One surprising question: What has saved more human lives than any vitamin, superfood or dietary supplement ever could?

The answer isn't hidden in an exotic berry or a miracle molecule.

It's clean drinking water.

The provision of safe drinking water is widely regarded as one of the greatest public health achievements in human history. Together with sanitation and vaccination, it has dramatically reduced the burden of infectious diseases such as cholera, typhoid fever and dysentery, contributing to one of the greatest increases in life expectancy ever recorded.

For most of us, however, safe drinking water has become so commonplace that we rarely stop to think about it. We simply turn on the tap, fill a glass and continue with our day.

Yet every sip begins an extraordinary physiological journey.

Water is not merely something that quenches thirst. It is the most abundant nutrient in the human body and the medium in which virtually every biological process takes place.

Without food, humans can survive for several weeks.

Without water, survival is usually measured in days.

Few nutrients are as ordinary—and as indispensable—as the one flowing from your tap.

The science

Water: much more than H₂O

At first glance, water appears to be one of the simplest molecules in nature: two hydrogen atoms bound to one oxygen atom. Its simplicity, however, is deceptive.

The polarity of the water molecule allows it to form hydrogen bonds with neighbouring water molecules. These hydrogen bonds are responsible for many of water's remarkable properties, including its ability to dissolve countless biological compounds, transport heat efficiently and maintain the three-dimensional structure of proteins and other biomolecules.

In fact, without these unique physicochemical properties, life as we know it would simply not exist. Water is therefore far more than a passive solvent. It is an active participant in human physiology.

In healthy adults, total body water represents approximately 50–65% of body weight, although this varies according to age, sex and body composition. Lean tissues such as skeletal muscle contain around 75% water, whereas adipose tissue contains only 10–20%. This explains why body water gradually decreases with increasing body fat and ageing.

Body water is carefully distributed between two major compartments. Approximately two-thirds is located inside the cells (intracellular fluid), while one-third resides outside the cells (extracellular fluid), including blood plasma and the interstitial fluid bathing every cell. Maintaining this delicate balance is one of the body's highest physiological priorities. Even relatively small disturbances in water balance can influence cellular function, cardiovascular performance and thermoregulation.

Water: the body's universal transport network

Imagine trying to run a country without roads, railways or rivers. Even if factories continued producing goods, nothing would ever reach its destination. Water plays exactly that role inside the human body.

Blood plasma consists of approximately 90–92% water and serves as the principal transport medium for an astonishing variety of substances. Every minute, nearly five litres of blood circulate through the body, delivering oxygen, glucose, amino acids, vitamins, minerals and hormones to billions of cells. At the same time, blood transports antibodies, immune cells and signalling molecules that coordinate communication between organs.

The journey back is equally important. Carbon dioxide produced by cellular respiration is transported to the lungs, while metabolic waste products such as urea, creatinine and uric acid are carried towards the kidneys for elimination. Even molecules that do not readily dissolve in water, such as cholesterol and triglycerides, ultimately depend on water-based transport systems. They are packaged into specialised lipoproteins that allow them to travel through the bloodstream.

Water is therefore much more than a liquid. It is the highway upon which every nutrient, hormone and immune cell travels.

Water: the body's natural climate control

One of water's most remarkable characteristics is its exceptionally high specific heat capacity. In simple terms, water can absorb and release large quantities of heat with only minimal changes in its own temperature. This property is fundamental to human survival.

Despite fluctuations in environmental temperature, healthy humans maintain a remarkably stable core temperature of approximately 37°C. This stability allows enzymes, hormones and metabolic pathways to function within their optimal physiological range.

When muscles generate heat during exercise, warm blood transports this excess heat towards the skin. Sweat glands then release water onto the skin surface. As this water evaporates, it removes considerable amounts of thermal energy from the body, effectively cooling us down.

Conversely, in colder environments, circulating water distributes heat generated by metabolically active tissues throughout the body, helping preserve core temperature.

Without water, humans would be unable to maintain thermal homeostasis and even moderate physical activity would rapidly become life-threatening.

Water: the architecture of life

Water is often viewed as something surrounding our cells. In reality, it forms an essential part of the cells themselves. Approximate water content of different tissues illustrates this beautifully:

Tissue	Water content
Blood plasma	90–92%
Brain	73–75%
Skeletal muscle	75%
Liver	70%
Bone	20–30%
Adipose tissue	10–20%

Water maintains cell volume through osmotic regulation, preserves tissue elasticity and provides the medium through which nutrients and signalling molecules diffuse. Without sufficient intracellular water, proteins lose their optimal environment, membranes function less efficiently and cellular metabolism begins to deteriorate. Water is therefore not simply present within tissues.

