The Myth of 90% Water
For years, a pervasive health myth has circulated, suggesting that the human body is up to 90% water. This figure is a significant overestimation and is not supported by scientific evidence. The confusion likely stems from several sources. First, some aquatic organisms, such as jellyfish, can be well over 95% water, and the human body's reliance on water might have led to an exaggerated comparison. Another possible origin is a misunderstanding of the percentage of water found in specific tissues, such as blood plasma, which is over 90% water, or the early fetal stage, which can reach nearly 97% water. However, when considering the entire organism, these figures are misleading. The truth is far more complex, with the actual percentage changing throughout a person's life and differing between individuals.
The Real Figures: How Water Percentage Varies
The amount of water in the human body is not a fixed number but a dynamic figure influenced by numerous factors. The average adult male is composed of about 60% water, while the average adult female is around 50–55%. This difference is primarily due to differences in body composition. Women typically have a higher percentage of body fat, and since fatty tissue contains significantly less water than lean muscle tissue, their overall percentage is lower. For an elderly person, the percentage can decrease even further, dropping to as low as 45%. The highest water percentage is found at birth, where infants' bodies are about 75-78% water, gradually decreasing to adult levels by about one year of age.
Where is all the water located?
Water is not just a free-flowing liquid within the body but is meticulously distributed and contained within different fluid compartments. Roughly two-thirds of the body's total water is located inside the cells and is called intracellular fluid. The remaining one-third is extracellular fluid, found in the spaces between cells (interstitial fluid) and in blood plasma. Water's presence varies dramatically by tissue type. Here is a breakdown of the approximate water content in different body parts:
- Lungs: Approximately 83% water.
- Blood: Roughly 92% of blood plasma is water.
- Brain and Heart: Composed of about 73% water.
- Kidneys and Muscles: About 79% water.
- Skin: Contains approximately 64% water.
- Bones: Only about 31% water.
The Crucial Functions of Water
Water is called the 'elixir of life' for a reason; its functions are integral to every system in the body. It is a vital nutrient that acts as a building material, solvent, and transporter. Without adequate hydration, the body's physiological functions cannot be maintained. Here are some of water's essential roles:
- Temperature Regulation: It helps maintain a stable body temperature through sweating.
- Waste Removal: It flushes out metabolic waste products via urine and feces.
- Nutrient and Oxygen Transport: It is a critical component of blood, which carries nutrients and oxygen to cells throughout the body.
- Lubrication and Protection: It lubricates joints and acts as a shock absorber for organs, the spinal cord, and the fetus in the womb.
- Cellular Health: It is crucial for maintaining the health and integrity of every cell.
The Body’s Water Balance
Maintaining the right balance of fluid is vital. The body has sophisticated mechanisms to regulate its water content, primarily driven by the kidneys and a thirst signal from the brain. When dehydrated, the body conserves water, and thirst signals prompt drinking. In contrast, if there is an excess of water, the kidneys produce more urine to excrete the surplus. Chronic dehydration can lead to serious health issues, while overhydration, though rare, can also be dangerous, causing conditions like hyponatremia.
Water Content by Age and Sex: A Comparison
To highlight the variability, the following table compares the typical water percentages across different demographics, showing how the "one-size-fits-all" myth is simply inaccurate.
| Demographic | Approximate Water Percentage |
|---|---|
| Infants | ~75-78% at birth, decreasing to 60-65% by one year |
| Adult Males | ~60% |
| Adult Females | ~50-55% |
| Elderly (Over 60) | Can be as low as 45% |
| Athlete (High muscle mass) | May be higher than average for their sex |
| Obese Individuals (High fat mass) | May be lower than average for their sex |
Conclusion: The Importance of Accurate Information
The notion that our bodies are 90% water is a widespread exaggeration that has little basis in fact. While water is undoubtedly the most abundant substance in the body and essential for all life processes, the true percentage is considerably lower for an adult, hovering around 60%. The percentage is not static and changes throughout our lifespan, driven by our age, gender, and overall body composition. Understanding the actual figures and the vital roles water plays helps to appreciate the complexity of our own biology. Instead of fixating on an incorrect percentage, focusing on maintaining proper hydration by listening to your body's thirst signals is the key to supporting overall health and well-being.
Visit the USGS Water Science School for more information on the role of water in the human body.
