Contrary to the popular wisdom that we are 70% water, the actual range is from about 40% to 70% of our body's mass, or about 42 liters. It makes up 65% to 75% of muscle mass and 10% of the fat mass. Because body fat has a low water percentage, those with more fat have a lower overall percentage of their body weight as water.
For an average 80-kg male, their body mass is about 53% water. 62% is fluid contained within cells (or intracellular) such as muscles, bones, and fat. 38% is extracellular- a wide ranging category that includes fluid between cells, lymph, saliva, fluid in the eyes and digestive tract, fluid bathing the spinal cord nerves, and fluid excreted by the skin and kidneys. Extracellular fluid also includes blood plasma. These amounts and percentages are quite dynamic because fluid exchange between the two major categories of water storage.
Exercise impacts this balance in several ways. A hard workout will move fluids from blood plasma into cells and the spaces between them (thanks to higher fluid pressure within the circulatory system). However, higher fitness levels will tend to increase the amount of water within cells due to increases in muscle mass.
Why We Need Water
Without water, we would die in a matter of days. It has many crucial functions within our bodies such as:
- It is the body's transport and reactive medium. Nutrients and gases travel in it, waste uses it to exit
- Water (in conjunction with certain proteins) lubricates joints and cushions many organs such as the heart, lungs, and eyes
- It gives structure and form because it is noncompressible
- It is very heat stabilizing, able to absorb lots of heat without increasing in heat significantly itself. Great for keeping our body temperature the same in heat and exercise
Where Water Comes from in our Diets
One thing we may not realize is that water comes from a variety of sources. A sedentary adult will usually require 2.5 liters of water, which can increase to between 5 and 10 liters daily for an active adult in a humid environment. In a typical diet for a sedentary person, food actually provides about 1 liter of our fluid needs. Our metabolism creates another 350 mL because water is a byproduct from the chemical reactions of fat, protein, and carbohydrate breakdown. And drinking fluids typically provides around 1200 mL.
And of course, what comes in also goes out. There is the obvious: the same individual urinates about 1250 mL of fluid and 100 in feces. However we also lose about 850 in skin thanks not only to sweating but also something called insensible perspiration- the continuous seeping of water from the deeper tissues to the surface of the skin. And fun fact: we can sweat up to 12 liters in a day!
Last but not least, we lose about 350 mL through our lungs. This is from exhaled moistened air. During strenuous exercise for a physically active person, 2 to 5 mL of water can be lost every minute. Hotter more humid conditions result in less water lost, whereas more is lost in colder temperatures (which are also dryer and need more moisturizing by our lungs) or at altitude (due deeper breaths in and out).
Don't Sweat the Small Stuff (too much)
When it comes to sweat, the water loss itself is actually the most serious consequence. Sweating is determined by three major factors: the intensity of physical activity, the temperature, and the humidity. Intensity and temperature are obvious- as those increase sweating does.
But the greater the humidity the harder it is for the body to sweat due to the moisture already in the air. More moisture in the air means it is less able to carry more- which means sweat has a harder time evaporating and instead condenses into the droplets. Sweat cools through evaporation, which means humidity reduces the body's capacity to cool down.
Sweat also leads to the loss of minerals such as chromium, copper, manganese, and zinc. However, this does not mean that minerals should be supplemented above recommended levels in order to improve exercise performance or help with the training process. Ensuring a balanced diet and remaining well nourished in general is a far more impactful route to balance against mineral loss.
Additionally, other other minerals in the form of electrolytes can be sweated out during exercise. Electrolytes (sodium, potassium, and chlorine) are dissolved electrically charged particles called ions. Sodium and chlorine are the main minerals in blood plasma and extracellular liquid. Potassium is the main intracellular liquid. Collectively, they are important because they modulate fluid exchange within the body's fluid compartments. As such, they promote a constant, well-regulated exchange of nutrients and waste products between the cell and its external fluid environment.
During vigorous exercise, our kidneys reduce sodium and water loss due to the release of the hormones vasopressin and aldosterone, along with the enzyme renin. Including salt in fluids or food ingested during or after exercise can help replenish electrolytes.
Vigorous Exercise and Hyponatremia
A quick word about hyponatremia (water intoxication): one can drink too much water in during vigorous exercise. Hyponatremia can be caused by excessive sweating alongside drinking too much water. This can reduce sodium in our body to dangerous levels. It is therefore crucial that fluid intake does not exceed fluid loss.
Physiologically, sodium loss in our body during exercise is caused by several things. Drinking large quantities of water draws sodium from the extracellular compartments into our intestines, which dilates concentrations of sodium. Then exercise makes this worse because urine production goes down during exercise thanks to more water going to other uses such as sweating. When we are not urinating we cannot get rid of excess water to bring our body back into balance.
McArdle, Katch, and Katch in their text Exercise Physiology make the following recommendations:
- Drink 400 to 600 mL of fluid before 2-3 hours before exercising
- Then drink 150 to 300 mL about 30 minutes beforehand.
- Drink no more than 1000 mL of water every 15 minutes during or after exercise
- Add a small amount of sodium (¼ to ½ tsp per 32 oz) to the ingested fluid
- Do not restrict salt in the diet
- Include some glucose in the rehydration drink to facilitate intestinal water uptake via the glucose-sodium transport mechanism
And that's all the water talk for today. Stay tuned for a future article on fluid replacement in exercise!