thirst mechanism is activated.
It is also worth bearing in mind that adopting this approach to fluid consumption is somewhat one-dimensional, based upon the notion that ‘all fluids are equal’. The concern being, for instance, that a litre of fizzy drink be perceived as being just as good for hydrating the body as a litre of pure water. Clearly there are issues of quality as well as quantity that should be considered when looking at fluid consumption.
The Dehydrating Effects of Alcohol
Lay wisdom has long extolled the notion that alcohol, tea and coffee are all dehydrating to the body and increase the body’s need for water.
It is universally accepted that alcohol has a dehydrating effect. Alcohol acts as a diuretic, suppressing the production of anti-diuretic hormone, thus leading to the excretion of water from the body. For example, Eggleton (1942) found that for every 1g of alcohol consumed, urine excretion increased by 10ml.
The Dehydrating Effects of Tea and Coffee: Myth or Reality?
For many years it has been well-versed nutritional advice that tea, coffee and other caffeinated beverages have a dehydrating effect through a mild diuretic action.
For example, Neuhauser-Berthold et al (1997) studied the effects of caffeine on 12 healthy men and women. These regular coffee drinkers abstained from all caffeine for a five-day period prior to the study. Results showed that ingestion of six cups of coffee per day resulted in a 41% increase in 24-hour urine excretion, a negative fluid balance and weight loss, all indicative of dehydration. Total body water as measured with bioelectrical impedance analysis decreased by 2.7%. Interestingly, despite the apparent dehydration, only 2 of the 12 subjects experienced thirst.
Recent research has challenged this perspective as being no more than a nutritional myth, demonstrating that caffeinated drinks can indeed count toward the daily fluid requirement. For those who virtually never drink caffeine, a caffeinated drink may have a slight dehydrating effect. However, with regular consumption, the body is likely to get used to these drinks, and can thus be counted toward daily fluid consumption.
An interesting study to note here is the work of Grandjean et al (2000). The study examined how different combinations of water, coffee and caffeinated cola affected levels of hydration in the body. During one phase of the study, the volunteers consumed only water, whilst in another phase 75% of the intake was from caffeinated beverages. No difference at all was found in almost every test devised to measure hydration. The study thus concluded, “This preliminary study found no significant differences in the effect of various combinations of beverages on hydration status of healthy adult males. Advising people to disregard caffeinated beverages as part of the daily fluid intake is not substantiated by the results of this study.”
Indeed, the authors of the extensive report by the Institute of Medicine of the National Academies of Science (2004) stated, “While concerns have been raised that caffeine has a diuretic effect, available evidence indicates that this effect may be transient, and there is no convincing evidence that caffeine leads to cumulative total body water deficits. Therefore, the panel concluded that when it comes to meeting daily hydration needs, caffeinated beverages can contribute as much as noncaffeinated options”
This conclusion found further support in the results of a study of fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption (Armstrong et al 2005). In the study, healthy males consumed 3mg of caffeine on days 1 to 6. On days 7 to 11, subjects consumed either 0mg (placebo), 3mg or 6 mg of caffeine in a capsule form, with no other dietary intake. On days 1, 3, 6, 9 and 11 the following variables were unaffected by different caffeine doses: body mass, urine osmolality, urine specific gravity, urine colour, 24-h urine volume, 24-h Na+ and K+ excretion, 24-h creatinine, blood urea nitrogen, serum Na+ and K+, serum osmolality, hematocrit, and total plasma protein. The researchers concluded that the findings “question the widely accepted notion that caffeine consumption acts chronically as a diuretic.”
So where does that leave us? One way to understand the differing conclusions of these studies is to consider the concept that caffeine tolerance can develop with habitual consumption, thus reducing its dehydrating effect. The subjects in the Grandjean et al (2000) study were all habitual caffeine consumers, where as the subjects in the Neuhauser-Berthold et al (1997) study were effectively “caffeine naïve” subjects who had lost caffeine tolerance through pre-study abstinence.
6. Water and Performance
Water is crucial to optimum sporting performance. Indeed, the muscles that drive performance are 75% water.
In a review of the negative effects of dehydration on exercise performance, Barr (1999) noted that even modest levels of dehydration (less than 2% loss of body weight) reduced aerobic endurance and resulted in increased body temperatures, heart rate and perceived exertion. Dehydration of 2-3% loss of body weight places a significant strain on circulatory function due to a reduction in plasma volume. This adversely affects both the capacity for exercise and thermoregulation. Dehydration of 5% loss of body weight results in increased rectal temperature and heart rate and decreased sweating rate, exercise capacity and VO2max (a measure of aerobic fitness). With more severe dehydration (6-10% of body weight), heat stroke and heat exhaustion become life threatening.
