Diet & health, but bitesize.

Part 1Why "balanced"?

Your body is a bit like a building site. To run properly it needs fuel to keep working, building materials to grow and mend itself, and a few special tools that make everything else run smoothly. No single food gives you all of these. That's why a healthy diet has to be balanced.

A balanced diet means eating the right amount of food, with the right variety in it, so you get all seven things your body needs. Miss one out, or have too much of another, and something starts to go wrong.

There are seven components of a balanced diet: carbohydrates, proteins, lipids (fats and oils), vitamins, minerals, fibre and water.

Part 2What each group does

You need to know what each component is for and a food you'd find it in. Here's the lot:

Carbohydrates are your main source of energy — they're the fuel that keeps you moving and your body working. Found in bread, rice, pasta and potatoes.

Proteins are for growth and repair — building new cells and mending damaged ones (muscle, skin, hair). Found in meat, fish, eggs, beans and cheese.

Lipids (fats and oils) are an energy store and also insulate you, keeping you warm. Found in butter, oil, nuts and oily fish.

Vitamins are needed in tiny amounts to keep you healthy — for example, vitamin C (in oranges) keeps skin and gums healthy, and vitamin D (in oily fish) helps build strong bones.

Minerals are also needed in small amounts — for example, calcium (in milk) for strong bones and teeth, and iron (in red meat and spinach) for healthy blood.

Fibre can't be digested, but it's still vital: it keeps food moving through the gut and stops you getting constipated. Found in wholemeal bread, vegetables and cereals.

Water makes up most of your body and is needed for every chemical reaction in your cells. You get it from drinks and from food.

Part 1Food is fuel

Everything your body does — moving, growing, keeping warm, even thinking — needs energy. That energy comes from the food you eat, mostly from carbohydrates and fats.

The energy in food is measured in kilojoules (kJ) or kilocalories (kcal). You'll see both on food packets. A kilocalorie is just an older unit — when people say a food has "200 calories" they really mean 200 kcal.

If you take in the same amount of energy as you use up, your weight stays steady. The trouble starts when these two don't match — but more on that in a moment.

Part 2Different people, different needs

Not everyone needs the same amount of energy. How much you need depends on:

Age — a growing teenager needs more than a young child; needs drop again in older age.
Sex — on average, men need a bit more energy than women.
Activity level — a builder or an athlete uses far more energy than someone who sits at a desk all day.
Pregnancy — a pregnant woman needs extra energy because she is feeding a growing baby too.

That's why a recommended daily amount is only a rough guide. The right amount for you depends on who you are and what you do.

Part 3Reading a bar chart

In this unit you'll often be given the energy needs of different people as a bar chart. Reading one is a key skill, so let's go through it slowly.

A bar chart has two axes. The x-axis (along the bottom) shows the different people or groups. The y-axis (up the side) shows the energy needed, with a scale and a unit. To read a value: find the top of a bar, then read straight across to the y-axis.

From the chart above you can read off, for example, that the man needs 10 500 kJ a day, while the child needs 4500 kJ. You can also compare: the athlete needs the most, the child the least, and the athlete needs about three times what the child needs (14 000 vs 4500).

Part 1The idea: burn the food

How do scientists work out how much energy is in a piece of food? They burn it. When food burns, the energy stored in it is released as heat. If you use that heat to warm up some water, then the more energy the food contains, the hotter the water gets.

This experiment is called calorimetry. The set-up is simple: hold a measured amount of water in a boiling tube, set fire to a food sample underneath it, and watch how much the water's temperature rises.

Part 2Making it a fair test

To compare two foods fairly, you can only let one thing change between them — the food itself. Everything else must be kept the same. So for each food you should keep these the same:

· the same mass of food burned
· the same amount (volume) of water
· the same starting temperature of the water
· the same distance between the flame and the tube

If you keep all of those constant, then any difference in temperature rise must be down to the food — which is exactly what you want to test. That's what makes it a fair test.

Part 3Working out the temperature rise

The maths here is simple. The temperature rise is just the difference between the final and starting temperatures:

temperature rise = final temperature − start temperature

Whichever food gives the bigger temperature rise released the most energy.

Part 1Too much, or too little

From Topic 2 you know that staying healthy means matching the energy you take in to the energy you use up. When that balance is wrong, problems follow.

If you take in more energy than you use, the extra is stored as fat. Over time this can lead to obesity — being very overweight. Obesity raises the risk of serious health problems such as type 2 diabetes, heart disease and high blood pressure.

If you take in far too little food, your body runs out of fuel and breaks down its own stores. This is starvation, and it leaves you weak, with no energy, and unable to grow or repair properly.

Part 2Deficiency diseases

Some problems come not from how much you eat but from missing a particular nutrient. A disease caused by lacking a nutrient is called a deficiency disease. Two you need to know:

Scurvy is caused by a lack of vitamin C. It leads to bleeding gums, sore joints and slow healing. It's why sailors on long voyages used to fall ill — until they started taking citrus fruit.

Rickets is caused by a lack of vitamin D (which helps the body use calcium). Without it, bones grow soft and weak, and a child's legs can bow outwards.

Part 1What is a drug?

A drug is a substance that changes the way your body or brain works. Some drugs are medicines that help you when you're ill. Others are harmful, and some are illegal. Many drugs are addictive, which means the body comes to crave them and it's very hard to stop.

Smoking, vaping and drinking alcohol all involve taking drugs into the body — nicotine in cigarettes and vapes, and alcohol in drinks. Let's look at how each one harms you.

Part 2How they harm the body

Smoking. Cigarette smoke contains tar, which coats the lungs and causes lung damage and lung cancer. It also contains carbon monoxide, a gas that stops the blood carrying oxygen properly, so the heart has to work harder. And it contains nicotine, the drug that makes smoking so addictive.

Vaping. Vapes were sold as "safer than smoking", but they are not harmless. They still deliver addictive nicotine, and there are real concerns about the effect of the chemicals in vapour on young, growing lungs.

Alcohol. Alcohol slows down the brain. In the short term this slows your reaction time and affects judgement (which is why drink-driving is so dangerous). In the long term, heavy drinking damages the liver — the organ that breaks alcohol down — and can lead to liver disease.