Like many of us, I’ve been trying to stay hydrated lately. I remember to drink when I exercise, but often forget when I’m busy doing other tasks. My experience is not uncommon – many of us cart around colourful water bottles that barely get touched during the day. We feel guilty because we’ve heard that we need to drink at least 8 cups of water per day to be healthy. But is this number right? How much water should we drink?

This question surfaced recently when a running friend reported that she was drinking 4 litres daily at the recommendation of her nutritionist. Four litres or 16 cups is a lot of water, especially when you consider that this 16 cups of water is in addition to all other dietary liquid (like tea, coffee, juice, ice cream, milk, or soup). This practice seems like a bit of a burden, doesn’t it? But is it necessary? Does it have value or not?

The Difficulty with General Fluid Suggestions

Photo by Monica Grabkowska

When I worked at a hospital, I had the luxury of calculating fluid requirements for people living in a standardized environment, who were provided with fluids (by intravenous or tube feeds). Even in such a regulated environment, it quickly became evident that fluid needs are highly individual. Not every 80-year-old grandmother requires 25 ml of fluid per kg of body weight. Some need more, some need less. Fluid needs are also affected by genetics and how in shape you are. The fitter you are, the more you sweat, and the higher your needs.

So how do we make suggestions for people who are sometimes in the heat all day, and sometimes not, who exercise intensely for three hours one day, and then chill by the pool the next? It gets more complicated to predict fluid needs when your daily living patterns vary.

At the very least, I’m sure you can already see that you’re not going to need to drink the same amount of fluid every day. Because your fluid requirements vary daily, blanket recommendations are not useful or for that matter sustainable. Not everyone needs 8 cups of water every day and 16 cups of water every day may simply be too much for many people.

What About Individual Needs?

Photo by Autri Taheri

You can calculate your individual baseline fluid needs. Several websites provide age-specific equations (such as mynetdiary.com/water-needs.html). Even so, it is best to listen to your thirst, drink fluids throughout the day, and check your urine to ensure it is pale yellow (straw-coloured). Your exercise specific fluid guidelines from are as follows*:

4 hours before exercise:

Drink 1 to 2 cups of fluid

2 hours or less before exercise:

Drink ½ to 1 ½ cups of fluid if you are dehydrated (i.e. you have not urinated or you have only produced a small amount of dark urine).

During exercise:

Sip fluid during your activity. Avoid gaining weight

Immediately after exercise:

If you lose weight drink 2 to 3 cups for every lb of weight lost (or 500-750 ml for every 0.5 kg)

For more post exercise fluid suggestions check out my blog about refuelling after exercise. Among other variables, the amount you need to drink will vary depending on humidity, temperature, and exertion.

More is… More

Photo by Quin Stevenson

As with most things, more is not necessarily better with water. Sometimes more is just more – and sometimes more is dangerous. Certain medical conditions may require fluid limits such as kidney disease and congestive heart failure. But, even in otherwise healthy individuals, excessive fluid can cause serious complications.

Exercise associated hyponatremia (EAH) is one such condition that can affect active, healthy people. EAH is common in marathoners, road cyclists, swimmers, and ultra runners. EAH is a condition caused by overconsumption of water without adequate sodium to replace that lost in sweat and is more common in people who are salty sweaters. Salty sweaters sweat more salt than the average person making them more susceptible to EAH.

To limit your risk for developing EAH it is suggested that you:

Drink enough to prevent excessive dehydration
Don’t over drink
Consider electrolyte replacement (salt) when exercise is prolonged, particularly if you run slower than a 4-hour marathon. You can determine if you are a salty sweater by wearing black clothing for a run and looking for salt stains after you exercise

The best way to ensure that you are not over drinking is to weigh yourself before and after you exercise. If you gain weight, you are drinking too much. If you lose more than 2% of your body weight, you are not drinking enough.

What About 4L?

This brings us back to the original question. Are we all a bit dehydrated? I don’t know how many of us are dehydrated, but I’ll bet most of us are drinking reasonable amounts of fluid. Could some of us use more? Probably. But the source of fluid is most likely the bigger issue. Many of us get much of our fluid needs from less healthy drinks. A more important message than drink 8 cups of water a day would be to replace sweet drinks like soda pop and juice with water.

What about quantities? Would drinking 4 L of water a day re-energize our skin, our digestive system, and our bodies? Many people do not require 4 L of fluid every day. A 30-year-old, 59 kg (130 lb.) woman probably needs roughly 1.8 to 2.1 L of fluid a day. If she is fit and she sweats a lot, she may need much more on heavy exercise days.

The Bottom Line

Water and hydration are essential for your general health and your exercise regime. Be cognizant about including fluid regularly in your day. Water is best, but other fluids also count towards your daily requirements. Recognize that you need to drink more on days that you sweat more, eat more and exercise more, and less on sedentary days. Above all, know that you have a thirst mechanism for a reason and trust that for everyday situations, it will get you close to where you need to be.

Hydration Hype