A quick way of answering some of the most frequently asked questions related to hydration in general and to hydration and physical activity, the effect of hydration on concentration and the special needs of sensitive populations

General Questions on Human Hydration

How much does an adult need to drink each day?

According to the European Food Safety Authority (EFSA)* under moderate activity and environmental conditions, the adequate total daily water intake (comprising the sum of water content coming from all types of beverages and foods) is about 2.5 L for adult males and 2.0 L for adult females, but some individuals will need more than this and some will need less. It seems obvious that bigger people will have greater requirements than small people. It is also important to note that needs are not constant – they will be affected by day-to-day changes in diet, weather conditions, exercise levels and many other factors. Foods typically provide about 20% of the water requirement but this may increase to 50% depending on the foods that are chosen.

* EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA); Scientific Opinion on Dietary reference values for water. EFSA Journal 2010; 8(3):1459. [48 pp.]. doi:10.2903/j.efsa.2010.1459. Available online: www.efsa.europa.eu.

Why are fluids so important to overall health?

Water from the beverages we drink and the foods we eat are needed by the body to regulate body temperature, and transport oxygen and other essential nutrients to the body cells. The body is constantly losing water – for example, through breath as we exhale, through the skin as we sweat or perspire, and through urine. This water must be replaced to maintain good health.

How do beverages fit into an overall healthy lifestyle?

Just like the food we eat, beverages are important. Beverages can help keep us hydrated and some beverages can also provide vital nutrients that our body needs. Some beverages contain energy so should be chosen with our energy needs in mind.

What are the consequences of under-hydration?

Under-hydration occurs when our total water intake is insufficient for the body needs. Under-hydration can lead to dehydration, which results in dry mucous membranes, decreased sweat, saliva and tears; muscle weakness, rigidity or tremors; confusion, hallucinations, and delirium; abnormal respiration; coma; and eventual death in very extreme circumstances. Death in this way is extremely rare, but this condition has attracted a lot of attention because the consequences are so serious.

How can we know if we are under-hydrated?

Urine frequency, volume and colour are probably the best and most useful indicators of body hydration level. If urine frequency and volume are less than normal, then probably water intake has been inadequate and is present. Dark yellow urine means under-hydration, pale to light yellow urine colour is normal and is a sign of normal water balance (euhydration) and totally clear urine normally means over-hydration.

What should we do if I think we are dehydrated?

If dehydration is confirmed, an increased water intake is necessary. If we know that we have not drunk enough that day and we have not passed urine or if our urine is dark in colour, then drinking some additional water or other fluids should be sufficient to restore our body water level. However, in more severe dehydration, as may occur if we have been suffering from diarrhoea, vomiting or if we have been active in very hot or humid conditions then the need for rapid and effective rehydration is more urgent. Water alone is poorly absorbed when drinking, so water with a little sugar (glucose) and/or a small amount of salt (sodium chloride) should be ingested instead. These rehydration solutions can be obtained from a pharmacy, but isotonic sports drinks may be used if these are more readily available. The glucose in these drinks helps absorption of sodium and water and the presence of glucose and sodium in the plasma will help retain the water in the body rather than just losing it in the urine. This is recognised as the standard treatment in cases of dehydration caused by diarrhoea, but is also useful in other situations where dehydration is present.

Is thirst a good indicator of dehydration?

This is true in normal conditions, but thirst is not always a reliable indicator of dehydration. We can ignore thirst signals – when it is not convenient to drink, for example – and if we do this on a regular basis we may not be able to rely on thirst as a signal that we should drink. During intense exercise,high levels of tiredness or other types of stress, the body also turns off the sense of feeling thirsty.

Can I get dehydrated when it’s cold outside?

Although people tend to perspire more in hot weather, dehydration can also be a problem during the winter months. High rates of energy expenditure in winter activities such as snowboarding, ice skating, and skiing and use of heavy clothing can cause significant sweating and water and mineral salt loss.

So, we have to watch out for dehydration, which can lead to fatigue and headaches, even in the winter months. The best way to protect ourselves is by drinking fluids regularly during the day and ensuring that we replace the sodium lost in perspiration.

