Nov 20 2014

How Much Water?

Water is the focus of a great deal of medical myths and snakeoil. This is perhaps due to water’s health halo – it is the very essence of purity, the elixir of life. I wonder if there is an evolved psychological connection to water. The idea of drinking dirty or contaminated water seems to be especially disgusting, while pure clean water is the most wholesome thing in the world.

Perhaps this is why there are so many “magic water” scams out there.  There is structured water, energized water, and ionized water. Homeopathy is essentially magic water with memory.

There is also a persistent belief that simply drinking more water has untold health benefits. You have probably heard that you should drink eight 8 oz glasses of water per day to stay healthy. This, however, is a myth, in that the 64 oz figure is not based on any evidence.

Your water requirements will vary, depending upon your body size, level of activity, humidity and ambient temperature. It will also depend on the type of food you eat. We get on average about 20% of our water intake from our food, but this will also vary depending on the type of food you eat.

Fortunately  you don’t have to do any complex calculations or measurements to determine and track your water intake. Your body will do it all for you automatically and then inform you when you need to take in more water by making you feel thirsty. Thirst is our primary method of fluid management. On the back end, our kidneys will adjust how much water we retain or lose through micturition (technical speak for pee) to fine-tune our fluid balance.

So for most people in most situations, listening to your thirst is an adequate strategy for getting enough water. However, in extreme situations water loss can be rapid and dehydration can occur before thirst can correct it. In these situations it does make sense to stay ahead of your hydration, and to make sure you have ready access to fluid. This includes high altitudes where the air is dry and water loss through breathing increases significantly. Other hot or dry environments. Or during physical activity that is likely to produce a great deal of sweating and heavy breathing.

When water loss occurs through heavy sweating, such as when running a marathon, another issue crops up – is it better to drink pure water, or a sports drink that contains electrolytes? There are other situations where this is an important issue, such as those rehydrating after vomiting, or who are simply dehydrated from prolonged nausea and decreased food and fluid intake.

In such cases you are usually better off drinking something with electrolytes and perhaps also calories. When you sweat or vomit you lose water and electrolytes, and if you hydrate with pure water your electrolyte levels can drop. Even if you hydrate with an electrolyte drink, if you drink a large volume (such as during a marathon) then you can still dilute your electrolytes.

Evidence suggests that marathon runners, for example, need about 450mg of sodium per hour to replace what is lost through sweating and to maintain their electrolyte homeostasis.

There is also evidence that you can overdo hydration. Marathon runners who drink as much water as they can tolerate, even electrolyte drinks, can become hyponatremic (too little sodium in the blood) and impair performance or even risk their health. The current recommendation is to drink enough to maintain homeostasis, but not enough to maintain body weight during prolonged physical activity.

Sometimes people drink too much water because they think they should, or because they have a pathological desire to do so (called primary polydipsia). In such cases dangerous levels of hyponatremia can result, causing delirium and even seizures.

Even worse, drinking large volumes of water is sometimes recommended by health gurus or pop advice. I recently came across this article, for example, where a woman allegedly decided to force herself to drink 3 liters of water per day. She claimed remarkable health benefits and as evidence we are presented with before and after pictures. 

The before and after picture is a common scam. Even without Photoshop, it is easy to manipulate pictures to change someone’s appearance. In this case, it seems obvious that the lighting is different in the two pictures, and that may be sufficient to explain the apparent difference. The right picture has a front light to hide all the shadows (just look at the eyes).

Misleading articles such as this one might motivate some people to dramatically increase their water intake, which will not help them but may cause health problems, and can even be fatal in extreme cases.


As is often the case, simple advice is often more practical (and may be more accurate) than complex advice that is not evidence-based. When it comes to fluid intake, listen to your thirst is all you need to know for most situations. As indicated above, when high levels of water or salt loss are anticipated you do need to take special precaution to keep well hydrated. In extreme athletic situations, like running a marathon, then special care should be taken to optimize water and electrolyte intake over time.

There are also special medical conditions, such as heart failure and kidney disease. If you have a medical condition that affects your fluid and electrolyte needs, then listen to your doctor. The general rules here may not apply to you.

Water is vital for life, and access to clean water (something we take for granted in the industrialized world) is a problem in much of the world. But there is no magical water, and just like anything else, you can have too much of a good thing. So don’t force yourself to drink more water than your body is telling you that you need.

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