Water is essential for the human body to function properly. In fact, water makes up a significant portion of the human body. On average, around 60% of an adult’s body weight comes from water. This percentage can vary based on factors like age, health status, and body composition.
Total body water content
The total body water content is known as total body water (TBW). This is comprised of two compartments:
- Intracellular fluid (ICF) – fluid inside cells
- Extracellular fluid (ECF) – fluid outside cells
Here is a breakdown of the average water content of the human body by compartment:
| Compartment | Percentage of body weight |
|---|---|
| Intracellular fluid (ICF) | 28-38% |
| Extracellular fluid (ECF) | 20% |
| Total body water (TBW) | 60% |
As shown, intracellular fluid makes up the largest share of total body water, accounting for 28-38% of body weight. Extracellular fluid makes up about 20%. Together, this equals the total body water percentage of around 60%.
Factors that influence body water percentage
While 60% is the average, several factors can cause the body water percentage to vary between individuals:
- Age – Infants have higher body water percentages, around 75% at birth. This decreases to 60% by age 1. Elderly individuals tend to have lower percentages, declining to 50% by age 80.
- Sex – Men generally have more muscle mass than women, so they tend to have a marginally higher percentage of body water.
- Body composition – Individuals with higher muscle mass and lower fat mass tend to have higher percentages of water.
- Health conditions – Certain diseases like diabetes and kidney disorders can alter fluid balances.
- Medications – Some drugs like diuretics reduce body water by increasing urine output.
- Exercise and heat – Intense exercise and heat exposure cause sweating and temporary water loss.
Changes in body water with age
As mentioned, the body water percentage changes over a person’s lifetime. Here is an overview:
- Infants – ~75% of body weight
- Children – 65-70% of body weight
- Young adults – 60-65% of body weight
- Middle age – 50-60% of body weight
- Elderly – 45-50% of body weight
The main reasons for declining body water percentages with age include:
- Decreasing muscle mass and increasing body fat as we age
- Kidney function deteriorating, reducing the body’s ability to concentrate urine
- Decreased thirst signals and fluid intake
- Hormonal changes
Water distribution in infants
Infants need a high proportion of water to support essential bodily functions and metabolism. Here’s how the 75% water content is distributed in infants:
- Extracellular fluid – 40% of body weight
- Intracellular fluid – 35% of body weight
The large share of extracellular fluid provides padding, support, and nutrients to the cells during this crucial growth and development stage.
Changes during childhood and adolescence
As infants turn into children and then adolescents, the percentage of body water progressively declines while body fat increases. Some key changes include:
- Extracellular fluid decreases to 25% of body weight
- Intracellular fluid decreases to about 40% of body weight
- Boys accumulate more muscle and hence maintain higher water percentages than girls
- Onset of puberty causes slight dips in body water percentages
Regulating fluid balance
The body maintains fluid balance through carefully regulated input and output. The key processes involved are:
- Fluid intake – Beverages and foods provide the input of water into the body. Thirst mechanisms prompt us to consume fluids.
- Kidney filtration – The kidneys filter the blood and remove excess fluid and waste products as urine.
- Sweating – The sweat glands in the skin allow water loss for thermoregulation.
- Respiration – Some water is lost when breathing out humidified air.
- Digestion – Fecal matter eliminates some fluid from the body.
Hormones like anti-diuretic hormone (ADH) and aldosterone help fine-tune this balance by regulating kidney function and thirst signals in the brain.
Maintaining fluid balance
To keep the body’s fluid levels in balance:
- Drink fluids regularly without waiting for thirst. Water is best.
- Moderate caffeinated and alcoholic beverages which have diuretic effects.
- Eat a balanced diet with fruits and vegetables which have high water content.
- Replace fluids lost from exercise, heat, diarrhea, vomiting, etc.
- Get medical help for persistent fluid imbalances causing very dark urine or inadequate urination.
Dehydration
Dehydration occurs when the body loses more fluid than it takes in. Symptoms can include:
- Thirst
- Dry mouth
- Dizziness
- Dark urine
- Fatigue
- Confusion
Severe dehydration rapidly reduces the body’s water percentage and can become life-threatening without prompt treatment. Causes include diarrhea, vomiting, strenuous exercise, diabetes insipidus, and not drinking enough water.
Treating dehydration
Treatment focuses on replacing lost fluids and electrolytes. This can be done by:
- Oral rehydration therapy – Drinking water, juices, or electrolyte solutions
- IV fluids – For acute dehydration, IV saline solution may be needed
- Identifying and treating the underlying cause – infections, diabetes, etc.
Overhydration
Overhydration occurs when fluid intake exceeds the kidney’s ability to excrete it. This causes an excess buildup of fluid in the body’s tissues and blood vessels. Symptoms can include:
- Swollen hands and feet
- Distended neck veins
- Congested lungs
- Shortness of breath
- Confusion
- Fatigue
In rare cases, severely high fluid volumes can lead to brain swelling and seizures. Causes include kidney failure, liver disease, and some hormonal disorders.
Treating overhydration
Treatment involves:
- Restricting fluid intake
- Medications to increase urine output like diuretics
- Treating underlying medical conditions
- Dialysis in kidney failure
Measuring body water percentage
There are several techniques that can estimate total body water and percentages:
- Bioelectrical impedance analysis (BIA) – Sends a small electrical current through the body and measures resistance to calculate fluid volumes.
- Blood tests – Measures blood levels of electrolytes like sodium and proteins.
- Urine tests – Urine osmolality and specific gravity indicate hydration status.
- Plethysmography – Measures changes in body volume in specialized chambers.
- Isotope dilution – Known amounts of isotopes like deuterium oxide are tracked to calculate fluid volumes.
These methods help determine if an individual’s fluid levels are in the normal range and guide hydration and treatment recommendations.
The importance of water
Maintaining adequate total body water is vital for health. Here are some key functions of water in the human body:
- Transports nutrients, electrolytes, and gases
- Removes wastes and toxins
- Protects and cushions organs
- Maintains muscle and skin integrity
- Lubricates joints
- Regulates body temperature
- Supports digestion and absorption
Given water’s far-reaching roles, even small losses or excesses can disturb body functions. Drinking enough fluids daily is essential to stay healthy.
Conclusion
On average, water makes up about 60% of the human body. However, this percentage varies with age, sex, health status, and other factors. Infants have the highest percentage at ~75%, which declines to ~45-50% in the elderly.
Body fluid balance is tightly regulated through thirst, kidney function, and hormones. Dehydration and overhydration occur when input and output become mismatched. Maintaining adequate water levels is vital for normal functioning.
Estimating total body water percentages helps assess hydration status and guide treatment. Drinking enough fluids daily, along with healthy kidney function, are key to sustaining ideal water levels in the body.
