Water is essential for life, and most people have heard the claim that the human body is made up of around 70% water. But is this really true? Let’s take a closer look at the evidence.
Quick Answer
The often quoted figure that the human body is made up of 70% water is not entirely accurate. The percentage of water in the human body varies according to age, gender, and other factors. However, it is true that water makes up a significant proportion of the human body.
What Does the Research Say?
Most researchers agree that the 70% water figure is an oversimplification. Here’s a summary of what the research tells us:
- Newborn infants have the highest percentage of water, around 75-78%. This is because they have more extracellular fluid and relatively less protein and mineral content compared to adults (1).
- The percentage of water decreases with age. By one year of age, the water content drops to around 65%. In adults, it ranges from 50-65% depending on gender and body composition (2).
- Adult males tend to have a lower percentage of water than females. This is because females naturally have a higher percentage of body fat. Fat tissue has a higher water content compared to lean tissue (3).
- Athletes and very muscular individuals tend to have a lower percentage of water, around 50-55%, due to having more lean tissue and less fat (4).
- Obese individuals have a higher percentage of water, up to 70%, due to increased fat tissue (5).
Based on this research, it’s clear that the 70% water figure is a rough average that doesn’t apply equally to all people.
Water Content of Specific Tissues and Organs
Looking beyond the whole body percentage, the water content varies widely between different tissues and organs. Here are some examples (6):
- Blood is approximately 83% water.
- Muscle tissue is around 75% water.
- The brain is about 73% water.
- Skin tissue is approximately 70% water.
- Adipose (fat) tissue is around 10-40% water.
- Bone tissue is around 22% water.
As you can see, some parts of the body like blood and muscle are mostly water, while others like bone and fat tissue have a lower percentage. This helps explain why using a single figure to represent the entire body can be misleading.
Water Balance in the Body
Water balance in the human body is carefully regulated. The percentage of total body water (TBW) is maintained within a narrow range, even with fluctuations in water intake and output. Here’s how it works (7):
- Fluid intake: Around 80% comes from drinking liquids, 20% from food.
- Fluid loss: Occurs through urine, feces, sweat, and evaporation through breathing.
- Thirst mechanism: Triggers an urge to drink when the body water drops slightly below the normal range.
- Kidney regulation: The kidneys can adjust the amount of water lost in urine depending on intake and other needs.
- Hormonal regulation: Hormones like ADH (antidiuretic hormone) help adjust kidney function and maintain fluid balance.
In healthy individuals, this system of intake, output and internal regulation keeps body water percentage at appropriate levels even with moderate changes in the amount and timing of drinking fluids. Severe dehydration or overhydration are uncommon under normal circumstances.
Functions of Water in the Human Body
Given that water makes up such a large share of the human body, it serves many crucial functions (8):
- Transportation: Water transports nutrients, hormones, glucose, oxygen and waste throughout the body.
- Temperature regulation: Sweating and evaporation help regulate body temperature.
- Digestion and absorption: Water helps digest food and absorbs the nutrients.
- Lubrication: It lubricates joints and internal organs.
- Structure: Water gives structure to cells and is a key component of tissues.
- Metabolic reactions: It participates in metabolic processes as a solvent and reactant.
Maintaining adequate hydration is vital for allowing the body to perform these critical functions. Even small degrees of dehydration can impair performance and health if allowed to persist.
Impacts of Dehydration
Since water is so crucial, even mild dehydration of 1-2% of body weight can produce adverse effects including (9):
- Thirst and discomfort
- Reduced physical performance – up to 20% decrease in endurance capacity
- Impaired cognitive function – difficulties concentrating, headaches
- Constipation, digestive issues
- Increased perceived effort during exercise
- Altered thermoregulation – reduced ability to regulate body temperature
More severe dehydration of 5-10% body weight can result in serious medical issues including heat injury, seizures, respiratory distress, and even death in extreme cases. That’s why maintaining proper hydration is so important for health.
Recommendations for Water Intake
Due to differences in age, gender, activity level and other factors, recommended water intake can vary significantly between individuals. Here are some general recommendations from health authorities (10):
| Group | Recommended Intake (L/day) |
|---|---|
| Infants 0-6 months | 0.7 |
| Infants 7-12 months | 0.8 |
| Children 1-3 years | 1.3 |
| Children 4-8 years | 1.7 |
| Children 9-13 years | 2.4 |
| Adolescent girls 14-18 years | 2.3 |
| Adolescent boys 14-18 years | 3.3 |
| Adult females | 2.7 |
| Adult males | 3.7 |
Higher intakes may be needed for very active individuals or those in hot climates. Fluid needs can also increase during illness or pregnancy. Monitoring urine color is a simple way to assess if more fluids are required.
