It is interesting that when we think of good nutrition, water is not usually the first thing that comes to mind. Yet of all the aspects of nutrition it is the most crucial. Water accounts for between 55-75% of a person's overall bodyweight. Listed below is the amount that water contributes to the make-up of some of our vital organs.

Almost all functions in the body rely heavily on water at some point. It is essential to the structure of our body’s cells, cardiovascular function, assists in the transportation and utilisation of nutrients, lubricates joints and helps regulate body temperature. A person can go relatively long periods of time without protein, fat, carbohydrates and many other vitamins and minerals, but within several days of no water intake, a person can be in a life-threatening situation.

Water Balance

We take water into our bodies through drinking fluids and through the food we eat. On average the liquids we drink account for about 80% of our water intake and the food we eat makes up the other 20%. We also lose water on a daily basis. It is estimated that adults lose around 2.5-3L on average per day through sweating, urination, excretion of waste and breathing. To maintain balance of this input and output of water, the current recommendation in Australia is that adult men should be drinking 2500mL (10 glasses) and women 2000mL (8 glasses) per day. This recommendation is based on a temperate climate and assumes a moderate activity level. We will look more closely at what factors affect this later in this article. For now, it is worth checking out this table of recommendations as they differ for children of various ages, lactating women and the elderly.

These guidelines are quite straightforward and uncontroversial, and for the most part they represent the best practice in health advice. Why is it then that a quick web search on “How much water should I drink in a day?” can return different results.

Research Populations

Conclusions about hydration have in some circumstances been calculated with consideration given to population surveys. If you were to look at the culture in the United States where super-sized soft drinks are the norm and compare it with another country without a high intake of soft drink, you will observe a marked difference in fluid intake. It is easy to see how weighting this approach to determine a baseline norm for hydration can be skewed. In fact, if you compare the daily fluid intake of a person in 1960 to the daily fluid intake today, there has been significant increase and the majority of this is attributable to soft drinks. This may in part explain why the recommended adequate intake of liquid in the US is 3000mL for men and 2700mL for women, compared to 2500mL for men and 2000mL for women in Australia. 

Food and Culture

Surveys conducted in different areas of the world and in different cultures have also demonstrated the significance of diet in hydration. If we were to assume that an average American drinking 12 glasses of fluids, mostly soft drink, is better hydrated than a rural Italian farmer drinking 8 glasses of water per day, we would probably be wrong. Because around 20% of the daily intake of water comes from the food we eat, foods with a high-water content will significantly reduce the need to drink water. In this comparison a Mediterranean diet that includes large amounts of fruit and vegetables is far superior for hydration compared with a diet of burgers, fries and other processed foods. The lower salt and protein content in the Mediterranean diet also places less stress on the kidneys and requires less water to process. The main point to take away is that good hydration is not just about how much you drink, it is about what you drink and eat, and how your body processes it. When considering your own hydration, it is worth taking a look at the water content of various foods.

Quantifying Normal Hydration in Populations

Most research relating to hydration has occurred surrounding exercise and the effects of dehydration. The lesser researched aspect, but arguably more important, is looking at hydration in the context of well-being and overall health. This type of research has shown that low water intake is related to kidney disease, diabetes and incidence of urinary tract infections. However, there are currently questions surrounding hydration, in that there is no widely accepted scientific consensus of what constitutes ideal hydration and how to precisely measure it. For example, The National Health and Nutrition Examination Survey (NHANES) in the US showed that people who were drinking the least amount of water (1694 mL/day) and the people drinking the most (7934 mL/ day), have essentially the same biomarkers for hydration in their blood (280 vs 279mmol/kg plasma osmolality). Many researchers (PDF, 608 KB) have noted that there is still a long way to go to thoroughly understand all the dynamics of measuring and determining optimal hydration in large populations. Potentially in the future, quantifying optimal hydration could be as simple and widespread as diagnosing Blood Glucose or Blood Pressure. For now, however, this lack of certainty provides room for interpretation, and this naturally results in different opinions.

What do we know?

The body uses sweating to cool down during exercise. The process of sweating results in a loss of water and when this amount approaches 1-2% of bodyweight, performance decreases and risk of heat illness increases. Between 1-2% of water loss is considered to be mild dehydration. When exercising or playing sport it is common for someone to lose between 400mL to 2L of sweat per hour. This amount can be increased significantly by other factors such as

• Increased temperature and humidity
• Increased intensity during exercise
• Excess clothing that does not permit sweat to evaporate
• Increased fitness as fitter people begin sweating earlier
• Larger body mass which increases surface area of skin and thermoregulatory load

