How much water do you really need, and is possible to drink too much? In this podcast, I explain why the advice to drink ‘8 glasses of water each day’ is a myth that needs to be busted. And then I’ll outline that while even though it is very rare, it is possible for a healthy person to drink too much water with the consequences of this being potentially life threatening. So, endurance athletes take note, this could apply to you. 

Water makes up about 60 per cent of an adult’s body weight. All that water is there for a reason as without it, essential life processes could not occur. Water is needed to:

Water is vital. You get that. But how much of it do you need each day? Surely it is just a matter of ‘drinking your 8 glasses of water’ each day and you’re good to go? Not so fast. If I had to nominate one pervasive myth in nutrition, it would be the popular advice that we should drink at least eight glasses of water a day. Of course, our bodies need water, otherwise we would die from dehydration. But the amount needed is extremely variable and depends on a person’s body size, physical activity levels, climate and what types of food they are eating.

It is believed that the “eight glasses” myth was a US Recommended Dietary Allowance dating back to 1945. The guide said a suitable allowance of water for adults was 2.5 litres a day, but most of this water could be found in prepared foods. If that last, crucial part is ignored, the statement could be interpreted as clear instructions to drink eight glasses of water a day.

Even a comprehensive search of the scientific literature finds no evidence to support the eight-glasses-a-day advice. The clear reason that evidence for such prescriptive advice doesn’t exist is that a person can get all the water they need without consuming a single glass.

Drinks like soft drink, fruit juice, tea and coffee, milk, and foods like fruit, yoghurt, soups, and stews all have appreciable amounts of water that contribute to fluid intake. Don’t be concerned about seeing coffee listed as a fluid – the “coffee makes you dehydrated” mantra is another myth that needs to be busted. Drinks such as coffee, tea and cola do have a mild diuretic effect from the caffeine, but the water loss caused by this is far less than the amount of fluid consumed in the drink in the first place. It’s only alcoholic drinks which have a dehydrating effect.

Australian dietary recommendations also bust the eight-glass myth as the official Nutrient Reference Values state “there is no single level of water intake that would ensure adequate hydration and optimal health for the apparently healthy people in the population.”

Because water needs vary depending on diet, activity, environmental temperature and humidity, a general water requirement is difficult to establish. Recommendations are sometimes expressed in proportion to the amount of energy expended under average environmental conditions. The recommended water intake for a person who expends 8400 kilojoules a day, for example, is 2 to 3 litres. Total water includes not only drinking water but water in other beverages, such as milk, and in foods as well. In general, drinking water and other beverages contribute about 70 to 80 per cent to water intake, and foods contribute the remainder. So not all of that water needs to come straight from the tap, and for the average person, it is a myth that you somehow need 8 glasses of pure water on top of all the foods and non-water beverages you drink.

So how do you know if you are drinking enough water? Well. You can check this for yourself every few hours. If your urine is lightly coloured or clear, you’re drinking enough. If it’s dark, then you should drink more. How simple is that?

Water overload

Okay, that’s how much water you need. And you get it that it is not good to be dehydrated. But can you drink too much of it? The answer is most definitely ‘yes’. Water overload (a condition called hyponatremia) is a potentially life-threatening condition.

Hyponatremia occurs when the sodium concentration in the blood drops to a dangerously low level. Sodium serves an important role in the body by acting as an electrolyte and helping with muscle contraction. If more water is drunk than the kidneys are able to deal with then this can dilute sodium in the body just the same as adding fresh water to saltwater makes it less salty.

Before you get too concerned every time you have a glass of water, take heart knowing that water overload in healthy people is incredibly rare. Most cases occur because of medical problems where water cannot be excreted efficiently, such as in people with impaired kidney function. Some isolated case reports have also described of hyponatremia occurring in people with mental illnesses who consumed massive quantities of water in a short period of time.

Hyponatremia has also been described in otherwise well-hydrated people who have drunk around 3 litres of fluid over an hour in an attempt to foil drug tests. In some cases, however, hyponatremia can occur in otherwise healthy people who drink too much water during long-duration endurance exercise.

Who is at risk?

People who compete in endurance events over many hours such as marathons and ironman triathlons are especially at risk of water overload. A ground-breaking study published in 2005 in the New England Journal of Medicine measured the sodium levels of 766 runners at the finish line of the 2002 Boston Marathon. Thirteen per cent of the runners were found to have hyponatremia and 0.6 per cent had such a low level of blood sodium that it was considered critical. I’ll link to the study in the show notes. http://www.nejm.org/doi/full/10.1056/NEJMoa043901 

The researchers surveyed the runners about their fluid consumption during the race. Hyponatremia was found to be most often associated with the following characteristics of runners:

The risk factors for hyponatremia in marathon runners equally apply to any activity that involves continuous strenuous activity for more than 4 hours (bushwalking or even military manoeuvres for example) when combined with excess fluid consumption.

Athletes with a small body size (typical of many endurance sportspeople) are thought to be at even greater risk especially if they sweat a lot and are ‘overzealous’ in their drinking habits during an event.

