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Fitness

The Importance of Hydration

The Importance of Hydration

This article is for educational purposes only and does not replace the advice of a qualified healthcare professional.

Water is so fundamental to human life that it is easy to take for granted. It does not have the novelty of a trending supplement. It is just water. And yet adequate hydration underpins virtually every physiological process in the body, and chronic mild dehydration is far more common than most people realise, quietly affecting energy, cognition, mood, digestion and physical performance.

What Water Does in the Body

The human body is approximately 60% water, though this varies by age, sex and body composition ranging from 75% in infants to 55% in the elderly.

Water transports nutrients and oxygen to cells and carries waste away, regulates body temperature, cushions joints and organs, supports kidney function, maintains blood volume and blood pressure, and is essential for digestion from saliva production to the movement of food through the intestines. 

It facilitates every chemical reaction in the body, including energy production at the cellular level.

The brain is particularly sensitive to hydration status, being approximately 75% water, and even small deficits have measurable effects on cognitive function, mood, and the subjective experience of energy.

How Much Do You Actually Need?

The "eight glasses a day" rule is one of the most persistent health myths in circulation. Actual fluid needs depend on your body size, activity level, environment, dietary patterns, health status, and whether you are pregnant or breastfeeding.

The European Food Safety Authority recommends a total water intake, from both food and drinks, of around 2 litres per day for women and 2.5 litres for men under average conditions. These are population-level reference values, not prescriptions: someone who is physically active or sweats heavily will need considerably more. 

Roughly 20 to 30% of daily fluid intake typically comes from food, particularly fruits and vegetables, so the amount you need to drink is less than total intake figures suggest. The most practical way to assess your hydration is urine colour.

Pale straw yellow indicates good hydration, while dark yellow or amber suggests you need more fluid, though this may require medical attention in some cases. Completely clear urine can actually indicate overhydration and unnecessary flushing of electrolytes.

What Happens When You Are Dehydrated

Dehydration occurs on a spectrum, and the effects begin well before you feel thirsty. Thirst is a relatively late signal: by the time you notice it, you may already be mildly dehydrated.

Research published in the Journal of Nutrition found that mild dehydration of just 1.36% of body weight in women caused measurable impairment in mood, concentration and the perception of task difficulty, with a separate study finding similar effects in men at around 1.59%, including increased fatigue and working memory impairment. These are levels that occur easily during a normal day.

At mild levels you might notice fatigue, difficulty concentrating, headaches, dry mouth, reduced urine output, and constipation. As it becomes more significant, physical performance declines, heart rate increases, and temperature regulation suffers. Severe dehydration is a medical emergency, but it is the low-level, chronic, barely-noticed kind that affects the most people daily.

Hydration and the Brain

The cognitive effects of dehydration are frequently mistaken for other problems. That mid-afternoon brain fog, the headache that arrives by 3pm, the difficulty staying focused in a long meeting: these are commonly attributed to stress or poor sleep, when hydration may be a significant contributing factor.

Mild dehydration can impair short-term memory and increase anxiety and fatigue, and research in older adults links chronic low fluid intake with increased risk of delirium, cognitive impairment, and urinary tract infections, a major driver of hospital admissions in elderly people.

Consistent fluid intake throughout the day, rather than large amounts drunk sporadically, appears most effective for maintaining cognitive performance.

Hydration and Physical Performance

Even mild dehydration reduces endurance, increases perceived effort, impairs coordination and slows reaction time. A loss of just 2% of body weight through sweat can reduce aerobic performance by up to 10 to 20%, according to research from the American College of Sports Medicine.

Sports drinks become relevant during exercise lasting more than 60 to 90 minutes. Sweat contains not just water but electrolytes, mainly sodium and chloride, and these need replacing alongside the fluid itself. Longer sessions also benefit from the carbohydrate these drinks provide.

For everyday exercise under an hour, water is sufficient, and drinking to thirst is the most practical approach. Hot conditions and heavier sweating can shift that threshold earlier.

Hydration and Digestion

Water is essential for every stage of digestion. In the large intestine, water is reabsorbed from digested material, and when intake is insufficient the colon absorbs more water from the stool to compensate, resulting in dry, hard stool that is difficult to pass.

This is one of the most common and correctable drivers of constipation, and increasing fluid alongside dietary fibre is essential, as fibre requires water to function.

Adequate hydration also supports kidney function and reduces the risk of kidney stones, which are associated with chronic low fluid intake.

What Counts Toward Your Fluid Intake?

Water does most of the work, though almost everything you drink contributes. Milk is a good example, shown in research to be retained better than water thanks to its protein, lactose and electrolyte content. Juices, teas, soups and smoothies all count too, though whole fruits and vegetables are preferable for their fibre.

Alcohol is the exception as it actively promotes fluid loss by suppressing antidiuretic hormone, thought to be one of the reasons for the headache and fatigue of a hangover.

Individual Factors That Increase Fluid Needs

Pregnancy increases fluid needs due to increased blood volume and the demands of fetal development, and breastfeeding requires an additional 500 to 700ml per day. Hot or humid weather significantly increases losses, air travel is dehydrating due to low cabin humidity, illness involving fever, vomiting or diarrhoea causes rapid fluid and electrolyte loss, and certain medications, including diuretics, increase losses too.

Older adults are at particular risk because the sensation of thirst diminishes with age, kidney function declines, and many deliberately reduce fluid intake to manage urinary urgency, which makes proactive, scheduled fluid intake more important as we age.

Practical Ways to Stay Well Hydrated

Rather than counting glasses, build hydration into the structure of your day.

  • Start the morning with a glass of water: you wake mildly dehydrated after hours without fluid, and replenishing early sets a good foundation.
  • Keep a water bottle visible at your desk, drink a glass before each meal, and eat plenty of fruits and vegetables.
  • If plain water is unappealing, sliced fruit, cucumber or mint make it more enjoyable, and herbal teas also contribute to hydration levels.
  • Check your urine colour periodically, particularly in warm weather or during increased activity.

The Bottom Line

Hydration is not complicated, but it is foundational. Adequate fluid intake supports energy, cognition, mood, digestion, physical performance and kidney health, and chronic mild dehydration is common, easily corrected, and frequently at the root of symptoms attributed to other causes.

You do not need a fancy app, a gallon jug, or a rigid schedule. You need consistent, attentive habits and a diet rich in water-rich whole foods.

References: 

https://academic.oup.com/nutritionreviews/article/68/8/439/1841926
https://www.nature.com/articles/ejcn2009111
https://www.sciencedirect.com/science/article/pii/S0022316622028899?via%3Dihub
https://www.bda.uk.com/resource/fluid-water-drinks.html
https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/mild-dehydration-impairs-cognitive-performance-and-mood-of-men/3388AB36B8DF73E844C9AD19271A75BF
https://usecadence.com/blogs/science/how-dehydration-affects-physical-cognitive-performance
https://www.nhs.uk/conditions/kidney-stones/prevention/
https://pmc.ncbi.nlm.nih.gov/articles/PMC11783823/