It is one of their principal structural components.

Water: the medium of metabolism

Every second, trillions of biochemical reactions occur inside the human body. Virtually all of them take place in an aqueous environment. Water is frequently described as a solvent, but this understates its importance.

It also participates directly in metabolism.

For example, digestion depends heavily on hydrolysis reactions, in which water molecules are used to break chemical bonds within carbohydrates, proteins and lipids. Without water, digestive enzymes would simply be unable to release nutrients from food.

Water also contributes to:

• ATP production and cellular energy metabolism;
• acid-base regulation;
• enzyme activation;
• intracellular signalling;
• maintenance of osmotic equilibrium;
• transport across biological membranes.

Human physiology is, quite literally, water chemistry.

Water supports immunity and waste removal

Every day, your immune system patrols billions of cells in search of invading microorganisms. This remarkable surveillance system depends almost entirely on water:

• White blood cells circulate continuously through blood and lymphatic vessels.
• Antibodies travel towards sites of infection.
• Inflammatory mediators coordinate communication between tissues.

Without adequate water, this transport network becomes less efficient.

Water also plays a central role in waste removal. Each day, the kidneys receive approximately 20% of the heart's resting cardiac output. Together, they filter around 180 litres of plasma every 24 hours. More than 99% of this filtered water is reabsorbed, leaving approximately 1.5 to 2 litres of urine containing urea, uric acid, creatinine and excess electrolytes. Contrary to popular belief, drinking excessive amounts of water does not "flush toxins" from the body. Healthy kidneys already regulate water balance with extraordinary precision. Adequate hydration supports this process. Excessive hydration does not enhance it.

What does this mean for you?

How much water do we actually need?

One of the most persistent nutritional myths is that everyone should drink exactly eight glasses of water every day. Human physiology is far more sophisticated than that. Water requirements vary considerably according to age, body size, physical activity, climate, dietary composition and overall health.

The European Food Safety Authority (EFSA) therefore recommends adequate Intakes rather than fixed requirements.

For healthy adults these are approximately:

• Women: 2.0 litres of total water per day
• Men: 2.5 litres of total water per day

Importantly, these recommendations refer to total daily water intake, not merely the water we drink.

On average, around 20–30% of our daily water intake already comes from food. Fruits, vegetables, soups, yoghurt and many other foods contribute substantially to our hydration status. Consequently, someone consuming a diet rich in fresh produce generally needs less drinking water than someone whose diet consists predominantly of dry, processed foods.

Likewise, water requirements increase substantially during hot weather, prolonged physical activity, fever, pregnancy, breastfeeding and gastrointestinal illnesses.

Diet composition also matters. Protein metabolism generates nitrogen-containing waste products that must be converted into urea before being excreted by the kidneys. Individuals consuming high-protein diets therefore generally require somewhat greater fluid intakes. Similarly, diets rich in sodium increase obligatory water losses because the kidneys require water to excrete excess salt while maintaining electrolyte balance.

Hydration is therefore not determined by a single universal number. It is a dynamic physiological process that continuously adapts to our lifestyle and environment.

Does the quality of water matter?

From the perspective of hydration, virtually all potable water hydrates equally well. From a nutritional perspective, however, not all water is identical.

Natural mineral waters vary considerably in their concentrations of calcium, magnesium, sodium, bicarbonate, sulphate and other naturally occurring minerals. Although these minerals are present in much smaller quantities than in food, regular consumption over many years can contribute meaningfully to total nutrient intake.

The World Health Organization (WHO) has even recognised that drinking water should not merely be considered a source of hydration, but may also contribute to the intake of essential minerals, particularly calcium and magnesium.

For most people living in countries with a safe public water supply, the first priority should always be quality and safety. Clean, microbiologically safe drinking water remains one of the greatest achievements of modern public health.

Once that criterion is fulfilled, the mineral composition becomes the next interesting aspect.

Example. Calcium and magnesium: a physiological partnership.

Among all minerals naturally present in drinking water, calcium and magnesium deserve particular attention. Calcium is best known for its role in maintaining healthy bones and teeth, but its physiological importance extends far beyond the skeleton. It is essential for muscle contraction, blood coagulation, neurotransmitter release, intracellular signalling and normal cardiac function.

Magnesium is equally indispensable. It functions as a cofactor for more than 300 enzymatic reactions and is involved in ATP production, protein synthesis, DNA repair, glucose metabolism and neuromuscular regulation. 

Interestingly, these two minerals rarely act independently. In many physiological processes they complement one another.

