Water and Hydration

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.

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Why Is Water Essential?

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.

Functions of Water in the Body

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

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How Much Water Do We Need?

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.

Sources of Water

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.

Water Content of Common Foods

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.

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Water Loss from the Body

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

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When Is Extra Water Needed?