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You’ve probably heard or read about how you can survive for weeks without food but that you can only survive for days without water. Your parents probably told you repeatedly when you were young to drink at least eight cups of water a day. Water is known to help cleanse out the toxins in your body and to quench your thirst on a hot summer day. But what role does water play when it comes to body composition?

While most of us are predominantly preoccupied with total body weight, body fat percentage, or even BMI, total body water and hydration status are often overlooked.

When was the last time you thought about getting enough fluids in your body to reach your health and fitness goals? 

Staying well-hydrated is not just important for survival. Your hydration status is equally as important as getting enough rest and quality food for muscle growth and improved physical performance. Moreover, meeting your daily hydration needs could be the difference between accomplishing your desired body composition goals and not seeing body composition improvements when you think you’re doing everything right.

In this article, we’re going to put a spotlight on the following:

  • What body water is and why keeping an eye on your body water balance is important
  • What happens to your body when you’re dehydrated
  • Why it’s important to stay hydrated if you’re looking to build more muscle
  • The link between dehydration and fat loss
  • How much one should drink daily to stay well-hydrated

What is Body Water?

At one point, you’ve probably wondered what makes us truly human?

You’ll probably get various answers from people with different backgrounds. There’s consciousness, free will, having a set of morals, the idea of having a soul, and being fully aware of our own mortality.

Although these answers require further deep thought and investigation, one thing’s for sure: as much as two-thirds of your body weight is water.

Trivia: Ninety-five percent of the body of a jellyfish is water.

Even though much of our body is made up of water, the percentage of our body composition that is water changes based on our functional needs. Some of the essential functions of water in our bodies include the following:

  • It’s a building block to almost every cell in the body.
  • It regulates body temperature via sweating and respiration.
  • Carbohydrates and proteins used for energy are transported by water in the bloodstream.
  • It assists in the removal of metabolic waste, mainly through urination.
  • It acts like a shock absorber to protect our brain and spinal cord.
  • It makes up saliva and fluid that lubricates your joints.

The amount of water in your body depends on various factors including age, gender, physical activity, and even where you live. It’s often referred to as Total Body Water (TBW).

For example, infants are born with roughly 78% of their entire weight being water. By one year of age, TBW decreases to about 65% of weight. In healthy adult males, TBW averages 60% of their weight because they generally carry more lean mass. On the other hand, women will see that roughly 55% of their weight is TBW.

Most of this water comes from that lean body mass mentioned above. This includes your blood, organs, and muscle. Here’s a snapshot of the major body organs’ water content:

  • Brain and heart – 73%
  • Lungs – 83%
  • Skin – 64%
  • Muscles and kidneys – 79%
  • Bones – 31%

Your TBW can be further segmented into two compartments: extracellular water (ECW) and intracellular water (ICW).

Intracellular Water (ICW)

ICW is the amount of water found in your cells. In healthy adults, ICW makes up two-thirds of your total body water. Essential cellular processes take place in the ICW, such as creating the energy necessary to fuel each cell for their specific functions.

Extracellular Water (ECW)

ECW is the amount of water located outside of your cells. It makes roughly a third of the total body water in healthy adults. ECW helps control the movement of electrolytes, delivers oxygen to the cells, and clears waste from metabolic processes.

In terms of body composition, an increase in ICW can signify increased muscle mass. On the other hand, an increase in ECW could indicate inflammation, fluid retention, or disease.  In fact, inflammation resulting from having too much visceral fat can trigger activation of the renin-angiotensin-aldosterone-system (RAAS) which in turn increases ECW. When activated, this hormone system acts to maintain blood pressure and fluid balance. However, when overstimulated, the RAAS can lead to symptoms associated with obesity, kidney disease, and the metabolic syndrome.

It’s worth noting that TBW is constantly changing with gains and losses of fluid even in healthy adults. The volume is regulated through a complex system of exchange of fluids, solutes, and ions within the compartments in the body — the RAAS and the ADH.

