The food we eat provides our body with the energy we need to live. As we saw in the previous chapter, energy from food is measured in Calories (Cal) and kilojoules (kJ).
The Australian Government has a department that focuses on developing dietary guidelines to advise all Australians about the amount, and kinds of food, that we need to eat to stay healthy. All this information can be found on their website. These guidelines offer an approach to eating but everyone's needs are different especially for those who are sick or pregnant.
The energy comes mostly from the protein, carbohydrates, fat and dietary fibre in different amounts per gram of food consumed. If you consume a lot of high kilojoule foods you may be getting more energy than you need. The excess energy is stored as fat.
The recommended amount of kilojoules your body needs each day depends on your age, gender, weight and level of physical activity. You can go here to get an estimate on your daily energy requirements.
Practice question
Question 1
Bob is in his 40's and leads an inactive lifestyle. Today he has a small Greek salad, which contains $446$446kJ.
| Age | Activity Level | Women (kJ/day) | Men (kJ/day) |
|---|---|---|---|
| $18-35$18−35 | inactive | $8000$8000 | $10500$10500 |
| active | $9000$9000 | $12500$12500 | |
| very active | $10500$10500 | $14800$14800 | |
| $36-70$36−70 | inactive | $8000$8000 | $10000$10000 |
| active | $8800$8800 | $11800$11800 | |
| very active | $10400$10400 | $14300$14300 |
Find the recommended kilojoule intake.
What percentage of his recommended kilojoule intake is this?
Give your answer as a percentage correct to one decimal place.
Food labels
Food manufacturers are required to provide nutritional information about their products on the packaging to help consumers to monitor their energy intake. The "Per $100$100 g" column helps us to compare nutrients for similar products more easily while the "Per serve" column tells you how much of each nutrient you are consuming per serve. More information about how to read the nutrition table can be seen below.
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 (in kJ)?
One packet is $200$200 g and the energy value for $100$100 g is $2159$2159 kJ therefore we multiply this amount by $2$2 to get the total.
$2159\times2=4318$2159×2=4318 kJ
(b) How many kilojoules are consumed if you eat $4$4 biscuits?
The label states that $1$1 serving is equal to $4$4 biscuits. Therefore the amount kilojoules is $453$453 kJ if you eat $4$4 biscuits.
(c) Ben is restricting his energy intake to $7500$7500 kJ a day. How many biscuits can he consume if he eats nothing else?
We can calculate the number of kilojoules per biscuit by dividing $453$453 kJ by $4$4 biscuits. $453\div4=113.25$453÷4=113.25 kJ per biscuit
Then we can divide $7500$7500 kJ by $113.25$113.25 kJ to find the number of biscuits Ben can eat. $7500\div113.25\approx66.23$7500÷113.25≈66.23
Because we want the number of whole biscuits that do not exceed $7500$7500 kJ, we need to round 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?
To find the amount of protein in each biscuit we take the amount of protein per serving and divide it by $4$4 biscuits. $1.5\div4=0.375$1.5÷4=0.375 g per biscuit
We can then find what $0.375$0.375 g is as a percentage of $45$45 g.
| Percentage daily protein | $=$= | $\frac{0.375}{45}\times100%$0.37545×100% |
| $=$= | $0.8\overline{3}%$0.83% |
Therefore, one biscuit will provide $0.8\overline{3}%$0.83% of the recommended dietary intake.
Practice question
Question 2
Uther is looking at the nutritional label on the soy milk he just purchased. The label is shown below.
| Nutrition information | |||
| Serving per package - $4$4 | Serving size - $250$250 g | ||
| Per Serve | Per $100$100 g | ||
| Energy | $135$135 kJ | $54$54 kJ | |
| Protein | $8.25$8.25 g | $3.3$3.3 g | |
| Fat | |||
| Total | $4.50$4.50 g | $1.8$1.8 g | |
| Saturated | $0.50$0.50 g | $0.2$0.2 g | |
| Carbohydrate | |||
| Total | $15.00$15.00 g | $6$6 g | |
| Sugars | $10.00$10.00 g | $4$4 g | |
| Sodium | $128$128 g | $51$51 g | |
How many kilojoules are there in $1$1 serving of soy milk?
How many kilojoules are in $60$60 g of soy milk?
How many kilojoules are there in the entire carton of soy milk?
Energy expenditure
Our bodies need a certain amount of energy to sustain basic physiological functions (e.g. digestion, breathing, body temperature) and also to perform various physical activities everyday. As mentioned before the amount of energy required each day depends on a person's age, weight and lifestyle. For example, someone who has an active lifestyle will require more energy than someone of the same age who lives an inactive lifestyle.
Different activities will expend energy at different rates.
Worked example
Example 2
The table below shows the estimated kilojoules burned per kilogram of body weight per $30$30 minutes of activity.
www.weightloss.com.au
Use the table above to estimate the energy burned by the following people's bodies during the given activities:
(a) A $50$50 kg female walks plays badminton for $30$30 minutes.
Badminton for $30$30 minutes will expend approximately $13.82$13.82 kJ/kg. So we multiply the rate by the weight of the person to estimate the energy burned.
$13.82\times50=691$13.82×50=691 kJ
Therefore $691$691 kJ of energy is expended.
(b) A $65$65 kg male plays a basketball game for $1$1 hour.
Playing a basketball game for $30$30 minutes will expend $20.27$20.27 kJ/kg. To find the energy burned in $1$1 hour for a $65$65 kg male we multiply $20.27$20.27 by $2$2 ($1$1 hour $=2\times30$=2×30 minutes), then multiply by the weight.
$20.27\times2\times65=2635.1$20.27×2×65=2635.1 kJ
Therefore approximately $2635.1$2635.1 kJ is burned.
Practice questions
Question 3
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 4
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 |
How long would John have to run to burn $1500$1500 Calories?
How long would John have to ski to burn $125$125 Calories?
Question 5
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 |
According to the table, how much energy did he use in total for these tasks?