• Calcium promotes muscle contraction. Magnesium facilitates muscle relaxation.
• Calcium stimulates nerve excitation. Magnesium helps stabilise neuronal activity.
• Calcium initiates numerous intracellular signalling pathways. Magnesium regulates many of the enzymes involved in those same pathways.

For optimal physiological function, the body therefore depends not only on an adequate intake of each mineral individually, but also on an appropriate balance between them. Although there is no universally accepted "ideal" calcium-to-magnesium ratio, many nutrition scientists consider a dietary ratio of approximately 2:1 to 3:1 to be physiologically favourable.

Modern Western diets, however, often provide abundant calcium but relatively little magnesium due to the reduced consumption of nuts, legumes, whole grains and green leafy vegetables. Choosing a magnesium-rich mineral water may therefore represent a simple strategy to modestly improve magnesium intake, particularly for individuals whose diet is relatively poor in magnesium-rich foods.

When comparing mineral waters, don't just look at the amount of calcium. Look at magnesium too. Sometimes, what appears to be the richest water on the label is not necessarily the most balanced one.

Still or sparkling?

Sparkling water is simply water into which carbon dioxide (CO₂) has been dissolved under pressure. When dissolved, part of the carbon dioxide reacts with water to form carbonic acid:

CO₂ + H₂O ⇌ H₂CO₃

This weak acid lowers the pH of sparkling water, typically to values between 3.5 and 5.5 depending on the level of carbonation.

This often leads to one of the most persistent nutritional myths: that sparkling water "acidifies" the body. Fortunately, human physiology is far more sophisticated. Blood pH is tightly regulated between approximately 7.35 and 7.45 by several powerful buffering systems involving bicarbonate, respiration and kidney function. The small amount of carbonic acid present in sparkling water is rapidly neutralised and has no clinically meaningful effect on systemic acid-base balance in healthy individuals. Scientific evidence also shows that sparkling water hydrates just as effectively as still water.

Likewise, there is no convincing evidence that carbonated water weakens bones. Earlier concerns originated largely from studies involving cola beverages, where phosphoric acid and displacement of calcium-rich drinks—not carbonation itself—were the likely explanations. In fact, sparkling mineral waters naturally rich in calcium and magnesium may contribute positively to daily mineral intake.

Nevertheless, sparkling water is not ideal for everyone. Some individuals with gastro-oesophageal reflux disease (GERD), bloating or irritable bowel syndrome may find that still water is simply more comfortable. For healthy individuals, however, the choice between still and sparkling water is primarily one of personal preference.

Does water temperature matter?

From a hydration perspective, cold, room-temperature and warm water all contribute equally to fluid balance. Once ingested, the body rapidly adjusts the temperature of the water towards its core temperature of approximately 37°C.

Nevertheless, digestion is a carefully regulated biochemical process. The stomach functions as a sophisticated reaction chamber in which temperature, acidity and enzyme activity are continuously regulated to optimise digestion. Drinking a glass of cold water does not significantly alter core body temperature, nor has it been shown to impair digestion in healthy individuals.

However, very cold beverages temporarily cool the stomach and upper gastrointestinal tract before they are warmed to body temperature. Although this effect is rapidly compensated, many people experience ice-cold drinks as less comfortable during meals or immediately after intense exercise. From a physiological perspective, there is therefore little reason to seek out extremely cold beverages during routine daily hydration.

Room-temperature water or slightly warm water generally requires minimal physiological adjustment and is often better tolerated. The same applies to unsweetened herbal teas. Rather than chasing extremes, the body usually performs best under stable conditions.

Does timing matter?

Overall hydration throughout the day is considerably more important than following rigid drinking schedules. Nevertheless, timing can influence comfort, digestion and sleep quality.

After waking, a glass of water helps compensate for the modest fluid losses that occur overnight through breathing, perspiration and urine production.

During prolonged exercise, regular fluid replacement becomes essential to compensate for sweat losses.

Large volumes immediately before bedtime, on the other hand, may increase nocturnal urination and disturb sleep.

What about drinking during meals?

Perhaps no topic generates more debate than this one.

The stomach is not simply a storage bag. It is a highly specialised biochemical reactor. During digestion, specialised cells secrete hydrochloric acid, lowering gastric pH to approximately 1.5–3.5. This acidic environment activates digestive enzymes such as pepsin, initiates protein digestion and helps destroy many microorganisms entering with food. Drinking a moderate amount of water with a meal is perfectly compatible with healthy digestion.

