A lunch to think about
Like most people, Janet didn't think about food groups such as proteins, fats and carbohydrates as she tucked into her meal. The hamburger looked good and tasted even better. The golden, crispy, brown French fries were also delicious, especially when they were dipped in the sauce. The whole meal was a delight for Janet, but was it healthy?

Let us look at each part of the meal in turn. As Janet munched her way into the hamburger first, it may seem the best place to start. The hamburger is sandwiched between two halves of a bread roll. The bread is made from flour. Flour is a white powder that is made by grinding up wheat grains. A wheat grain contains a tiny wheat plant and its store of food. The flour is formed from the food store. If the grain had been planted in the ground, the tiny wheat plant would have used the energy in the food store to help it sprout or germinate. This stored energy which is now in the flour in the bread roll is starch. When it is digested inside Janet's body it will be broken down into sugar, travel round in the blood and provide instant energy to any cells that need it.

If Janet had removed the top of her hamburger, she would have seen a piece of lettuce resting on the top of the meat. The lettuce is a piece of a broad leaf. It has veins running through it, which are made of fibre. They support the thinner, weaker parts of the leaf. Inside Janet's body, the fibre in the lettuce leaf will not be digested but it will perform a useful task. It will give the food bulk. As the other substances are digested, they give the muscles in the gut less and less to push on. This makes the movement of food along the gut inefficient. The fibre will remain solid throughout its passage along the gut and will give the muscles some substance on which to push. This will help move all the food along the gut.

Beneath the lettuce leaf is the hamburger itself. This is made from meat and contains protein and fat. When the protein is digested, it will provide materials for the body to grow and repair itself. The digested fat will form a store of energy and be deposited under Janet's skin and around other organs inside her body.

Underneath the hamburger is a slice of tomato, a piece of gherkin and some chopped onions. The tomato contains vitamin A, which will help Janet's body defend itself against germs. The onion and the gherkin contain fibre, which will help the food move along the gut just like the fibre in the lettuce leaf.

Occasionally, Janet stopped munching her hamburger and ate some French fries. She dipped some of them in tomato sauce before she ate them. French fries are small strips of potato. A potato is a tuber produced by the potato plant. It grows on an underground stem. There are buds on the side of a potato which we call 'eyes'. Potatoes are rich in starch. If a potato is planted in the ground, the energy in its starch is used to make the buds sprout and grow into a new plant.

When potatoes are chipped and fried to make French fries they soak up some of the fat that they are fried in. When Janet ate her French fries she received a double helping of energy - from the starch and from the fat.

The last part of Janet's meal was the tomato sauce. Although tasty it is also rich in iron. This will help Janet's red blood cells carry oxygen round her body.

So looking back at Janet's meal as she put her napkin and cartons in the bin, was it healthy? Well, yes and no. There were plenty of useful substances like protein, starch fibre and iron but there was also plenty of fat too. How could Janet balance her diet for the rest of the day? She could have meals with more fruit and vegetables and definitely no more fat. It is alright to have a meal which contains a little too much of some food groups occasionally, provided that you balance it with other more healthy meals for the rest of the day.

Are snacks such as crisps and chocolate healthy meals?
If eaten occasionally they do no harm. They both are rich in fats and if eaten frequently the energy they contain may be stored as fat in the body. This makes the body overweight and puts extra strain on the heart. Sweets are also harmful if eaten in large amounts. They are rich in energy, which can be stored as fat and the sugar can damage the teeth.

What are healthy snacks?
Fruit makes a very healthy snack. It does not contain fat, contains some vitamins and fibre and does not contain a great deal of energy which could be stored as fat. Celery and raw carrot make healthy crunchy snacks, which are rich in fibre and also help to clean the teeth.

Should all people eat the same diet?
No. The diet should match the person's life style. If a person does a lot of physical work in their job, such as people who work on building sites, they should have a diet which contains a large amount of energy. This will give them the power to do their work without making them fat. If a person has a less active job, such as a person in an office, they should have a diet which contains much less energy.

What happens if a person becomes less active but keeps the same high energy diet?
They do not burn off the extra energy in activities so it is stored as fat in the body and the person gains weight. If the person becomes seriously overweight they could put extra strain on their heart which in turn could lead to heart disease.

Can the energy in food be measured?
Yes, it can. The Calorie is a unit of measuring energy that is widely known. In science the unit for measuring energy is called the kilojoule. This word is shortened to kJ just as gram is shortened to g. Scientists have measured the energy in different foods by burning them in pure oxygen and recording the amount of heat that is given out. For example a peanut contains 25 kJ of energy.

Can the energy people need for an activity be measured?
Yes, it can. Scientists have worked out that as you sit at the computer reading this you are using 5 kilojoules in a minute. If you ate a peanut it would give you the energy to sit here for five minutes. When you get up and walk away, you are using energy at the rate of just over 15 kJ per minute. When you go for a swim you use 30 kilojoules of energy every minute.