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This lesson covers the role of water in the body and the importance of adequate hydration, as required by AQA GCSE Food Preparation and Nutrition (8585), specification section 3.2.2. Water is the most abundant substance in the human body, making up approximately 60% of adult body weight. Although it provides no energy (calories), it is essential for virtually every bodily function. Understanding the functions of water, daily requirements, and factors affecting hydration is important for the exam.
Water is involved in a remarkable range of bodily processes. It is not a nutrient in the traditional sense (it is not a vitamin, mineral, protein, fat, or carbohydrate), but it is classified as an essential dietary component because the body cannot function without it.
| Function | Detail |
|---|---|
| Solvent | Water dissolves more substances than any other liquid. It acts as the universal solvent in the body, allowing nutrients, minerals, glucose, and other substances to dissolve and be transported in the blood and other body fluids. |
| Transport | Water is the main component of blood plasma (approximately 90% of blood plasma is water). It transports nutrients, oxygen, hormones, and antibodies to cells, and carries waste products to the kidneys and lungs for removal. |
| Waste elimination | Water is essential for the kidneys to filter waste products from the blood and produce urine. Adequate water intake helps flush out toxins and waste. It also helps prevent constipation by softening stools. |
| Temperature regulation (thermoregulation) | When the body overheats, water is lost through sweating. As sweat evaporates from the skin surface, it removes heat, cooling the body down. This is the primary mechanism for temperature regulation during exercise or in hot weather. |
| Digestion | Water is a key component of digestive juices (saliva, gastric juice, bile, pancreatic juice, intestinal juice). It helps break down food physically and chemically and is needed for the absorption of nutrients. |
| Lubrication | Water is a major component of synovial fluid (lubricates joints), saliva (lubricates the mouth and aids swallowing), mucus (lubricates and protects internal surfaces), and tears (lubricate and protect the eyes). |
| Chemical reactions | Many metabolic reactions in the body occur in aqueous (water-based) solution. Water is a reactant in hydrolysis reactions (breaking down large molecules into smaller ones, e.g., digestion of starch into glucose). |
| Cell structure | Water gives cells their shape and structure. The cytoplasm of cells is largely water. Without adequate water, cells shrink and cannot function properly. |
| Cushioning and protection | Water in cerebrospinal fluid cushions the brain and spinal cord. Amniotic fluid protects the developing foetus during pregnancy. |
graph TD
A["Functions of Water"] --> B["Transport<br/>(blood, nutrients)"]
A --> C["Waste Elimination<br/>(urine, kidneys)"]
A --> D["Temperature<br/>Regulation<br/>(sweating)"]
A --> E["Digestion<br/>(digestive juices)"]
A --> F["Lubrication<br/>(joints, saliva)"]
A --> G["Chemical<br/>Reactions"]
A --> H["Cell Structure<br/>(cytoplasm)"]
style A fill:#2980b9,color:#fff
style B fill:#3498db,color:#fff
style C fill:#3498db,color:#fff
style D fill:#3498db,color:#fff
style E fill:#3498db,color:#fff
style F fill:#3498db,color:#fff
style G fill:#3498db,color:#fff
style H fill:#3498db,color:#fff
The UK Eatwell Guide recommends that adults drink 6 to 8 glasses of fluid per day (approximately 1.5 to 2 litres). This includes water and other drinks such as tea, coffee, milk, and fruit juice. Food also contributes to water intake — many fruits and vegetables have a high water content.
| Source | Contribution |
|---|---|
| Drinks | Water, tea, coffee, milk, fruit juice, squash — the main source of fluid intake (approximately 70–80% of total water intake) |
| Food | Many foods contain significant amounts of water: fruit (oranges ~87%, watermelon ~92%), vegetables (cucumber ~96%, lettuce ~95%), soups, yoghurt, rice, pasta (when cooked) |
| Metabolic water | A small amount of water is produced as a by-product of metabolism — when the body breaks down macronutrients for energy, water molecules are released. This contributes a small but constant supply. |
| Food | Approximate Water Content |
|---|---|
| Cucumber | 96% |
| Lettuce | 95% |
| Tomatoes | 94% |
| Watermelon | 92% |
| Strawberries | 91% |
| Oranges | 87% |
| Milk | 87% |
| Yoghurt | 80% |
| Cooked rice | 70% |
| Banana | 74% |
| Bread | 36% |
| Cheese | 37% |
| Biscuits | 3% |
Exam Tip: If asked how someone can increase their water intake, include both drinks (water, herbal tea, milk) and food (fruit, vegetables, soups). AQA examiners award marks for recognising that food contributes to hydration.
