The physical activity recommendations suggest that children should engage in 60 minutes of moderate-to-vigorous physical activity (MVPA) on most days of the week to help prevent numerous health outcomes (1). It has recently been shown that engaging in 15 minutes of MVPA was negatively associated with obesity in the ALSPAC cohort at 12 years old (2). The physical activity recommendations and the previous ALSPAC findings do not address the potential health benefits associated with light physical activity (LPA). There are indications that LPA may be of benefit to health outcomes, with a recent study noting that light physical activity was independently associated with improved blood glucose regulation in adults (3). The implications of LPA in terms of preventing obesity have not yet been extensively studied.

Considering the principles of energy balance - reducing or increasing the intensity of physical activity, while altering the duration of physical activity, to match the energy expended with 15 minutes of MVPA, should result in the same negative association with obesity in the ALSPAC children at 12 years old. In line with this hypothesis, a cross-sectional study involving children, determined that 45 minutes of moderate physical activity (MPA) and 15 minutes of vigorous physical activity (VPA) were both associated with lower body composition measures (4). However, different physical activity intensities may be negatively associated with obesity at varying levels of energy expenditure and this hypothesis requires further research.

Aim:

To determine the influence of the intensity and duration of physical activity with regard to obesity among a sample of 12 year old children participating in ALSPAC.

Objective 1:

To determine if LPA is independently associated with obesity in the ALSPAC children at 12 years old.

Objective 2:

To determine if the energy expended with 1 MET-hour (15 minutes of MVPA) is negatively associated with obesity, when achieved through LPA, MPA and VPA in the ALSPAC children at 12 years old.

Objective 3:

To determine if the energy expended with 2, 3 and 4 MET-hours is negatively associated with obesity, when achieved through LPA, MPA, MVPA and VPA in the ALSPAC children at 12 years old.

Dependent variable:

- Obesity (top 10% fat mass)

Independent Variables:

- LPA (objective 1)

- 30 minutes of LPA (objective 2)*

- 20 minutes of MPA (objective 2)*

- 15 minutes of MVPA (objective 2)*

- 10 minutes of VPA (objective 2)*

- 60 minutes, 120 minutes & 180 minutes of LPA (objective 3)*

- 40 minutes, 60 minutes & 80 minutes of MPA (objective 3)*

- 30 minutes, 45 minutes & 60 minutes of MVPA (objective 3)*

- 20 minutes, 30 minutes & 40 minutes of VPA (objective 3)*

*See appendix 1 for the calculation of the MET-hours

Covariates:

- Gender (model 1)**

- Social class, maternal education, smoked during pregnancy, maternal obesity, birth weight & gestational age (model 2)**

- TV viewing (38mo) & sleep pattern (30mo) (model 3)**

- Pubertal status (model 4)**

**Repeat models with the inclusion of sedentary behaviour and MVPA as covariates (objective 1)

Repeat models with the incusion of sedentary behaviour and either LPA, MPA, MVPA or VPA

(objectives 2 & 3)

Data Analysis:

- Descriptive statistics

- Logistic regression (obese: yes or no)

References:

(1) Haskell WL, Lee IM, Pate RR et al. Physical activity and public health: updated recommendation for adults from the American College of Sports Medicine and the American Heart Association. Med Sci Sports Exerc. 2007;39(8):1423-34

(2) Ness AR, Leary SD, Mattocks C et al. Objectively measured physical activity and fat mass in a large cohort of children. PLoS Med. 2007;4(3):e97

(3) Healy GN, Dunstan DW, Salmon J et al. Objectively measured light-intensity physical activity is independently associated with 2-h plasma glucose. Diabetes Care. 2007;30(6):1384-9

(4) Wittmeier KDM, Mollard RC & Kriellaars DJ. Physical Activity Intensity and Risk of Overweight and Adiposity in Children. Obes 2008; 16: 415-420

Appendix 1- Calculation of MET-hours:

MVPA (4 METs):

15 minutes of MVPA = 1 MET-hour

30 minutes of MVPA = 2 MET-hours

45 minutes of MVPA = 3 MET-hours

60 minutes of MVPA = 4 MET-hours

LPA (2METs):

30 minutes of LPA = 1 MET-hour

60 minutes of LPA = 2 MET-hours

90 minutes of LPA = 3 MET-hours

180 minutes of LPA = 4 MET-hours

MPA (3 METs):

20 minutes of MPA = 1 MET-hour

40 minutes of MPA = 2 MET-hours

60 minutes of MPA = 3 MET-hours

80 minutes of MPA = 4 MET-hours

VPA (6 METs):

10 minutes of VPA = 1 MET-hour

20 minutes of VPA = 2 MET-hours

30 minutes of VPA = 3 MET-hours

40 minutes of VPA = 4 MET-hours

Appendix 2 - Variables Required to match current data:

30, 60, 90 & 180 minutes of LPA; Accelerometry; Children; 11 year clinic

20, 40, 60 & 80 minutes of MPA; Accelerometry; Children; 11 year clinic

30, 45 & 60 minutes of MVPA; Accelerometry; Children; 11 year clinic

10, 20, 30 & 40 minutes of VPA; Accelerometry; Children; 11 year clinic.