The eatwell plate

A balanced diet is a diet in which all the components needed to maintain health are present in appropriate proportions.

The eatwell plate shows the proportions of the different components that should be eaten daily in a well-balanced diet.

Food wheel showing healthy diet. Fresh fruit and vegetables should be largest part of diet, then carbohydrates, proteins and dairy products. Fats and sugars should be smallest part of the diet.

A balanced diet should include appropriate proportions of carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre.

Energy requirements

The amount of energy we need varies. This is due to the following factors:

  • age
  • activity levels
  • pregnancy

Age

The amount of energy we need tends to increase as we approach adulthood.

Age in yearsDaily energy requirement in kJ
0 (newborn)2,000
25,000
67,500
13 (girl)9,000
13 (boy)11,000
16 (girl)9,000
16 (boy)12,000
Adult hand holding a young baby’s hand
A baby needs much less energy than a teenager or an adult

The energy needs of adults go down as they age. For example, people in their 50s need about 17 per cent less energy in their diet than people in their 30s.

Activity levels

People who are active tend to need more energy than sedentary people. For example, an adult office worker might need 10,000 kJ per day, but a manual worker might need 15,000 kJ per day.

Pregnancy

In general, the greater a person’s mass, the more energy they need. Men tend to need more energy than women, and a woman’s energy needs increase when she is pregnant. This is mainly because she is carrying extra mass.

The energy content of food

The energy content in a food sample can be measured using simple calorimetry.

To carry out this experiment, you need to:

  1. pour 10 cm3 cold water into a boiling tube
  2. record the starting temperature of the water
  3. record the mass of the food sample
  4. heat the food in a Bunsen flame until it catches fire
  5. heat the water using the flame from the burning food
  6. record the final temperature of the water
A test tube filled with water with a thermometer inside the tube. Burning food on a mounted needle is held below the tube.Investigating the energy content in a food sample

Calculations

To work out the energy per gram of food transferred to the water, use this equation.

\text{energy transferred (J) =}~\frac{\text{mass of water (g)}\times 4.2~\text{J/g~}{^\circ\text{C}}\times\text{temperature increase~} {\text{(}{^\circ\text{C)}}}}{\text{mass of food (g)}}

Question

Two crisp breads are tested to see which provides the most energy per gram.

0.7 g of crisp bread A is burned and increases the temperature of 10 cm3 of water by 15°C.

0.5 g of crisp bread B is burned and increases the temperature of 10 cm3 of water by 20°C.

Using the equation, calculate the energy transferred from each sample. If the answer is more than 1,000 joules, you should express the answer in kilojoules: 1,000 J = 1 kJ.

Note that 1 cm3 of water has a mass of 1 g.

\text{energy transferred (J) =}~\frac{\text{mass of water (g)}\times 4.2~\text{J/g~}{^\circ\text{C}}\times\text{temperature increase~} {\text{(}{^\circ\text{C)}}}}{\text{mass of food (g)}}

Crisp bread A

\frac{10\times4.2\times15}{0.7}{~=~900\text{ J/g~or~}0.9\text{~kJ/g}}

Crisp bread B

\frac{10\times4.2\times20}{0.5}{~=~1,680\text{~J/g~or~}1.68\text{~kJ/g}}

Crisp bread B has the highest energy content.