Does air conditioning affect hydration levels?

Air conditioning drops the humidity indoors considerably, and therefore has a drying effect on the skin which is particularly sensitive to dehydration. Skin dehydration can be observed through parched lips, dry eyes, itchy eyes or irritable skin. Increased amounts of water are lost through the lungs when the air is dry. If we work in an air conditioned environment, we should try to drink enough water or other drinks to compensate for the moisture we are losing. The dryness caused by an air conditioning unit can be avoided by increasing the air’s moisture levels and hydrating the skin.

Are soft drinks hydrating?

All commonly-consumed beverages will help to keep the body hydrated. The most important thing is the amount of fluid consumed. Most people will drink more of a drink that they like the taste of, so there are good reasons for including a range of different drinks in each day’s intake to increase healthy levels of hydration.

Are low amounts of sweat on the skin a good indicator of dehydration?

No, this will sometimes be misleading. If the air is dry, the sweat will evaporate quickly and there may be no sign of sweat on the skin even when sweat rates are actually quite high.

Is it true that fizzy drinks dehydrate?

No, fizzy drinks do not dehydrate. All beverages contribute to the body’s hydration needs, Regular fizzy drinks contain about 90% water, and diet versions may contain up to 99% water. Some fizzy drinks also contain caffeine, but caffeine is a safe ingredient and the amount contained in most caffeine-containing soft drinks is relatively small. It has been demonstrated that caffeinated beverages also contribute to the daily total water intake.

Is it true that caffeinated beverages dehydrate?

It is true that caffeine is a diuretic, stimulating urine output. However, in the amounts present in most commonly-consumed drinks, this effect is small and trivial, and the water in the drinks will more than compensate for any effect of the caffeine. Avoiding caffeine-containing drinks may actually increase the risk of dehydration because of a decreased intake of fluids. According to guidelines published by the Institute of Medicine of the National Academy of Sciences (IOM-NAS), caffeine-containing beverages such as coffee, tea and soft drinks contribute to total daily water intake to a similar extent as beverages without caffeine.

Is it true that alcohol dehydrates?

Alcohol is a diuretic. This means that most alcohol-containing drinks encourage the body to increase the formation of urine by halting the production of the body’s anti-diuretic hormone. After drinking alcoholic drinks you often feel the need to urinate, thus speeding up the loss of water from the body that may lead to dehydration. Any recommendation of alcohol consumption for this aim should be made with caution. In the case of high sweat losses a source of salt should be consumed along with any fluid in order to secure effective hydration.

What are over-hydration and hyponatraemia?

Proper diagnosis requires a blood test to measure levels of substances that indicate how well the kidneys are functioning.

Over-hydration occurs when our total water intake is excessive for our body needs. Usually, the kidneys are very effective in getting rid of excess water, but occasionally they cannot cope. As the water content of the blood increases, the salt content is diluted (a condition referred to as hyponatraemia), decreasing the amount of salt available to body tissues and changing the distribution of water in the different body compartments: this can lead to problems with brain, heart and muscle function. A sustained decrease in plasma sodium concentration alters osmotic balance across the blood-brain barrier, leading to an increased movement of water into the brain, which in turns leads to swelling of the brain. This can progress to confusion, seizure, coma and even death in very extreme circumstances

Hyponatraemia is a rare condition that occurs when the sodium concentration in the blood falls to dangerously low levels. This can happen if we lose large amounts of both salt and water through sweat, but replace only water but this occurs only in extreme situations, such as in men doing hard work in desert climates. Occasionally it happens if people make a conscious effort to drink large amounts of plain water or other drinks with a low salt content.

Hydration and Physical Activity

What causes dehydration during exercise?

The primary cause of dehydration during exercise is sweat loss. In hard exercise in the heat, sweat losses of 1-2 litres per hour – and occasionally even more – are possible. Similar amounts of sweat may be lost in cold weather when large amounts of clothing are worn. Few people drink that much, so it is normal to finish an exercise session slightly dehydrated. Low levels of dehydration are normal and are not harmful.

How should lost fluids be replaced?