The body constantly loses water through several routes. These losses must be replaced to maintain hydration:
| Route | Detail |
|---|---|
| Urine | The kidneys produce urine to remove waste products. This is the largest route of water loss — approximately 1,500 ml/day on average, depending on fluid intake. |
| Sweat | Water is lost through the skin as part of temperature regulation. Sweat loss varies enormously — from a few hundred ml on a cool, inactive day to several litres during intense exercise or in hot conditions. |
| Breathing (exhaled air) | Water vapour is lost with every breath. You can see this on a cold day when your breath condenses. Approximately 400 ml/day is lost this way. |
| Faeces | A small amount of water is lost in stools. This increases significantly during diarrhoea. |
| Insensible losses | Small amounts of water are constantly lost through the skin by evaporation (not sweating) — this is called insensible perspiration. |
flowchart LR
A["Water In"] --> B["Body Water<br/>(~60% body weight)"]
B --> C["Water Out"]
A --- D["Drinks<br/>Food<br/>Metabolic water"]
C --- E["Urine<br/>Sweat<br/>Breathing<br/>Faeces"]
style B fill:#2980b9,color:#fff
style D fill:#27ae60,color:#fff
style E fill:#e74c3c,color:#fff
Certain conditions increase water loss or demand, meaning more fluid must be consumed:
| Condition | Why More Water Is Needed |
|---|---|
| Physical exercise | Sweating increases significantly during exercise. Athletes can lose 1–2 litres of sweat per hour during intense activity. Water must be replaced to prevent dehydration and maintain performance. |
| Hot weather | The body sweats more to cool down, increasing water loss. |
| Illness (fever, vomiting, diarrhoea) | Fever increases metabolic rate and sweating. Vomiting and diarrhoea cause rapid and significant fluid loss. Dehydration is a serious risk during illness, especially in young children and older adults. |
| Pregnancy and breastfeeding | Blood volume increases during pregnancy. Breastfeeding women lose additional fluid through breast milk (approximately 700–800 ml/day). |
| High altitudes | Breathing rate increases at altitude, leading to greater water loss through exhaled air. |
| Dry or air-conditioned environments | Low humidity increases water loss through the skin and breathing. |
| High-protein or high-fibre diets | Protein metabolism produces more waste products that require water for excretion. Fibre absorbs water in the gut. |
Dehydration occurs when the body loses more water than it takes in. Even mild dehydration can impair physical and mental performance.
| Severity | Signs and Symptoms |
|---|---|
| Mild | Thirst, dark yellow urine (concentrated), dry mouth, headache, reduced concentration |
| Moderate | Dizziness, fatigue, very dark urine, reduced urine output, dry skin, rapid heartbeat |
| Severe | Confusion, very rapid heartbeat, low blood pressure, sunken eyes, fainting. Severe dehydration is a medical emergency and can be fatal if untreated. |
One of the simplest ways to monitor hydration is urine colour:
| Urine Colour | Hydration Status |
|---|---|
| Pale straw / light yellow | Well hydrated |
| Dark yellow | Mild dehydration — drink more fluids |
| Amber / honey | Moderate dehydration — increase fluid intake promptly |
| Brown / very dark | Severe dehydration — seek medical attention |
Exam Tip: Mentioning urine colour as a practical indicator of hydration status shows applied knowledge, which AQA rewards. It is a simple, real-world monitoring tool.
Although rare, it is possible to drink too much water. This condition is called hyponatraemia (low blood sodium), and it occurs when excessive water intake dilutes the sodium in the blood to dangerously low levels.
The Eatwell Guide recommends 6–8 glasses of fluid per day and includes guidance on the types of drinks:
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