However, consuming very large volumes of fluid during a meal temporarily increases gastric volume and slightly dilutes gastric contents. The stomach compensates remarkably well by secreting additional acid until the optimal pH is restored. Although this compensation occurs efficiently in healthy individuals, there is little physiological advantage to making the stomach work harder than necessary. Many people also experience greater fullness or bloating after consuming large quantities of fluid during meals.

For this reason, a practical approach is to remain well hydrated throughout the day, enjoy a moderate glass of water with meals if desired, but avoid drinking excessively while eating. Hydration is best viewed as a continuous process—not something that needs to occur all at once at the dinner table.

Five practical tips

1. Make hydration a habit.

Don't wait until you're thirsty before reaching for a drink. A simple strategy is to enjoy a glass of room-temperature or lukewarm water—or a cup of unsweetened herbal tea—every hour or two, adjusting the amount according to your activity level, climate and diet. Small, regular amounts are generally better tolerated than drinking large volumes all at once.

2. Choose quality over quantity.

Hydration is about more than volume. Whenever possible, choose clean, good-quality drinking water. If you regularly drink mineral water, occasionally compare the labels. A balanced mineral composition can make a meaningful contribution to your daily intake over time. A calcium-to-magnesium ratio of approximately 2.5 : 1 is generally considered a favourable balance within the overall diet.

3. Avoid extremes in water temperature.

Very cold water is perfectly safe, but room-temperature or lukewarm water generally requires less physiological adjustment and is often better tolerated, especially during meals. Your body works hard to maintain an internal temperature of approximately 37°C. Helping it maintain that stability is a simple habit worth adopting.

4. Drink with meals—but in moderation.

A glass of water during a meal is perfectly compatible with healthy digestion. However, avoid drinking excessive amounts while eating. Large volumes temporarily increase stomach volume and dilute gastric contents, requiring the stomach to restore its optimal acidic environment for digestion. Hydrate throughout the day, rather than all at once during meals.

5. Hydrate your body during the day.

Aim to consume most of your fluids during the morning and afternoon. Likewise, reducing your fluid intake during the final two to three hours before bedtime may help minimise night-time urination and improve sleep quality.

Scientific references

EFSA Panel on Dietetic Products, Nutrition and Allergies. (2010). Scientific Opinion on Dietary Reference Values for Water. EFSA Journal, 8(3), 1459.

World Health Organization. (2022). Guidelines for Drinking-water Quality (4th edition, incorporating the first and second addenda).

Institute of Medicine. (2005). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press.

Guyton AC, Hall JE. (2021). Guyton and Hall Textbook of Medical Physiology. 14th Edition. Elsevier.

Maughan RJ, Watson P, Cordery PA, et al. (2016). A randomized trial to assess the potential of different beverages to affect hydration status: Development of a Beverage Hydration Index. American Journal of Clinical Nutrition, 103(3), 717–723.

Rosanoff A, Dai Q, Shapses SA. (2016). Essential nutrient interactions: Does low or suboptimal magnesium status interact with vitamin D and/or calcium status? Advances in Nutrition, 7(1), 25–43.

Heaney RP. (2006). Effects of carbonated beverages on bone health. American Journal of Clinical Nutrition, 84(4), 936–939.

Next Week

We've learned that nutrients (lesson 2) and water (lesson 3) keep our cells functioning.

But every second those cells are working, they also produce molecules capable of damaging themselves.

The oxygen that keeps you alive...

is also one of the reasons you age.

Next week, we'll explore the fascinating world of free radicals and antioxidants.

YOUR BUILDING BLOCKS

Building Block	What You've Learned	In One Sentence
✅ Lesson 1	The Forgotten Question of Nutrition	We shifted our perspective from asking "What should I avoid?" to asking "What does my body actually need?" We also discovered that food is more than fuel—it provides the molecules from which every cell is built.	Good nutrition begins with understanding what the body needs, not just what it should avoid.
✅ Lesson 2	The Nutrients We Cannot Live Without	We learned that some nutrients can be made by the body, while others must come from food. We also discovered that the body has remarkable nutrient reserves, allowing some deficiencies to develop over weeks and others only after years.	Your body is an extraordinary chemical factory—but even the best factory depends on a continuous supply of essential raw materials.
✅ Lesson 3	Water: The Most Underestimated Nutrient	We discovered that water is far more than a thirst quencher. It serves as the body's transport network, temperature regulator, structural component and the medium in which virtually every biochemical reaction takes place. We also learned that hydration depends not only on how much we drink, but also on the quality, timing and mineral composition of the water we choose.	Water doesn't simply keep you hydrated—it allows every other nutrient in your body to do its job.