In cases where the exercise session is short or where sweat losses aren’t severe, athletes can replace these fluids by drinking plain water. There are many research studies to show that drinking water is better than drinking nothing at all. In any exercise task lasting longer than about 30-40 minutes carbohydrate depletion, elevation of body temperature and the reduction in the circulating fluid volume may be important in causing fatigue. All of these can be manipulated by the ingestion of fluids, but the most effective drink composition and the optimum amount of fluid will depend on individual circumstances. Water is not the best fluid for ingestion during endurance exercise, and there is compelling evidence that drinks containing added carbohydrate and electrolytes are more effective in improving performance.

Is it possible to drink too much?

Yes. While most runners and cyclists know the importance of hydrating, some don’t realize that over-hydrating can dangerously lower blood sodium levels, a condition known as hyponatraemia. If you can weigh yourself before and after an exercise session, you can get an idea of whether your current drinking strategy is appropriate. As a rough guide, the aim should be to drink enough so that weight loss during an exercise session is not more than about 2% of your starting body weight. There should not be a need – except in a few exceptional circumstances – to drink so much that your weight increases during an exercise session.

What are electrolytes and what do they do for us?

Electrolytes or mineral salts, such as potassium, sodium and chloride, are important for maintaining the body’s water balance. They are also important in the normal functioning of all of the body’s cells, especially muscle and nerve cells. Electrolytes are lost through sweat in variable amounts, and these losses need to be replaced. You can replace them through a wide range of foods and drinks.

What are energy drinks?

Energy drinks contain larger amounts of caffeine than the majority of other beverages and are consumed because of their proposed stimulating effect to keep us awake.

How much should we drink after exercise to replace sweat loss if we don’t drink during exercise?

An easy way to estimate how much water we have lost during exercise through sweat is to weigh ourselves before and after exercise. One kilogram of weight loss is roughly equivalent to 1 litre of sweat loss. It is often recommended that we should drink about 1.2-1.5 litres for each kilogram of weight loss: this provides enough fluid to allow for ongoing losses of water in urine and through the skin.

Why do sports drinks contain sodium?

Sports drinks contain sodium to help retain body water and maintain hydration status. It also can help endurance athletes reduce their risk of developing hyponatraemia, a rare condition occurring primarily in endurance athletes who lose both salt and water through sweat, but replace only the water. Hyponatraemia can lead to disorientation, confusion and seizures. The sodium also helps to retain water in the body by reducing the amount of urine formed when a large amount of fluid is consumed.

Is water the best choice for rehydration after sport?

For low-intensity exercise lasting no more than about 30-40 minutes, where sweat losses are not high, water is adequate as long as any sodium lost through sweat is replaced through foods in order that effective hydration is maintained. Where sweat losses are high, a drink containing some salt and a small amount of sugar may be useful to replenish lost sodium and therefore achieve hydration.

Hydration and Children

How much fluid do children need each day?

The European Food Safety Authority (EFSA)* has concluded that adequate intakes of water for children are as follows): 1,300 mL/day for boys and girls 2 to 3 years of age; 1,600 mL/day for boys and girls 4 to 8 years of age; 2,100 mL/day for boys 9 to 13 years of age; 1,900 mL for girls 9 to 13 years of age. Adolescents of 14 years and older are considered as adults with respect to adequate water intake and the adult values apply.

While children can meet their daily fluid intake requirement with water alone, keep in mind that all beverages, including water, milk, juice, soft drinks and sports drinks, as well as foods with a high water content like soup, fruits and vegetables, contribute towards this recommendation. On average, about 80 percent of an individual’s total water needs are met by drinking beverages and about 20 percent is provided by food, but these fractions can vary greatly.

* EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA); Scientific Opinion on Dietary reference values for water. EFSA Journal 2010; 8(3):1459. [48 pp.]. doi:10.2903/j.efsa.2010.1459. Available online: www.efsa.europa.eu

What are the signs and symptoms of dehydration in children?

We should be concerned if children have an excessive loss of water by vomiting or diarrhoea, or if the child refuses to eat or drink. Presence of the following symptoms may be a sign of dehydration,but many of these are not specific and cannot be used as definitive signs:

  • the urine will become concentrated and darker yellow in colour;
  • decreased frequency of urination or dry diapers;
  • sunken eyes;
  • sunken soft spot on the top of the head in babies;
  • no tears when the child cries;
  • dry or sticky mucous membranes (the lining of the mouth or tongue);
  • lethargy (less than normal activity);
  • irritability (more crying).

