The Science of Hydration: How Water Intake Affects Overall Health

Water is essential for life. Depending on age, the body is 55-75% water. However, about half of people worldwide don't meet daily water intake recommendations, and up to 75% of Americans are chronically dehydrated.

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Understanding Hydration and the Human Body

Optimal hydration is paramount for maintaining overall health and well-being, and its importance extends far beyond simple thirst quenching. Water is required for the optimal physiological functioning of every organ system in the body. Water comprises 73% of the brain and heart, 31% of bones, 79% of muscles and kidneys, 64% of skin, and 83% of the lungs (14). Water is essential for maintaining cellular homeostasis, which refers to the body's ability to maintain a stable internal environment despite external fluctuations. The body keeps the balance of water and solutes within its cells and body fluids through osmoregulation. Maintaining an adequate hydration status influences this delicate balance, ensuring that cells neither swell nor shrink excessively, preserving their structural integrity and functionality. (33) 

Water is crucial for nutrient transport and absorption. It is a universal solvent, facilitating the dissolution and transportation of oxygen and essential vitamins, minerals, and other nutrients throughout the body. (25) 

Hydration is also integral to temperature regulation. The body relies on water to dissipate heat through perspiration, preventing overheating and maintaining a stable internal temperature. This is why staying well hydrated is especially important in hot weather. (14) 

Water acts as the body's natural lubricant. It cushions the joints to promote flexibility and mobility. Water also moistens the tissues of the eyes, nose, mouth, glands, skin, and digestive tract to prevent them from drying out. (25) 

Water also aids digestive processes and helps to rid wastes from the body by supporting the detoxification and elimination functions of the kidneys, liver, and intestines (3).  

Cognitive function is influenced by hydration status. The brain needs water to create hormones and neurotransmitters responsible for thinking, processing, memory, and emotions. Water acts as a shock absorber for the brain and spinal cord, preventing injury. (14)

Effects of Dehydration on Health

Dehydration occurs when the body loses more fluids than it takes in. This imbalance often results from inadequate water intake, excessive sweating, vomiting, diarrhea, or the use of diuretic medications. Infants, children, and adults over age 65 are at the highest risk of dehydration. (14) 

Dehydration can have profound and detrimental effects on health, affecting various physiological processes. It can hinder the efficient transport of nutrients and oxygen to cells and cause electrolyte imbalances, negatively impacting metabolic processes and cellular functioning. Inadequate hydration can impair cognitive function, causing difficulties in concentration, memory, and overall mental performance. It also compromises thermoregulation, making individuals more susceptible to heat-related illnesses like heatstroke. Dehydration may exacerbate joint issues by reducing the lubricating synovial fluid, contributing to stiffness and discomfort. Insufficient water intake can strain the kidneys and liver, impeding their detoxification functions and potentially leading to the accumulation of waste products and toxins in the body. Dehydrated skin is more prone to dryness and lacks the resilience needed to maintain a healthy protective barrier, potentially increasing the risk of dermatological issues. Severe dehydration can lead to shock, coma, and even death. (3, 14)

Given the potentially severe health consequences, it's important to recognize the signs and symptoms of dehydration. These symptoms may occur with as little as a 2% water deficit: 

• Thirst
• Dry mouth, skin, and lips
• Fatigue
• Flushed skin
• Decreased urine volume or dark-colored urine
• Headaches
• Confusion
• Irritability
• Fast, racing heartbeat
• Low blood pressure
• Muscle cramps
• Dizziness, especially when changing positions
• Loss of appetite
• Constipation (3, 14)

Hydration and Chronic Disease Prevention

The National Institutes of Health conducted a study involving 11,255 adults over 30 years, examining the relationship between serum sodium levels, hydration, and health outcomes. The research indicates that well-hydrated adults tend to be healthier, with fewer chronic conditions, and may live longer than those with insufficient fluid intake. Elevated serum sodium levels, even within the normal range, during middle age were associated with accelerated biological aging, an increased risk of chronic diseases, and premature mortality. Specifically, adults with serum sodium levels above 142 mEq/L had up to a 64% increased risk of heart failure, stroke, atrial fibrillation, peripheral artery disease, chronic lung disease, diabetes, and dementia. Adults with serum sodium levels between 138-140 mEq/L had the lowest risk of developing chronic disease. These findings underscore the importance of proper hydration in slowing aging, preventing chronic diseases, and promoting a disease-free life. (16) 

Adequate hydration exerts a profound influence on urinary system health. By promoting regular urine flow, hydration aids in flushing bacteria from the urinary tract. Studies consistently reveal that individuals maintaining proper hydration levels are less prone to urinary tract infections (UTIs) than those with lower fluid intake. Concerning kidney stones, hydration contributes significantly to prevention by maintaining higher urine volumes, thereby reducing the concentration of stone-forming substances and impeding their crystallization. Authors of a 2015 meta-analysis concluded that increased water intake is associated with a reduced risk of developing kidney stones. According to this analysis, consuming 2,000 mL and 3,100 mL of water daily is associated with a kidney stone risk reduction of 8% and 26%, respectively, compared to a daily water intake of 1,500 mL.

Water has been shown to be a protective factor in various types of cancer. Studies consistently suggest maintaining proper hydration is linked to a lower risk of colorectal cancer (37-39). Adequate water intake may contribute to regular bowel movements and prevent constipation, reducing the contact time between potential carcinogens and the colon lining (2). Data are more inconsistent for other types of cancer, but some studies do suggest that adequate fluid intake may be associated with a risk reduction for breast and bladder cancer.

Optimal Water Intake: How Much Is Enough?

