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3.05 Food and energy consumption


Just as we use different units to measure length or distance (i.e. metres, kilometres, light years), different units are used for measuring the amount of energy in food and drink.


Kilojoules and calories

The most common unit of energy is the kilojoule (kJ). As we know, the prefix 'kilo' means one thousand, so $1$1 kilojoule (kJ) is equal to $1000$1000 joules (J).

Calories are an older unit of energy in food. The term 'calorie' can be confusing, because $1$1 Calorie (Cal), spelt with an uppercase C, is equal to 1 kilocalorie (kcal).

Although kilojoules (kJ) are the standard unit of energy for food and drink in Australia, kilocalories (kcal) still appear on some nutritional labels, and the term 'calories' is often used to describe them.

A kilocalorie (kcal) is defined as the amount of energy required to raise the temperature of $1$1 kg of water, by $1$1 degree. One kilocalorie (kcal) is equal to $4.184$4.184 kilojoules (kJ), or approximately $4.2$4.2 kJ.

While calories and joules are both metric units, only the joule is part of the International System (SI) of Units.



Nutritional energy units
$1$1 kilojoule (kJ) $=$= $1000$1000 joules (J)


$1$1 Calorie (Cal) $=$= $1$1 kilocalorie (kcal)


$1$1 kilocalorie (kcal) $=$= $4.184$4.184 kilojoules (kJ)


Practice Questions

Question 1

Convert $35$35 kilocalories into kilojoules, using $1$1 kilocalorie$=$=$4.184$4.184 kilojoules.


Convert $2363.96$2363.96 kilojoules into kilocalories, using $1$1 kilocalorie$=$=$4.184$4.184 kilojoules.


Food and energy consumption

The food we eat provides our body with the energy we need to live.

Energy in food comes mainly from proteins, carbohydrates, fats and dietary fibre. These nutrients are present in different amounts per gram of food consumed. If we consume a lot of foods that are high in kilojoules, our body may be getting more energy than it needs. Any excess energy will be stored in the body as fat.

The recommended number of kilojoules our body requires each day, depends on our age, gender, weight and level of physical fitness. To get an estimate of our daily energy requirements, we can use the following online calculator

The Australian Government also provides dietary guidelines for Australians on the type and quantity of food required to stay healthy. 


Practice question

Question 1

Carl has a sirloin steak with a banana and carrot for lunch, and a glass of milk to go along with it. Later in the afternoon he also has some yoghurt. If Carl is $43$43 years old and leads an inactive lifestyle, what percentage of his daily kilojoule requirements has he consumed already?

Give your answer as a percentage correct to one decimal place.

Age Activity level Daily energy requirements (kJ/day)
Women Men
$18-35$1835 Inactive $8000$8000 $10500$10500
Active $9000$9000 $12500$12500
Very active $10500$10500 $14800$14800
$36-70$3670 Inactive $8000$8000 $10000$10000
Active $8800$8800 $11800$11800
Very active $10400$10400 $14300$14300
Food Energy (kJ)
sirloin steak $1179$1179
banana $56$56
carrot $86$86
milk $622$622
yoghurt $583$583


Nutrition information panels

Food manufacturers are required to display nutritional information on the packaging of their products, to help consumers monitor their energy intake. 

These nutrition information panels display the amount of each nutrient in kilojoules (kJ), per $100$100 g and per serve. The 'per $100$100g' column allows consumers to compare nutrients across similar products, while the 'per serve' column gives an indication of the amount of each nutrient in a typical serving. 

The following diagram provides some explanation on the different parts of a nutrition information panel.


Worked example

Example 1

The food label below is provided on a $200$200 g packet of chocolate biscuit fingers. 


Use the label to answer the questions:

(a) What is the total energy intake of eating the whole packet of biscuits?

Think: Each packet of biscuits is $200$200 g and the energy value per $100$100 g is $2159$2159 kJ, therefore we multiply this amount by $2$2.


Energy in one packet $=$= $2159\times2$2159×2
  $=$= $4318$4318 kJ


(b) How many kilojoules are consumed by eating $4$4 biscuits?

Think: The label states that $1$1 serving is approximately equal to $4$4 biscuits.


Energy in $4$4 biscuits $=$= $453$453 kJ


(c) Ben is restricting his energy intake to $7500$7500 kJ a day. How many biscuits can he consume if he eats nothing else?

Think: First calculate the number of kilojoules per biscuit, then divide $7500$7500 kJ by this amount, to work out how many biscuits Ben can eat.

Do: The energy per biscuits is found by dividing the energy in $4$4 biscuits, by $4$4.

Energy per biscuit $=$= $\frac{453}{4}$4534
  $=$= $113.25$113.25 kJ


Then divide $7500$7500 kJ by $113.25$113.25 kJ to find the number of biscuits Ben can eat. 