If at all concerned, we should seek medical advice: some of the symptoms of dehydration are very similar to those of over-hydration and providing extra fluids to a child who is over-hydrated is potentially dangerous.

How can children be encouraged to drink more?

Several studies show that children consume about 45 to 50 percent more liquid when it’s flavoured than when they are given just plain water. This is useful when trying to encourage a child to stay hydrated. Other tips for increasing a child’s fluid intake include:

  • Allow older children to suck on ice or home-made iced lolipops made with low-calorie beverages.
  • Have your child pick out a fun drinking water bottle and keep it filled with water or other low-calorie beverages.
  • Offer more fluid-rich foods like fruits and vegetables and soups as meals or snacks.
  • Let children create their own flavourful beverages by adding fresh fruit.
  • Offer beverages that are neither very hot nor very cold. Beverages slightly cooler than room temperature tend to be consumed in greater volumes than fluids at either temperature extreme.
  • Make sure that a variety of different drinks is available.

Should children drink during class at school?

Children in classrooms can be given structured water breaks. It is NOT necessary to keep a water bottle at their desks and to sip throughout the day. By incorporating water or other drinks in the classroom, pupils are likely to be more attentive, and able to enjoy learning. In schools where drinks are provided throughout the day, there have been reported improvements in concentration levels, academic performance and pupil behaviour. The evidence is not entirely convincing, but there are no adverse effects, so it is certainly worth trying.

Hydration and the Elderly

Do hydration requirements change as people get older?

Older adults are more vulnerable to shifts in water balance, including both over-hydration and dehydration. This is partly because of normal age-related changes, but also because of the increased likelihood that they have medical conditions that will affect water and salt balance. As people get older, your body’s signals for thirst, hunger and other needs get weaker, therefore after age of 50 people may need to drink even when they are not thirsty. Potential consequences of dehydration, include constipation, increased risk of falls, medication toxicity, urinary-tract and respiratory infections, delirium, renal failure, seizure, electrolyte imbalance, hyperthermia, and longer time to wound healing.

What special precautions should elderly people or their carers take to ensure they remain adequately hydrated?

To ensure adequate hydration, elderly people should drink plenty of water or other drinks at meals and between meals, have more soups at meals. They should be careful to replace lost fluids after fever and after vomiting or diarrhoea. It may be helpful to keep a glass of their preferred drink to take while relaxing or watching TV, and they should be informed about any medications that might cause increased water loss.

During periods of hot weather are there any special precautions that should be taken to ensure adequate hydration?

Increased environmental temperature, whether because of natural heat or air conditioning, can affect the body’s natural control mechanisms to adjust to the heat: there are effects on the skin, the vascular system and perspiration. As these control mechanisms are likely to be affected by age, it is important to help keep the body cool by drinking plenty of water or other drinks, limiting or avoiding physical activity, and wearing clothing such as cotton that draws perspiration away from the skin.

Hydration and Concentration

Can dehydration affect concentration?

Water makes up about 80% of the brain and any large change in water content will affect brain function. Dehydration, if sufficiently severe, adversely affects mental performance. Mental functions are affected to a different extent according to the degree of dehydration. When dehydration reaches about 2% of body weight, a significant decrease in short-term and long-term memory, motor coordination, reaction times, and perceptive discrimination may be observed.

When could this have a particular impact?

Dehydration can cause headaches, tiredness and loss of concentration, affecting alertness, so adequate hydration is particularly necessary for any activity requiring a good mental performance, such as driving, studying or working.

What precautions can people take to ensure they remain adequately hydrated when driving, at school or at work?

Significant water is lost just sitting in an air-conditioned car or office, so it is important to drink frequently. Ensuring adequate hydration during a long car trip may help to reduce road fatigue, and the same applies to sitting in a desk, either working or studying. Other measures could be to adjust the air conditioning or the amount of clothing worn to limit water losses through sweating.