Pre Workout

In trained athletes exercising in the heat for long periods of time the amount of water lost can well exceed 2L. This is the main reason why drinking before a workout is beneficial. In order for water to be absorbed into the body it needs to pass from the stomach to the small intestine. This rate is referred to as gastric emptying, and it slows considerably beyond exercise at a moderate intensity (>70VO2Max). To demonstrate this, if a person who is 70kg loses 1L of water in the first 30 minutes of a 60 minute workout and waits until this point to start drinking water to compensate, they will be unlikely to replace the fluids effectively before becoming noticeably dehydrated. At that point their level of dehydration will be largely determined by how hydrated they were before beginning exercise. This is why it has been recommended to drink 2-3 glasses of water 2 hours before exercise and then 1 glass of water about 20 minutes before exercise. This is a good guideline to follow, but there is one additional tool that you can use pre workout to examine hydration. The colour of your urine is telling you something about your hydration level. This chart is another guide you can use to determine how much you should be drinking before a workout. Ideally you should be a 1-2 on the chart prior to exercise.

During Workout

Under some circumstances it is not necessary to measure and plan the exact quantity of fluid intake during a workout. Research has shown that in situations where significant dehydration is not likely, forcing people to drink specific amounts is not helpful and can actually reduce exercise performance. Take for example a cool, windy day where a reasonably fit person is playing a recreational game of tennis for 60 minutes. Forcing 200mL of water every 15 minutes is this situation is not warranted, they should just have their water available and drink when they like. Alternatively, in circumstances such as a 60-minute, high intensity Crossfit workout on a 30 degree day, 75% high humidity and low wind, a person should be prioritising a drink every 15-20 minutes. Most people will be limited to how much they can drink by gastrointestinal comfort, generally this amount would be around 100-150mL each time.

As noted previously, there are several factors that influence how much water a person will lose through sweating. It is not possible to give a figure accurate for everyone but if you want to, you can follow this guide for calculating your hourly sweat rate. Essentially you are weighing in before and after exercise, to determine how much water is being lost through sweating. One litre of water weighs 1kg. This number will let you know roughly what you need to drink to return to pre workout levels. It is a good tool, just remember there is a high variability of sweat rate even in the same individual, depending on the exercise intensity and the environmental conditions. If you experiment with it a few times under different conditions you can get an idea of what levels of sweat and exertion are producing what amounts.  As a final note on hydration during a workout, it is worth noting that the sensation of thirst is muted somewhat as exercise intensity increases. The sensation of thirst is far more efficient at rest, but during exercise it is experienced when 1-2% of water loss has already occurred. At this point dehydration is likely to be experienced, and drinking fluids becomes a game of catch up.

Post Workout

Replenishing the water lost during intense exercise often continues in the hours following a workout. The diagram below is a tool that has been used in exercise science and for athletes in post workout periods. Basically, it uses three factors to determine the likelihood of dehydration being present. The first is the colour of a person's urine. On this chart it is a colour of 4 or greater. The second is a decrease in bodyweight greater than 1%. The third is the sensation of thirst. Where two of these factors are present it is likely that the person is dehydrated. Where all three are present it is very likely they are dehydrated. If you calculated your sweat rate as mentioned previously, or if you know your weight prior to exercising, then you have a guide of how much to drink to return to your previous level. If neither of these can be used, a person could use the colour of their urine and their sense of thirst at rest to guide them. Generally speaking, mild dehydration occurs in the range of 1-2% of water loss. If a person who is 70kg suspects they are mildly dehydrated, it would be fair to consider a fluid intake of around 700mL-1.4L to be somewhere close to rehydrate effectively.

Sports Drinks

The general consensus in Australia for sports drinks is that they are not usually necessary during moderate exercise up to 60 minutes in duration. Above 60 minutes or in higher intensity exercise of lesser durations it is advisable to include them as a part of fluid intake. If a person had calculated that they would need to replenish around 2% of their water lost through sweat, a sports drink would definitely be an advisable component of that intake. Because sweating also causes a loss in sodium and other electrolytes, a sports drink will assist with replenishing these. Also, in prolonged exercise (>90min) it also becomes important to replenish carbohydrates stores and the sugar in sports drinks helps accomplish this. Outside of this context, are marginally better than soft drink purely by virtue of having about half the sugar. There is a wide range in how much sodium each individual loses through sweating and sports drinks alone may not restore this amount for some people. This is why it is important to have a good balanced meal after intense or prolonged exercise.

Dehydration

Before, during and after a workout it is important to recognise the signs and symptoms of dehydration. The most common ones are

• Headaches
• Fatigue
• Mood changes
• Slow reaction times
• Decreasing Coordination in exercise
• Dry nasal passages
• Dry or cracked lips
• Dark coloured urine
• Muscle cramps
• Weakness
• Confusion
• Hallucinations
• Thirst

If there is ever any concern about your own or someone else's condition seek medical attention immediately. The incidence of heat illness is also closely related to hydration and sweating. Learn more about training in the heat.

Drinking More Water

Some people find it difficult to drink water, especially in larger amounts before and after training. Read our article on four tips for drinking more water to help keep up an adequate intake.