Number of biscuits $=$= $\frac{7500}{113.25}$7500113.25
  $=$= $66.2251$66.2251... kJ
  $=$= $66$66 biscuits


Because we want the number of whole biscuits to not exceed $7500$7500 kJ, we had to round our answer down to $66$66 biscuits, as the most Ben can eat.

(d) The recommended dietary intake of protein for a $17$17 year old female is $45$45 g per day. What percentage of the recommended dietary intake comes from eating one biscuit? Express your answer to $2$2 decimal places.

Think: In a similar way to finding the amount of energy in each biscuit, we divide the amount of protein per serving, by $4$4 biscuits. We can then find what this amount is as a percentage of $45$45 g.


Protein per biscuit $=$= $\frac{1.5}{4}$1.54
  $=$= $0.375$0.375 g


We can then find what $0.375$0.375 g is as a percentage of $45$45 g.

Percentage of recommended dietary intake $=$= $\frac{0.375}{45}\times100$0.37545×100
  $=$= $0.8333$0.8333...
  $=$= $0.83%$0.83% ($2$2 d.p.)


Therefore one biscuit will provide $0.83%$0.83% of the recommended dietary intake. 


Energy expenditure

Our bodies need a certain amount of energy to sustain basic physiological functions (i.e. digestion, breathing, body temperature), as well as perform various daily physical activities. As mentioned before, a person's daily energy requirements depends on their age, weight and physical fitness. Someone who leads an active lifestyle will require more energy than someone of the same age, whose lifestyle is less active.

During different activities, our bodies will consume energy at different rates. We measure bodily energy consumption in kilojoules per kilogram (kJ/kg).


Worked example

Example 2

The table below shows the estimated number of kilojoules consumed per kilogram of body weight during $30$30 minutes of physical activity.


Use the table to estimate the energy consumed during the following activities.

(a) A $50$50 kg female plays badminton for $30$30 minutes.

Think: Playing badminton for $30$30 minutes will consume approximately $13.82$13.82 kJ/kg. Multiply this rate by the weight of the person to estimate the energy they consume.


Energy consumed $=$= $13.82\times50$13.82×50
  $=$= $691$691 kJ


(b) A $65$65 kg male plays a basketball game for $1$1 hour.

Think: Playing a basketball game for $30$30 minutes will consume $20.27$20.27 kJ/kg. To find the energy consumed in $1$1 hour, multiply $20.27$20.27 by $2$2, ($1$1 hour = $2\times30$2×30 minutes), then multiply by the person's mass.


Energy consumed $=$= $20.27\times2\times65$20.27×2×65
  $=$= $2635.1$2635.1 kJ


(c) A $70$70 kg boy canoes for $50$50 minutes.

Think: Canoeing for $30$30 minutes consumes $6.45$6.45 kJ/kg. To find the energy consumed in $50$50 minutes, multiply $6.45$6.45 by  $\frac{50}{60}=\frac{5}{6}$5060=56 of an hour, then multiply by the boy's mass.


Energy consumed $=$= $6.45\times\frac{50}{60}\times70$6.45×5060×70
  $=$= $376.25$376.25 kJ


Practice questions

Question 2

The graph shows the number of kilojoules that Kenneth burns in $10$10 minutes while doing each of these activities. How long would he need to spend playing golf to burn off the $3$3 cups of soda that contain $627$627kJ each? Give your answer to the nearest minute.

Question 3

The table shows the time needed for John, a fit $30$30 year old man, to burn $500$500 Calories for a number of different activities.

Activity Time
Bicycling $1$1 hour
Golf $2$2 hours
Running $56$56 minutes
Soccer $1$1 hour
Swimming $1$1 hour
Touch football $56$56 minutes
Skateboarding $1$1 hour
Skiing $1$1 hour
Housework $3$3 hours
Walking the dog $2$2 hours
Stair climbing $2$2 hours
  1. How long would John have to run to burn $1500$1500 Calories?

  2. How long would John have to ski to burn $125$125 Calories?

Question 4

Jack, who weighs $80$80 kg, goes for a run every morning for $1.5$1.5 hours. He then drives to work for $30$30 minutes, sits quietly whilst at work for $7.5$7.5 hours and then drives home, which takes another $30$30 minutes.

Activity Energy (kJ/kg/h)
Sitting quietly $1.7$1.7
Writing $1.7$1.7
Standing relaxed $2.1$2.1
Driving a car $3.8$3.8
Vacuuming $11.3$11.3
Walking rapidly $14.2$14.2
Running $29.3$29.3
Swimming ($4$4km/hour) $33$33
Rowing in a race $67$67
  1. According to the table, how much energy did he use in total for these tasks?



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