How Many Ounces of Water Should I Drink Daily?
8 ounces of water, 8 times per day
Unless you have been living under a rock your entire life, you must have encountered someone with the recommendation to consume at least 8 ounces of water, 8 times per day. Somehow this recommendation became the golden standard for optimal health.
Did you ever take a moment to wonder where this guideline came from?
As it turns out, the origin of the 8x8 rule is not very well known .
When we think about fitness, we commonly associate exercise and weight loss with a need to set a goal for how much water we should be drinking. I hear all kinds of recommendations ranging from 1 - 2 gallons per day.
Many of my clients ask me if it's necessary, so I set out to investigate...
Where does this advice come from?
The purpose of this literature is to investigate whether there is any true scientific credibility to the common water intake recommendations we hear. I am therefore not going to get into the physiological purpose and functions of water in the human body as this information is very well documented already [2, 3, 12].
Even though the overall origin is not entirely clear, the common justifications for routinely monitoring our water intake are :
- The human body consists of 50-70% of water, and our blood, muscles, brain, and bones are mostly made up of water; therefore we need to make sure we have an adequate intake to function and survive properly.
- Drink more water to prevent fatigue, arthritis, lack of mental alertness, angina, migraine, hypertension, asthma, dry cough, dry skin, acne, nosebleed, and depression.
- Population-based studies show correlations of increased water intake to decreased risk of cancers, heart disease, and other medical conditions.
- Constipation relief
- Thirst does not signal rehydration in time
- Increased water intake promotes weight loss
The first point above is an illogical argument; the human body does a perfectly fine job of regulating water balance in normal healthy individuals [2, 12]. The second point is just as nonsensical since there is absolutely ZERO evidence to support such claims.
The third point is about correlations, which do not prove causations. We need well-designed controlled studies to validate population-based correlations. At this moment, there is strong evidence to support that increased fluid intake will decrease kidney stone production in those diagnosed with Urolithiasis. Also, exercise-related asthma is linked to low fluid intake .
In normal healthy adults, stool output is not altered by fluid intake ; however those suffering from chronic constipation benefited from increasing fluid intake on top of consuming more fiber .
As for the last 2 points, let’s see how well they are scientifically backed. If so, how much water should we be aiming to consume each day --- AND is it worth the effort to mindfully track?
The Dehydration ---> Thirst Relationship
We typically think of our body in an EITHER – OR manner.
For example: we are either gaining or losing body-fat, or we are either gaining or losing muscle mass.
That’s just simply not true. At any given time we are BOTH losing AND gaining body-fat, muscle mass, and you hopefully guessed where I am going with this – water too.
In normal individuals who are moderately active, and in a 64 – 68 degrees Fahrenheit environment, body water levels remain relatively constant . Read this statement again... our body does a fine job of regulating water balance on its own without external interference!
To keep the scope of this text simple to answer our main question, I am intentionally leaving out many details, because you can follow my references for more biological information on the regulatory process.
Suffice it to say that our 2 primary sources for water are from beverages and from food, and deficit/surplus of water is counter-balanced by subtle hormonal changes.
"But dehydration is dangerous so how do we make sure this does not happen...?"
Signs of dehydration are not always so clear, and so governments make recommendations. The USDA sets our Dietary Reference Intake for water as follows :
Actual water requirements should not necessarily be defined this way, because there are too many variables that go into assessing an individual’s needs, and setting minimums might mislead certain people into thinking they have gotten enough water, when they actually haven’t .
Furthermore, there are currently no clear and effective ways to even measure a population’s dehydration status .
Most of us, however, are getting more than adequate water by following the current minimum recommendations .
Multiple labs have confirmed that physical activity will cause a water loss of 1 -2 liters per hour from sweat, so active people should certainly strive to meet recommended water needs [2, 4, 5, 6, 7, 8, 12, 13].
People who live in a hot climate also need to intake more water from beverages. Those who are very active and living in very hot conditions could lose up to 12 liters per day in sweat [5, 13].
If you’re monitoring your weight on a scale frequently, take note that 1 ml of water is equal to 1 g in weight. So your weight could easily swing from 4 – 26 pounds moment-to-moment based on sweat/rehydration rates [5, 6]. All the more reason to stop using the weight scale as a tool to measure fat-loss progress in the short-term.
A typical sedentary person with a temperature-regulated environment could actually get adequate water with just 500 ml per day, thereby making minimum recommendations potentially “overblown .” The average American is really not at risk for dehydration – if anything it’s really just young infants and elderly people that are at risk .
"So I can't rely on my thirst to tell me how much water I need?"
As previously said, the human body does a perfectly fine job of regulating water balance on its own. In normal healthy people, the thirst function operates effectively and efficiently, except in hot climates [2,12]. Moreover, before thirst is even elicited, an antidiuretic hormone (ADH) is released from the pituitary gland, causing the kidneys to increase water reabsorption .
During exercise, the thirst mechanism does not lead to adequate rehydration alone . If a beverage is not of a desirable temperature (i.e. warm water), someone could fail to rehydrate adequately as well, but who drinks warm water when thirsty ?
Infants and elderly may fail to rehydrate timely and properly due to various physiological reasons. Mental conditions and emotional states can also affect an elderly person’s ability to rehydrate correctly [2,12].
"Okay stop talking science and just tell me how many ounces of water to drink!"
There clearly is no definitive answer to this question. To target a specific amount of water each day is counter-productive and futile in many cases. It also supports an obsessive relationship with the diet, which is extremely undesirable for anyone looking to be fit.
Based on the data above, here is a breakdown of my opinion:
If you are a normal healthy sedentary person, not elderly or infant: drink when you are thirsty.
If you are living in a hot climate: follow USDA recommendations at MINIMUM... you should probably be drinking even more depending how much water you are losing through sweat.
If you are exercising: consider that you are losing 1-2 liters per hour in sweat... so replace it quickly as you don't want to lose beyond 2% of your weight in water.
"Wait... you didn't talk about the relationship of water to weight loss!"
How very observant of you. You can read all about that here: Can You Lose Weight by Drinking More Water?
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