Growing up in Scotland: overweight and obesity at age 10

2. Methods

2.1. Sample and data overview

The analysis presented in this report uses data from the first GUS birth cohort (Birth Cohort 1 or 'BC1'). BC1 is comprised of a nationally representative sample of 5217 children living in Scotland when they were 10 months old who were born between June 2004 and May 2005.

Starting in 2005/06, data were collected annually from when the children were aged 10 months until they were just under 6 years old, and then biennially at age 7-8 and when the children were in Primary 6 (age 10). At the time of writing (2018), the ninth sweep of face-to-face data collection with this cohort is underway at the time the cohort children are in their first year of secondary school (age 12). This report draws primarily on data collected, firstly, at the time the children were aged just under 6 and were in their first or second year of primary school (in 2010/11), and secondly, at the time they were aged 10 and in Primary 6 (in 2014/15). However, for a number of the factors examined it also draws on data collected from other time points.

Because the cohort is comprised of a nationally representative sample of children the results should be understood to represent all children of the respective age living in Scotland at the time point in question who were also living in Scotland when they were 10 months old. For example, the results presented for the GUS children at the time they were aged 10 are broadly representative of all children in Scotland who were aged 10 in 2014/15.

The main data collection on GUS takes place through annual or biennial 'sweeps' of face-to-face interviews with children and parents in their homes. This report draws on data collected from a number of sources: from the cohort child's main carer at various age points; from objective measures of the child's height and weight at the ages 4, 6, 8 and 10; from the children themselves when they were aged 10.

2.2. Measuring height and weight

Height was measured using a portable stadiometer with a sliding head plate, base plate and four connecting rods marked with a metric measuring scale. Participants were asked to remove shoes. One measurement was taken, with the participant stretching to the maximum height and the head positioned in the Frankfort plane^[12].

If the reading was between two millimetres it was recorded to the nearest even millimetre.

Weight was measured using either Seca or Tanita electronic scales, both of which use a digital display. Participants were asked to remove shoes and any bulky clothing. A single measurement was recorded to the nearest 100g.

In the analysis of height and weight, data from those who were considered by the interviewer to have unreliable measurements, for example those who had excessive clothing on, were excluded.

2.3. Child BMI classification

BMI is defined for children in the same way as it is for adults: weight (kg)/square of height (m²). The International Obesity Task Force concluded that BMI is a reasonable measure of adiposity in children and it is the key measure of overweight and obesity for children used in the Scottish Government monitoring data series – the Scottish Health Survey and the Child Health Surveillance Programme.

Despite the relatively wide acceptance of the use of BMI as an adiposity indicator, the establishment of an agreed specific obesity and overweight classification system for children and young people remains challenging. Constant changes in body composition during growth mean that the relationship between weight-for-height and adiposity during childhood and adolescence is age-dependent, and this relationship is further complicated by both ethnicity and gender (Daniels et al, 1997).

The classification of children's BMI used in this chapter, set out below, has been derived from BMI percentiles of the UK 1990 reference curves (Cole et al, 1990; Cole et al, 1998) referred to as the national BMI percentiles classification; these have been used in previous analysis of GUS to date. The national BMI percentiles classification has been shown to be reasonably sensitive - i.e. not classifying obese children as non-obese - and specific - i.e. not classifying non-obese children as obese (Reilly, 2002; Reilly et al, 2002). SIGN (2010) recommends that these reference curves and thresholds should be used for population surveillance in Scotland. The 85th / 95th percentile cut-off points are commonly accepted thresholds used to analyse overweight and obesity in children. These thresholds have been widely used to describe childhood overweight and obesity prevalence trends in the UK as follows:

• At or below 2nd percentile: at risk of underweight
• Above 2nd percentile and below 85th percentile: healthy weight
• At or above 85th percentile and below 95th percentile: at risk of overweight
• At or above 95th percentile: at risk of obesity

GUS uses a method developed by ISD Scotland and similarly applied on the Scottish Health Survey to plot the exact ages of the children in the sample against the reference population data. While children's exact age was used to calculate the BMI grouping prevalence rates (based on the interview date and the date of birth), results are presented using grouped ages based on age at last birthday.

Note that the terms 'overweight (including obese)', 'overweight or obese' and 'overweight/obese' are used interchangeably throughout the report to describe children whose BMI was at or above the 85^th percentile.