Signs of water overload

How could you know if you’re suffering from water overload? Symptoms of hyponatremia present as throbbing headaches, nausea, cramps, dizziness, confusion and lethargy. If the condition progresses and sodium levels fall even further, swelling of the brain can occur ultimately resulting in coma and possibly even death.

Unfortunately, the initial symptoms of hyponatremia mirror those of dehydration making the condition difficult to self-diagnose. If hyponatremia is misdiagnosed as simply dehydration, then consuming more water will only make the condition worse.

Mild cases of hyponatremia can be managed by taking extra salt combined with water restriction until most of the fluid is urinated out. Standard medical treatment of hyponatremia involves intravenous administration of a concentrated salt solution as well as using diuretics to speed water loss.

While a firm diagnosis of hyponatremia requires a blood test, something that would not be readily available at the finish line of an endurance event, some possible signs that a person may be fluid overloaded include swelling of the hands and feet or a watch or ring that has become much tighter.

Normally, our bodies are perfectly capable of maintaining water and sodium balance even if we drink several litres of water in a short period of time. Why hyponatremia occurs in endurance athletes is thought to be due to muscle damage causing a release of stress hormones. This metabolic response can result in reduced water excretion by the kidneys, causing water to accumulate in the body and thus lowering the sodium concentration.

Once the body’s water balance mechanism is impaired, drinking even a small amount of extra water can cause symptoms of hyponatremia. Importantly though, before an event it is thought that hyponatremia cannot occur because the body can excrete the extra water drunk so all athletes are encouraged to stay well-hydrated during their training and daily activities.

Pre-event, drinking about 500 mL of fluid two hours before race time will allow the body plenty of time to remove the extra water while still ensuring the athlete is well hydrated.

So how does a person walk the fine line between staying well hydrated during an endurance event while at the same time not risking becoming fluid overloaded? For events of several hours in duration, self-weighing may be a good option. If a person is well hydrated at the start of an event, then ‘weight gain’ during the event may mean a person is carrying too much extra fluid and should ease up on the drinks.

Knowing how your own body responds to activity and fluid hydration strategies will enable you to be better prepared for competition and keep your hydration status in the optimal zone.

Hyponatremia is a potentially dangerous condition caused by excessive drinking combined with abnormal water retention during exercise. People should be aware of the symptoms of water overload and aim for adequate, but not excessive, fluid intake during endurance-type sports. During the event, it is recommended that a person aims to consume about 300 to 500 mL of fluid (preferably a sports drink that contains salt) every 30 minutes. For slower, smaller athletes who are exercising in mild conditions, less fluid may be required while for well-trained athletes competing at high intensity in warm conditions more fluid may be needed.

Research Wrap Up

Now on to my research wrap up segment where I profile a study that has grabbed my attention during the week. And this one certainly has some relevance for all of us housebound living the iso life and making minimal trips out.  The research is all about the benefits of having a good stock of frozen fruit and vegetables with a new study finding that having convenient frozen fruit and vegetables always on hand in the freezer is a way to boost fruit and veg consumption with no trade-off in the nutritional quality of the diet.

Dietary surveys in Australia point to just 7 percent of people eating the recommended 5 servings of vegetables each day. While just half of Australians make it to the recommended two servings of fruit each day.

One possible way to help address the fruit and vegetable consumption gap is to promote ways to make them more accessible and convenient. The limited shelf-life of fresh produce can mean that it isn’t always possible to have plenty of fresh produce on hand and at home, especially if regular access to a supermarket or greengrocer isn’t always possible. The situation can be even more difficult for people living in remote areas or who find it difficult to get to a supermarket.

Several studies have already found that fruits and vegetables packaged as frozen or even canned are cost-effective and nutritious options for meeting daily vegetable and fruit recommendations in the context of a healthy diet. So, the question is do people who have plenty of frozen produce on hand at home actually eat more fruit and vegetables on average compared to the general population?

Using food and nutrient intake data from two large dietary surveys conducted in the United States, researchers were able to look at fresh and frozen fruit and vegetable intake of people. This data was able to then be compared to a whole diet quality across large segments of the population. And I’ll link to the study in the show notes https://europepmc.org/article/med/29290348

The key finding was that people who were regular consumers of frozen fruit and vegetables ate significantly more fruits and vegetables than people who only ate fresh produce. The end results of eating more frozen fruit and vegetables was that a person’s diet was overall healthier. More fibre, potassium, calcium, and vitamin D was consumed, and less salt eaten in people who had frozen produce as part of their diet.

The energy intake of people 18 and under was lower among frozen fruit and vegetable consumers which is a positive finding considering the rise in childhood obesity rates. Adult eaters of frozen produce were also significantly more likely to have a lower body mass index too.

So, frozen fruit and vegetables should be a backup option in the freezers of most Australians – especially as we stuck in iso for some time. Such foods are a nutritious, cost-effective and convenient way to get more fruit and vegetables into you every day.

So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.

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I’m Tim Crowe and you’ve been listening to Thinking Nutrition.