Here’s how the ADH system works when your body experiences a significant loss of body water:

  1. The hypothalamus —the gland responsible for regulating our body temperature and triggering the processes that balance the fluids in our bodies—detects dehydration.
  2. As a result, the hypothalamus signals the release of antidiuretic hormone (ADH) vasopressin which causes the kidneys to remove less water from the blood. This leads to peeing less and a darker, more concentrated urine. It also raises blood pressure.
  3. Meanwhile, your brain also tells you that you’re thirsty, and once we sip on some water or consume something that’s made of water, TBW levels return to normal. The same mechanism happens when your body temperature rises being in a warm environment, working out, or fever. Body temperature goes down with sweating which in turns helps you cool off.

As for the cellular compartments, a water deficit leads to an increase in the electrolyte concentration in the extracellular compartment. This higher electrolyte concentration draws water from the intracellular compartment improve the electrolyte-fluid balance in the ECW, causing the cells to shrink.

The RAAS and ADH systems work interchangeably to keep our fluid balance on track and properly-timed because our brains, kidneys, various glands, and hormones work together to monitor the amount of water that you’re taking in and losing.

All of these are analogous to a radiator which heats a room. You set it to turn on at a certain temperature. When the room falls below that temperature the radiator turns on. When the room temperature increases, the radiator turns off. In this case, the RAAS activates when the room temperature is too hot (fluid is too high) and the ADH activates when room temperature is too low (fluid reduced).

In short, your body is smart and sensitive enough to detect irregularities and compensate for losses and gains to make sure that everything’s well-balanced.

What Happens to Your Body When You’re Dehydrated

Fluid gains and losses throughout the day (when you breathe, sweat, and pee) are regulated by the hypothalamus, the renin-angiotensin-aldosterone system, and kidneys.

Dehydration doesn’t occur until you’re losing more water than you can replace — frequent episodes of diarrhea and vomiting, having too much alcohol the night before, or sweating and peeing like crazy.

When dehydration sets in, you’re likely to experience a wide range of symptoms. Early signs and symptoms include dizziness, headaches, dry mouth, and cool skin. When not addressed in the early stages, dehydration could lead to a lack of urge to pee. Remember that time when you were out hiking and you haven’t peed in hours?

Dehydration can lead to long-term health consequences as well. High blood pressure is also common in people who are chronically dehydrated. When the body’s cells are devoid of water, the brain sends a signal to the pituitary gland to secrete vasopressin (a chemical that causes constriction of the blood vessels), which in turn causes blood pressure to rise. Chronic elevations in blood pressure can eventually lead to heart failure. As blood vessels narrow, oxygen and blood supply to the brain are also put in jeopardy resulting in migraines and significant dips in concentration.

Why It’s Important to Stay Hydrated When You Want to Gain Muscle Mass

Research studies have shown that when cells lose water (and thus volume), protein production can slow down and protein breakdown speeds up. However, it’s worth noting that these studies are done in-vitro (isolated in test tubes rather than an animal or human) and further research is required on the relationship of hydration and its impact on muscle mass breakdown.

What might this mean for the average person? Forgetting your water bottle at home before leaving for the weight room could mean leaving muscle gains on the table due to poor performance.

For instance, when you’re moderately dehydrated (approximately 3% of body weight), exercise performance may be impaired, particularly with repeated bouts of anaerobic exercises (e.g. weight lifting, sprinting). As a result, it increases demand for aerobic metabolism. This is further supported by another study concluding that blood flow to the exercising muscles (in prolonged exercise) declines significantly with dehydration.

It seems like dehydration can also increase your likelihood of injury.

Research findings by the Chicago State University suggested that dehydration of 2.9% body mass decreases the ability to generate upper and lower body strength. The researchers recommended that coaches and athletes must understand that sports performance requiring strength and power may be impaired by inadequate hydration resulting in increased susceptibility to injury.

Finally, dehydration may cause stalled muscle growth.

A study performed in ten-weight trained males reported that passive dehydration resulting in approximately 1.5% loss of body mass, as a result of water loss, decreases muscle strength of one rep max bench press. When muscle endurance and power are compromised due to dehydration, you’ll likely end up not pushing for more reps in the weight room, making your workout less effective.

