Table 1

Modelling of reduction in BMI for a single 'average' individual new to active transport (Effects are averaged over one calendar year)

Prep to Grade 2

Comments

Boys

Girls


Height (m)

1.20

1.19

Mean height for 5-7 year age group [19]

Weight (kg)

23.50

23.48

Mean weight for 5-7 year age group [19]

Body mass index BMI (kg/m2)

16.24

16.47

Mean BMI for 5-7 year age group [19]

Estimated total energy expenditure (MJ/day)

6.43

6.64

Total energy expenditure (MJ/day) = [.107 × weight (kg)] + [2.91 × height (metres)] + .417 [20]

Estimated total energy expenditure (kJ/day)

6433

6404

Conversion to kilojoules - multiply by 1000

Increased METS -- walking versus sitting

2.5

2.5

Metabolic equivalents for sitting = 1.0, walking 3.5. Therefore, additional energy expenditure of active transport to school = 2.5 METS [13]

Extra time spent on walking to and from school (min)

28.30

28.30

Mean travel time for Victorian children participating both in morning and afternoon WSB [15]

Energy expenditure increase from WSB participation(kJ per day)

116

116

Increase in individual energy expenditure from walking (kJ/school day) = weight (kg) × increased METs × time (hrs) × factor to convert kcal to kJ (4.2)

Average number of days of WSB participation to and from school per week

1

1

Based on Victorian WSB experience 2004 [15], where 1,675 enrolled children engaged in 3,049 sessions of active transport per week. This translates to 1.82 sessions per week/child, or about equivalent of walking to and from school one day per week.

Number of potential weeks of WSB participation per year

40

40

Number of weeks in the school year.

Total number of days of WSB participation per year

40

40

Number of WSB days per week × number of school weeks

Energy expenditure increase from WSB participation (kJ)

13

13

Total increase in individual energy expenditure × number of WSB days per year divided by 365

Relative increase in energy expenditure with WSB participation (%)

0.20

0.20

Average individual energy expenditure from WSB as % of estimated total energy expenditure per day: (13/6433) × 100 and (13/6404) ×100 for boys and girls respectively

Conversion factor

0.45

0.45

Factor for conversion of relative change in energy balance to relative change in weight [14,21]

Relative lower weight with WSB participation (%)

0.09

0.09

[1-(energy expenditure1/energy expenditure2)0.45]*100

Absolute lower weight with WSB participation (kg)

0.02

0.02

0.09% of original weight

New weight (kg)

23.48

23.46

Original mean weight minus decrease in weight as a result of WSB participation

New BMI (kg/m2)

16.23

16.45

New weight divided by square of height

Reduction in BMI (kg/m2)a

0.014

0.015

Original BMI minus new BMI for new participants


aThese figures are point estimates, which do not take into account uncertainty around any of the input parameters. As a result, they are different to the BMI changes quoted in the results section.

BMI body mass index; METS metabolic equivalent units; WSB Walking School Bus; kJ kilojoules; kg kilograms; m2 metres squared

Moodie et al. International Journal of Behavioral Nutrition and Physical Activity 2009 6:63   doi:10.1186/1479-5868-6-63

Open Data