The U.S. National Academies of Sciences, Engineering, and Medicine have determined an adequate daily fluid intake is approximately 2.7 liters (91 ounces) and 3.7 liters (125 ounces) for healthy women and men, respectively. About 80% of people's total water comes from drinking water and other beverages, and 20% is from food. For example, spinach and watermelon are almost 100% water by weight. (31) 

The National Academy of Medicine has established the following recommendations for daily water intake based on age group: 

As a general rule of thumb, your fluid intake is probably adequate if you rarely feel thirsty and your urine is colorless or light yellow (46). Drinking between 0.5-1 ounce of water for each pound you weigh daily is commonly recommended. For example, the average person who weighs 150 pounds should aim to drink between 75 and 150 ounces of water daily. (1) 

Myths and Misconceptions about Hydration

Let's explore some common misconceptions about drinking water.

"Drink 8 Glasses of Water a Day."

You've probably heard the general advice of drinking eight glasses of water daily to stay hydrated. However, this goal may not be reasonable for everyone. Water needs will vary based on age, sex, and pregnancy/breastfeeding status. (44)

Other factors to consider when considering adequate water consumption include environmental factors, physical activity levels, and illness. Individuals who live in hot climates, are physically active, or are sick with fever, vomiting, or diarrhea will have higher fluid demands to replace lost water through sweat, vomit, and diarrhea. (24, 43) 

"I Can't Drink Too Much Water"

While it is rare in healthy people, overhydration can occur when people drink water in volumes that exceed their body's needs and the kidneys' capacity to get rid of it. When this happens, sodium becomes diluted in the blood. This is called hyponatremia, and it can be life-threatening. Athletes who drink excessive water to avoid dehydration or those with underlying medical conditions are at higher risk of overhydration. For example, overhydration can occur in people with reduced kidney function, psychogenic polydipsia, or syndrome of inappropriate antidiuretic hormone secretion (SIADH). (28) 

Hydration in Special Populations

Children have higher water requirements relative to their body mass compared to adults. Adequate hydration status in children positively affects their physical and cognitive growth and development. Research also suggests that learning healthy drinking habits from infants drives sustained healthy dietary patterns into adulthood. (5) Young children are at a higher risk of dehydration, given the inability to recognize the need to replace lost fluids and to communicate that they are thirsty (14, 33). Children are also more susceptible to severe dehydration-related consequences compared to adults (22). Dehydration is a major cause of morbidity and mortality in infants and young children worldwide. The  American Academy of Pediatrics (AAP) recommends that children meet their daily fluid intake requirements through healthy beverages, including breastmilk, water, pasteurized milk, and 100% fruit juice.

Older adults have a diminished sense of thirst, making it challenging to recognize when they need fluids. Additionally, they don't carry as much water in their bodies and may have comorbid chronic diseases (e.g., diabetes) that further increase the risk of dehydration. (3, 14) Older adults should be encouraged to drink small amounts of water throughout the day to prevent dehydration (36).

Pregnant and breastfeeding women have increased fluid needs to produce amniotic fluid, extra blood supply, and milk. In addition to potentially dangerous consequences for the mother, dehydration can also impede the normal and healthy growth of the baby. (30)

Hydration and Athletic Performance

Hydration is of paramount importance for athletes, influencing their performance, endurance, and overall well-being during physical activities. Intense exercise leads to losing fluids and important electrolytes (like sodium and potassium) through sweat, necessitating proactive replenishment to avoid dehydration. Given the increased risk of electrolyte imbalance, athletes are encouraged to replace electrolytes with supplements, electrolyte-rich foods, and/or sports drinks. (14)

Drinking water before, during, and after exercise is essential for a comprehensive hydration strategy. The amount of fluid needed to stay hydrated will vary based on how much you sweat, the climate, and exercise duration/intensity. The American Council on Exercise has proposed these general guidelines to follow (19): 

• Pre-Exercise: 17-20 ounces of water a few hours before and then 8 ounces of water during warm-up
• During Exercise: 4-8 ounces of fluid every 15-20 minutes during exercise
• Post-Exercise: 3 cups of water for every pound lost during a workout

The Quality of Water and Hydration

Water quality influences its taste and safety. Contaminants in water, such as microbial pathogens, chemicals, and pollutants, can pose health risks if consumed. Therefore, ensuring access to clean and safe water is essential for optimal hydration and overall health. (17 ) 

In the United States, the Environmental Protection Agency (EPA) sets standards and regulations for the presence and levels of over 90 contaminants in public drinking water. Private wells are not regulated by the EPA. Despite these regulations, water quality can vary based on location. (18, 45)  

The Food and Drug Administration (FDA) regulates the safety of bottled water. While reports of contamination are rare, they do occur. There have been reports of bottled waters testing positive for contaminants like heavy metals and per- and polyfluoroalkyl substances (PFAs). The bottle's plastic can also leach toxic bisphenols, phthalates, and microplastics into the water. (27) 

Filtered water, obtained through various methods such as carbon filters or reverse osmosis, offers individuals control over the quality and taste of their water. Filtration can effectively remove impurities, chlorine, and certain minerals, providing purified and often better-tasting drinking water. Different types of water filters will perform different functions. To determine the most suitable type for your needs, consider factors such as the specific impurities present in your water and the desired filtration level. (6) 

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How Water Intake Affects Our Health: Final Thoughts

Hydration plays a pivotal role in safeguarding overall health and preventing diseases. Adequate water intake is essential for the proper functioning of bodily systems, from supporting cellular processes to regulating temperature and facilitating nutrient transport. Understanding personal hydration needs is crucial, as individual requirements vary based on age, activity level, and environmental conditions. Making informed choices about water intake involves recognizing the signs of dehydration, listening to the body's thirst cues, and considering the quality of water consumed.