In summary, dehydration could potentially lead to stalled muscle growth, possible muscle mass breakdown, increased risk of injury, and impaired exercise performance.

Can Dehydration Hinder Fat Loss?

Now that you’re aware of the importance of being well-hydrated to gain lean body mass, you’re probably curious if your hydration status can also impact fat loss efforts.

In a study by published in Annals of Family Medicine, the researchers found out that people who are obese and have a higher body mass index or BMI (although BMI has its own limitations) are more likely to be inadequately hydrated and vice versa. The researchers further concluded that—although the correlation requires further probing— their findings should encourage clinicians to consider hydration as a cornerstone of a weight-loss diet.

Meanwhile, studies have found that increasing water intake can lead to greater weight and fat loss than dieting alone. Overweight and obese adults were assigned a reduced calorie diet where one group was required to drink 500ml of water prior to every meal and the other group had no fluid consumption requirements. At the end of 12 weeks, the water drinkers were found to have a 44% greater rate of weight loss!

While further research is required, more focus on hydration appears to help people who want to speed up their fat loss efforts.

How Do I Know If I’m Dehydrated?

Before learning how much fluids and water you need to stay well-hydrated, it’s equally important to know if you’re already dehydrated or not.

Besides feeling thirsty and tired, here are some signs of fluid deprivation that some people overlook:

  • Darker colored urine (medium yellow to a brown range) or lack of urine
  • Headaches that worsen and turn into migraines
  • Confusion and inability to concentrate
  • Irritability

Your Game Plan: How to Stay Well-Hydrated

As for how much water and fluids to drink, it is important to ensure that you’re getting enough as everyone’s hydration needs differ depending on a number of factors. Age, climate, activity level, and existing health issues can all impact daily fluid requirements.

The Institute of Medicine recommends that women and men consume roughly 2.7 liters (91 ounces) and 3.7 liters (125 oz), respectively, of fluid a day. This may sound like a lot of water, but it is important to note that these fluid recommendations include intake from both food and beverages. In fact, food is estimated to contribute to 20% of our fluid intake alone!

In a nutshell, if you’re having your daily intake of fruits and vegetables and drinking fluids, you’re probably getting the water you need.

However, pay more attention to your hydration needs and fluid-guzzling habits when you’re involved in prolonged workouts or hard exercise, and when you’re sick resulting in frequent bouts of diarrhea and vomiting.

In these situations where we are losing excess water, it is often recommended to consume beverages that offer more than just water. For instance, ultra-endurance athletes are encouraged to up their fluid intake and make sure to opt for drinks with electrolytes. Sodium is necessary during recovery in reducing urinary output and increasing the rate of fluid balance restoration so these types of drinks offer benefits that water alone cannot.

Watch Out for Overhydration

Although dehydration is a serious concern, overhydration or hyponatremia is something to watch for when considering fluid and electrolyte replacement.

Hyponatremia is an electrolyte disorder in which plasma concentration of sodium is too low. Most cases of hyponatremia are induced by an increase in total body water. Many instances of hyponatremia have occurred because of drinking too much water.

In fact, the New England Journal of Medicine reported in a study of Boston Marathoners that hyponatremia has emerged as an important cause of race-related death and life-threatening illness among marathon runners. The researchers further found that sudden weight gain during the race (which correlated with excessive fluid intake) was the strongest single predictor of hyponatremia.

Proper Hydration May Be the Missing Link to Improving Your Body Composition

Your hydration status and meeting your daily fluid needs are equally as important as getting enough sleep and eating right in helping improve your body composition and staying healthy.

Not drinking enough water before your morning jog, forgetting your water bottle before spin class, and binging on alcohol the night before you do serious legwork are surefire ways to dehydrate and hinder progress in your body composition goals. Why not drink up some water or have a quick fruit-filled snack?


Kyjean Tomboc is a nurse turned freelance healthcare copywriter and UX researcher.  After experimenting with going paleo and vegetarian, she realized that it all boils down to eating real food.

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