A Balanced Diet, Energy Stores and Lifestyle

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Food gives your body two things: energy (to move, grow, keep warm and keep every cell working) and nutrients (the raw materials to build and repair the body). Plants make their own food by photosynthesis, but animals cannot, so animals — including humans — must eat food. A balanced diet contains all the nutrients in the right amounts, and the right amount of energy for the person.

The seven parts of a balanced diet

Also needed: fibre (from vegetables, fruit, brown bread, beans) is not digested, but it keeps food moving through the gut and prevents constipation. Your objective does not list fibre, but it is part of a healthy diet and may appear in questions about meals.

What happens when a nutrient is missing (deficiency)

Link: iron and red blood cells. On the Blood page you learned that red blood cells carry oxygen. Without enough iron the body cannot make enough haemoglobin, so less oxygen reaches the cells for respiration, less energy is released and the person feels tired.

Energy: why animals eat, and how they store energy

Energy in food is measured in kilojoules (kJ). Food labels show the energy in 100 g and in one portion. Foods with a lot of fat contain the most energy for their mass.

• Animals cannot make their own food, so they consume (eat) other organisms to get energy and nutrients.
• If an animal takes in more energy than it uses, the extra is stored, mostly as fat, and the animal gains mass.
• If it takes in less energy than it uses, it uses its stores of carbohydrate and then fat, and it loses mass.
• Examples: a camel's hump is a store of fat (not water) that it can use for energy when food is scarce in the desert. Bears build up thick fat stores before winter. Seals and whales have a thick layer of fat (blubber) that is both an energy store and insulation.

Different people need different amounts of energy. A person needs more energy if they are more active, bigger, growing (teenagers), or pregnant. For example, a builder who lifts heavy loads all day needs much more energy than an office worker of the same age.

1. Method for food-label calculations: find the energy per 100 g on the label.
2. Divide by 100 to get the energy in 1 g.
3. Multiply by the mass you eat. (Shortcut: energy in portion = energy per 100 g × mass ÷ 100.)
4. To find a percentage of daily needs: energy in portion ÷ daily need × 100.

Lifestyle, growth and health

Your lifestyle is the way you live: what you eat, how active you are, whether you smoke, how much you sleep. Lifestyle choices affect how well children grow and develop, and how healthy people are now and later in life.

Smoking and health

• Smoking in pregnancy: carbon monoxide reduces the oxygen that reaches the baby through the mother's blood. The baby may grow less before birth (low birth mass) and may be born early.
• Second-hand smoke: people who breathe in other people's smoke — especially children — are also harmed (more asthma and chest infections).
• Shisha (water pipe) is not a safe choice: its smoke still contains tar, nicotine and carbon monoxide.

Exam technique — data about health: describe the pattern with numbers ("as the number of cigarettes per day increases, the death rate increases, from 10 to 230 per 100 000"). Data like this shows a link (correlation). Scientists also found how tar causes cancer, which is why we can say smoking causes lung cancer. Not every smoker gets lung cancer, and some non-smokers do — the risk is much higher for smokers.

Worked examples

Worked example 1 (functions) Complete the table. [3] Nutrient | Function calcium | ______ ______ | growth and repair of cells vitamin D | ______

• calcium → strong bones and teeth [1]
• protein → growth and repair [1]
• vitamin D → helps the body absorb calcium (for strong bones and teeth) [1]
• Common mistake: writing "vitamin D makes bones strong" — the mark scheme wants the idea of absorbing calcium.

Worked example 2 (food label) A breakfast cereal contains 1 600 kJ per 100 g. Hana eats a 40 g portion. (a) Calculate the energy in her portion. [1] (b) Hana needs 9 000 kJ each day. What percentage of her daily energy does the portion give? Give your answer to 1 decimal place. [2]

• (a) 1 600 × 40 ÷ 100 = 640 kJ
• (b) 640 ÷ 9 000 × 100 = 7.11… = 7.1% [1 for the method, 1 for the answer; ecf from (a)]
• Common mistake: using 1 600 kJ as the energy in the portion. Always check whether the label gives energy per 100 g or per portion.

Worked example 3 (energy stores) Camels can live for many days in the desert without food. Explain how the camel's hump helps. [2]

• The hump is a store of fat. [1]
• When the camel cannot find food, it uses the fat for energy (fat releases a lot of energy per gram). [1]
• Common mistake: "The hump stores water." It stores fat.

Worked example 4 (deficiency) Youssef often feels tired and is short of breath when he climbs stairs. A blood test shows his red blood cells contain too little haemoglobin. (a) Name the mineral he is probably short of. [1] (b) Suggest two foods he should eat more of. [1] (c) Explain why he feels tired. [2]

• (a) iron
• (b) any two of: red meat, liver, lentils, beans, spinach / dark green leafy vegetables
• (c) Less haemoglobin, so his blood carries less oxygen to his cells [1]; so his cells carry out less respiration and release less energy [1].
• Exam technique: in a 2-mark "explain", give the cause and the effect, linked with "so".

Worked example 5 (smoking in pregnancy) Explain why a woman who smokes during pregnancy may have a baby with a low birth mass. [3]

1. Cigarette smoke contains carbon monoxide, which joins to her red blood cells. [1]
2. Her blood carries less oxygen, so less oxygen reaches the baby. [1]
3. The baby's cells get less oxygen for respiration / less energy for growth, so the baby grows less. [1]
4. Common mistake: writing "the smoke goes into the baby's lungs". The baby does not breathe air before birth; it gets oxygen from the mother's blood.

Worked example 6 (evaluating a meal) Omar's lunch every day is fried potatoes, white bread and a fizzy drink. Give two reasons why this is not a balanced diet and suggest one improvement. [3]

• Too much fat and sugar / too much energy, which can lead to obesity. [1]
• Very little protein, vitamins (e.g. vitamin C, A), minerals (calcium, iron) or fibre. [1]
• Improvement: e.g. replace the fizzy drink with water or milk; add ful, lentils, eggs or chicken for protein; add salad or fruit for vitamins and fibre. [1]

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3