A Balanced Diet and Nutrition: KS3 Biology Explained

A balanced diet provides all the nutrients the body needs, in the right proportions, to maintain health, support growth, and fuel everyday activity. There are seven nutrient groups — carbohydrates, proteins, fats, vitamins, minerals, fibre, and water — each with a distinct role, and a deficiency in any of them leads to specific health problems.

What are the seven nutrient groups?

Nutrient | Main role | Good food sources
Carbohydrates | Primary energy source | Bread, rice, pasta, potatoes, fruit
Proteins | Growth and repair of cells and tissues | Meat, fish, eggs, beans, lentils
Fats (lipids) | Long-term energy store; insulation; cell membranes; fat-soluble vitamins | Butter, oils, nuts, oily fish
Vitamins | Regulate chemical reactions; support immune system, vision, blood clotting | Fruits, vegetables, dairy
Minerals | Structural (bones, teeth) and regulatory roles | Dairy (calcium), red meat (iron), bananas (potassium)
Fibre (dietary) | Prevents constipation; feeds gut bacteria; reduces bowel cancer risk | Wholegrain bread, vegetables, pulses
Water | Transport medium for all reactions; temperature regulation; waste removal | Water, all food and drinks

Carbohydrates, proteins, and fats are the three macronutrients — needed in large amounts. Vitamins and minerals are micronutrients — needed in very small amounts but equally essential.

What happens if you eat too little of a nutrient?

A deficiency disease is a condition caused by an insufficient supply of a specific nutrient. The body cannot function properly without it.

Nutrient lacking | Deficiency disease / condition | Symptoms
Vitamin C | Scurvy | Bleeding gums, slow wound healing, fatigue
Vitamin D | Rickets (children) / osteomalacia (adults) | Soft, deformed bones; bone pain
Iron | Anaemia | Fatigue, pale skin, breathlessness
Calcium | Weak bones and teeth; muscle cramps | Bone fractures, poor dental health
Iodine | Goitre (enlarged thyroid gland) | Swollen neck, fatigue, impaired brain development

Vitamin C is needed to make collagen — the structural protein that holds tissue together. Without it, blood vessels and skin break down. Sailors on long voyages in the 16th–18th centuries frequently died of scurvy because they had no access to fresh fruit. The link to citrus fruit was observed and then confirmed by the Scottish physician James Lind in 1747.

How much energy do you need and where does it come from?

Your body's energy comes mainly from carbohydrates and fats, with proteins used as a last resort. The energy content of food is measured in kilojoules (kJ) or kilocalories (kcal):

1 g of carbohydrate provides approximately 17 kJ (4 kcal)

1 g of fat provides approximately 37 kJ (9 kcal) — over twice as much

1 g of protein provides approximately 17 kJ (4 kcal)

Fat is a much more concentrated energy store than carbohydrate, which is why it is the body's long-term energy reserve. Carbohydrates are quicker to break down and are used for immediate energy demand.

The amount of energy you need depends on your age, size, sex, and activity level. A teenage boy who plays sport daily needs significantly more energy than a sedentary adult. In the UK, the average daily energy requirement is roughly 8,400 kJ (2,000 kcal) for women and 10,500 kJ (2,500 kcal) for men, but these are only guidelines — individual needs vary widely.

What is the Eatwell Guide?

The Eatwell Guide, produced by NHS England and Public Health England, shows the proportions of each food group a person should eat as part of a healthy balanced diet:

Fruit and vegetables: should make up just over a third of your diet — aim for at least 5 portions per day.

Starchy carbohydrates (bread, rice, pasta, potatoes): should make up just over a third — choose wholegrain where possible.

Proteins (beans, pulses, fish, eggs, meat): roughly 12–15% of the diet.

Dairy or dairy alternatives: smaller portion for calcium.

Oils and fats: use in small amounts; choose unsaturated oils.

Foods high in sugar and fat: not shown as a food group — eat as rarely as possible.

This proportional model replaces the old "food pyramid" and reflects current nutritional science more accurately.

What is the difference between eating too little and too much?

Both extremes cause health problems:

Undernutrition (not enough food overall, or missing specific nutrients):

Stunted growth in children

Reduced immune function

Muscle wastage (the body breaks down protein from muscles for energy)

Specific deficiency diseases (see above)

Overnutrition (eating too many calories overall):

Excess energy stored as fat → obesity

Type 2 diabetes (cells become resistant to insulin, unable to control blood glucose)

Cardiovascular disease (fatty deposits narrow arteries)

High blood pressure

A truly balanced diet is neither too little nor too much of any nutrient — it is sized to match actual energy needs and varied enough to cover all micronutrients.

Why is protein especially important for growing teenagers?

Protein is made up of amino acids — the building blocks used to construct new cells, enzymes, hormones, and structural tissues. Growing teenagers need relatively more protein per kilogram of body mass than adults because they are building new tissue rapidly.

The body can make some amino acids itself (non-essential amino acids), but eight to ten essential amino acids must come from food. Animal proteins (meat, fish, eggs, dairy) contain all essential amino acids in the right proportions — they are called complete proteins. Most plant proteins (rice, beans, wheat) are incomplete — they lack one or more essential amino acids. Vegetarians and vegans can meet their needs by combining different plant proteins (e.g. rice and beans together provide all essential amino acids).

Frequently asked questions

What are the seven nutrients needed for a balanced diet?

The seven nutrients are: carbohydrates (for energy), proteins (for growth and repair), fats (for long-term energy and cell membranes), vitamins (to regulate reactions and support immunity), minerals (for structure and regulation), dietary fibre (for a healthy digestive system), and water (for all body reactions and transport). A balanced diet includes all seven in the right amounts.

What is a deficiency disease?

A deficiency disease is caused by a long-term lack of a specific nutrient. For example, too little vitamin C causes scurvy (bleeding gums, slow wound healing), too little iron causes anaemia (fatigue and pale skin), and too little vitamin D causes rickets in children (soft, deformed bones). Deficiency diseases can be prevented or treated by restoring the missing nutrient.

Why do we need both carbohydrates and fats if both provide energy?

Carbohydrates are broken down quickly into glucose and used for immediate energy — they are the body's preferred fuel for short-term demands. Fats provide more than twice the energy per gram and are stored more efficiently for long-term reserves. The body also needs fats to build cell membranes, absorb fat-soluble vitamins (A, D, E, K), and insulate nerves. Both are therefore necessary.

How much water does the body need each day?

The NHS recommends around 6–8 cups (approximately 1.5–2 litres) of fluid per day for adults, and more in hot weather or during exercise. Water makes up about 60% of the human body and is essential for every chemical reaction in cells, transporting nutrients and waste in the blood, regulating body temperature through sweat, and lubricating joints.

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