It is thought that mild dehydration may have relatively little effect on muscle strength, whereas aerobic tasks are more adversely affected by dehydration. This may be explained by a reduction in blood flow to exercising muscles and hence an adverse effect on oxygen supply, compared with forms of exercise where metabolism is predominantly anaerobic.
In the field of sports nutrition, it is often recommended that to avoid overheating and to sustain performance, it is necessary to drink enough to sweat. Pre-hydrating, drinking during performance, and rehydrating afterwards are particularly important for endurance athletes.
However, there is no general consensus on the optimal frequency or volume of water for athletes/exercisers. This will largely be dependent on variable factors such as duration and intensity of exercise, environmental conditions and the characteristics of the individual.
It should be noted that over-consumption of water is potentially dangerous and can lead to the serious condition of hyponatraemic encephalopathy. Thus, whilst acknowledging the performance inhibiting effects of dehydration, exercisers should also be advised of the dangers of over-consumption of fluids before, during or after exercise.
11. Which Water?
There is much debate as to what is the healthiest type of water to drink. Water companies assert that tap water complies with strict safety requirements. Whilst drinking water may be safe in terms of the short-term risk of illness, it could be argued that the long-term effects of a cocktail of low-level contaminants remain largely unknown.
Distilled Water
Advantages
- Very efficient at removing impurities including micro-organisms and contaminants.
Disadvantages
- Expensive and bulky equipment required.
- High maintenance.
- Removes everything - good (calcium, magnesium) as well as bad.
- Questionable taste.
Reverse Osmosis
Water is forced under pressure through a semi-permeable membrane.
Advantages
- Very effective in removing contaminants.
- Cheap and easy to operate.
Disadvantages
- Cost of installation is expensive.
- Large storage space required for tank.
- Removes everything – good (calcium, magnesium) as well as bad.
- Environmentally unsound? (for every 1 glass of water produced, 3-4 will be wasted).
- Possible chemical contamination from synthetic membranes?
Activated Carbon Filters
These largely come in the form of built-in, tap-mounted and jug filters.
Advantages
- Inexpensive.
- Remove chlorine, heavy metals, and most organic compounds (including trihalomethanes).
Disadvantages
- How effective?
- May not effectively remove nitrates, fluoride or micro-organisms.
Bottled Water
It is important to distinguish between ‘spring water’ and ‘natural mineral water’ – ‘spring’ is essentially a meaningless term, whereas ‘natural mineral water’ must comply with stringent criteria set out in the Natural Mineral Waters Directive 1980. Mineral water cannot be subjected to chemical treatment and must be bottled at source. It is also free from any pesticide or nitrate contaminants found in the water table.
Advantages
- Stringent quality requirements (‘natural mineral water’).
Disadvantages
Environmentally unfriendly (transportation, packaging) – buying mineral water contributes to a landfill mountain with more than 500 million water bottles disposed of every year.
Xenoestrogen exposure from plastic bottles – Phthalates are the agents used to make flexible plastic bottles and have been linked to reductions in sperm count in men – might these leach into the water?
Other Water Filters
There are increasingly advanced water filter systems available for home use. Whilst not cheap to install, they remove bacteria, chlorine, organic and inorganic pollutants, sediments and reduce toxic chemicals and dissolved heavy metal. At the same time, nutrient minerals such as calcium, magnesium and chromium are retained.
What is the best source of water?
Controversy and conflicting information abounds. Water purity and quality is a contentious and confusing issue. These issues are not purely nutritional, but also environmental, and indeed commercial.
12. Conclusion
Issues of both the quantity and quality of the water we drink are hotly contested. In an increasingly contaminated environment, genuinely pure water appears to have become an expensive luxury.
Whilst there is no one, universal formula for water intake, there needs to be a balance struck between dehydration and over-consumption. This will vary from individual to individual. Whilst the ‘8-by-8 rule’ now looks to be an unfounded assumption, we should not lose sight of the importance of pure water for the maintenance and functioning of the many body systems that depend on it.
We should also not lose sight of the quality issue. In our quest for pure water, it is clear that there are pros and cons attached to different types of water and methods of purification. Issues of purity, practicality, price and environmental sustainability are all relevant to the method of water purification that we as individuals opt for.
Copyright BCNH & Glen Matten 2008
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