We've all heard the claim that we must drink 8 glasses of water a day, but how did this seemingly arbitrary number come to be? and is there any merit to it? Like most hard facts, the deeper we look, the more malleable these dogmas become.
In this article, we will take a look at:
- Water and the human body
- Different states of hydration
- How the recommended daily allowance (RDA) is determined
Water and the human body
Water is an essential nutrient for life; every cell in the body needs it to function adequately. The benefits of drinking water go beyond just nutrition and hydration.
At the cellular level:
- Water gives form and structure to every cell in the body.
- It plays a critical role in the formation of proteins inside a cell.
- It is why our DNA has its legendary double helix shape.
Without water, proteins simply couldn't function as they do (actually, they would not function at all). Cells wouldn't develop properly, and DNA wouldn't be able to pass its instructions to future cells- making cell division completely useless.
Our body composition comprises roughly 78% water when we are born. That number drops to around 65% within the first year of life. By the time we reach adulthood, our body will only be composed of roughly 60%-55% water.
Like most things, this whole water business can get murky the more we look into it. When it comes to proper hydration, there is a plethora of variables that may alter the level of fluid within the body:
Sex, age, lean body mass, fat level, genetics, diet, climate, amount of glycogen stored in the body, electrolytes, hormonal balance, standing too long, sitting too long, sleep... the list goes on and on.
- Water retention
We can develop an almost endless list of factors that affect our hydration; for the sake of time and peace of mind, let's just focus on one:
- Body composition
Lean body mass contains about 73% water, while adipose tissue (fat) holds roughly 10% water. This means that an athlete with a high lean body mass retains a higher level of water throughout the day than someone with lower muscle mass and a higher fat percentage.
However, suppose that same athlete had low glycogen levels, either from a low carbohydrate diet or a long bout of endurance exercise. In that case, the level of water retention will tend to approximate that of his more sedentary counterpart.
The difference in water composition between an athlete with full glycogen storage and one without is partly caused by the fact that each gram of glycogen is bound to 3-4 grams of water. Therefore, the more glycogen in the muscle, the more water we retain.
How much water do we hold?
If we take a man that weighs 170 pounds, roughly 102 pounds of that will be water weight. About 2/3 (75 lbs.) of that water weight will be intracellular (water located inside your cells), and the remaining 1/3 (37 lbs.) will be extracellular (water located outside your cells).
The extracellular weight is then subdivided into the plasma (20-25%) of the extracellular volume, and 75%-80% will be interstitial (surrounding the cell).
In short:
Roughly 2/3 of our body weight comes from water (technically fluids, but let's just call it water).
That water weight is then separated further: 2/3 of it is contained within our cells and 1/3 outside of them. Furthermore, the fluid outside of our cells is divided: 20%-25% of it being plasma fluid (the fluid that surrounds blood cells and platelets) and 75% to 80% being interstitial (the fluid that surrounds our cells)
Different States of Hydration
Scientists can get creative when naming the different pathophysiological states of water and electrolyte balance within the body. There are way too many states that mean more or less the same thing. When it comes to everyday use, let's just focus on four commonly used terms:
- Euhydration — the state of optimal total body water content (which is around 42 liters for the average guy (+-1%))
- Hyperhydration (2% or more over the normal hydrated state)
- Hypohydration (static state 2% or more below normal hydrated state)
- Dehydration (the active process of losing water)
Returning to the "proverb" of 8 glasses of water a day. Let's take a look at how this concept came to be:
Let's take the axiom, "Fluid needs are inversely proportional to fluid losses," as a general truth:
What does this mean? (let's get a bit nerdy)
It means that the daily requirement of water becomes a matter of balancing the scales of fluid gains vs. fluid losses to maintain the optimal level of hydration (remember the term Euhydration)
An important variable that tilts the scale away from the state of Euhydration is our metabolism.
Our metabolism is perpetually active—converting macronutrients (food) into energy (ATP), energy back into macronutrients, moving proteins to and fro', along with multiple other processes. When we look at the process of creating energy within the body (aerobic respiration), we discover that energy production generates water as a byproduct. We also discover that the opposite also holds true. That is, when our cells use the energy molecules in the body, those cells consume water.
To be in a euhydrated state, there must be a balance between fluids and energy consumption so that the volume of water required equates to the unit of energy spent (mL of fluid needed /Kcal spent).
Our fluid needs are based on metabolic demands. When at rest, we make around 0.1ml of water per Calorie expended; simultaneously, we lose water via respiration and evaporation (from the skin) at a rate of 0.5mL/Calorie.
Combining the two, we average roughly 0.4 ml of water lost per Calorie of energy spent while at rest.
Another important aspect of water loss that we have to consider is urine excretion. Accounting for the amount of water lost through the urine can be tricky since the kidneys can concentrate the urine (low volume) or dilute it (high volume) depending on numerous variables. Still, for simplicity, we can conclude that, on average, we excrete about 0.6ml of water/kcal in the urine.
Considering these figures, we arrive at the grand total of 1ml/Cal, equating to the RDA water requirement.
To breakdown some of the numbers above:
An average sedentary person spends about 2,000 Calories per day—those two thousand Calories cause a loss of about 2,000ml (2 liters) of water—which converts to around 64 fluid oz or, better yet, 8 glasses of water/day.
This figure doesn't consider activity level, diet, environment, medicines, individual metabolism, and numerous other factors that direly affect our hydration level.
… and that's how the 8 glasses of water a day came to be, based on an average sedentary person whose activity level is mainly based on chores such as shopping, cleaning, taking out the trash, and walking the dog.
The concept of drinking 8 glasses of water a day considers the metabolic demands of an average individual but fails to account for a tremendous number of variables. Variables such as body composition, exercise, sex, glycogen storage, and other variables are not considered. In reality, "8 glasses of water a day" serves as a simple genetic baseline from which any person should expand on based on his/her individual demands. The amount of water an individual should drink is not static, and it depends on a plethora of physiological variables and environmental factors.