Water vs Other Beverages
While plain water is best to meet everyday hydration needs, other beverages like milk and juices also contribute. However, some drinks like soda and alcohol can have dehydrating effects if consumed in excess. Here’s a comparison:
- Plain water – Hydrating and zero calories
- Milk – Hydrating with nutrients like calcium, potassium and protein
- 100% fruit juice – Hydrating with antioxidants but high in natural sugars
- Coffee/tea – Can be hydrating if consumed without too much caffeine
- Soda – Can have mild diuretic effects from caffeine and sugars
- Alcohol – Dehydrating due to increased urine output
For the best hydration, aim to meet most needs through water, milk, fruits/vegetables and unsweetened drinks. Limit caffeine and alcohol which act as diuretics. Monitor urine color and drink more if dark yellow or amber.
The Bottom Line
While the often-cited 70% water figure is not exactly scientifically accurate, it is certainly true that water composes a very substantial proportion of the human body. Maintaining proper hydration is essential for health and life.
The exact percentage of water varies based on age, gender, body composition and other factors. But studies show adult males are typically 55-60% water, adult females 50-55%, obese individuals 70%, and newborns 75-78%. No single number defines everyone.
Water serves vital functions related to temperature regulation, nutrient transportation, digestion, circulation and more. Even mild dehydration can impair these processes.
Recommended water intake depends on factors like age, activity level and climate. The best approach is to drink fluids regularly, monitor urine color and drink more if dark yellow or amber. Plain water is ideal, but other beverages can contribute to hydration as long as caffeine and alcohol are limited.
While saying the body is 70% water may be overly simplistic, keeping properly hydrated is clearly essential for good health!
References
1. Fomon SJ. Body water compartments in human infants. J Pediatr. 1960;56(4):520-529. doi:10.1016/s0022-3476(60)80271-8
2. Wang ZM, Deurenberg P, Guo SS, Pietrobelli A, Wang J, Pierson RN Jr, Heymsfield SB. Six-compartment body composition model: inter-method comparisons of total body fat measurement. Int J Obes Relat Metab Disord. 1998;22(4):329-337. doi:10.1038/sj.ijo.0800583
3. Chumlea WC, Guo SS, Zeller CM, Reo NV, Baumgartner RN, Garry PJ, Wang J, Pierson RN Jr, Heymsfield SB, Siervogel RM. Total body water reference values and prediction equations for adults. Kidney Int. 2001;59(6):2250-2258. doi:10.1046/j.1523-1755.2001.00732.x
4. Silva AM, Minderico CS, Teixeira PJ, Pietrobelli A, Sardinha LB. Body fat measurement in adolescent athletes: multicompartment molecular model comparison. Eur J Clin Nutr. 2006;60(8):955-964. doi:10.1038/sj.ejcn.1602406
5. Waki M, Kral JG, Mazariegos M, Wang J, Pierson RN Jr, Heymsfield SB. Relative expansion of extracellular fluid in obese vs. nonobese women. Am J Physiol. 1991;261(2 Pt 1):E199-E203. doi:10.1152/ajpendo.1991.261.2.E199
6. Jéquier E, Constant F. Water as an essential nutrient: the physiological basis of hydration. Eur J Clin Nutr. 2010;64(2):115-123. doi:10.1038/ejcn.2009.111
7. Maughan RJ. Impact of mild dehydration on wellness and on exercise performance. Eur J Clin Nutr. 2003;57 Suppl 2:S19-S23. doi:10.1038/sj.ejcn.1601897
8. Popkin BM, D’Anci KE, Rosenberg IH. Water, hydration, and health. Nutr Rev. 2010;68(8):439-458. doi:10.1111/j.1753-4887.2010.00304.x
9. Sawka MN, Cheuvront SN, Carter R 3rd. Human water needs. Nutr Rev. 2005;63(6 Pt 2):S30-S39. doi:10.1111/j.1753-4887.2005.tb00152.x
10. Institute of Medicine (US) Panel on Dietary Reference Intakes for Electrolytes and Water. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington (DC): National Academies Press (US); 2005. 2, Overview of Dietary Reference Intakes for Water and Electrolytes. Available from: https://www.ncbi.nlm.nih.gov/books/NBK109285/
