Growing up in Scotland: overweight and obesity at age 10

6. Factors associated with change in BMI category between ages 6 and 10

6.1. Introduction

The previous chapter considered – amongst the relevant data available in GUS - factors associated with a child being overweight or obese at age 10. As demonstrated in earlier sections, a reasonable proportion of children who were overweight or obese at age 10 had been a healthy weight at age 6. Similarly, some of those children who were overweight or obese at age 6 were a healthy weight at age 10. In relation to identifying issues relevant for tackling and preventing childhood overweight and obesity, both of these groups are of interest.

In this section we explore the factors associated with a move into and a move out of overweight and obesity between the ages of 6 and 10. The analysis uses the same variables explored in the previous chapter.

6.2. Key findings

• A higher proportion of children living in more deprived areas moved into overweight than did children living in less deprived areas. Seventeen percent of children living in areas in the most deprived quintile became overweight or obese compared with 11% of those living in areas in the least deprived quintile.
• Higher rates of moving into overweight were also seen amongst children whose mothers were obese, children who only occasionally or quite often ate breakfast, those with higher weekly screen time and those with a TV in their bedroom. For example, 16% of children with a TV in their bedroom moved into overweight compared with 11% of those who did not have a TV in their bedroom.
• A greater proportion of children with heavier birth weights than those with lighter birth weights moved out of overweight. Higher rates of moving out of overweight were also seen amongst those who snacked on unhealthy items less frequently and children who did not have a TV in their bedroom. For example, 10% of children who ate unhealthy snacks up to once a day moved out of overweight compared with 4% of those who consumed unhealthy snacks more often.
• The results of analysis undertaken to examine which factors were most closely associated with moving into overweight when all factors of interest were taken into account found that having a higher weekly screen time and a mother who was obese were statistically significantly associated with moving into overweight. The results also tentatively suggest that having a TV in the child's bedroom and not always eating breakfast may also be associated with moving into overweight, though the relationship was not statistically significant.
• Similar analysis found that moving out of overweight was associated with lower household income, less frequent consumption of unhealthy snacks and not having a TV in the child's bedroom.

6.3. Defining and identifying change

By comparing a child's BMI status at age 6 with the same measure at age 10, in section 3.4, we were able to identify the proportion of healthy weight children who become overweight or obese and the proportion of overweight or obese children who become a healthy weight. Using these two variables, a new variable was derived which captured transitions in BMI status between the two age points. This variable had the following categories:

• Remain healthy weight: children whose BMI was classified as healthy weight at both ages
• Move out of overweight: children who were overweight or obese at age 6 and healthy weight or underweight at age 10
• Move into overweight: children who were healthy weight or underweight at age 6 and overweight or obese at age 10
• Remain overweight: children who were overweight or obese at age 6 and age 10
• Other: children who were underweight at age 6 and age 10 or who moved between being underweight and healthy weight

The proportion of children in each category is shown in Figure 6‑1. Fifty-eight percent of all children were a healthy weight at age 6 and age 10. As may be expected given earlier results, children were more likely to move into than out of overweight. Fifteen percent moved into overweight whilst 5% moved out of overweight. A significant minority (19%) remained overweight whilst 3% fell into the 'other' category.

Figure 6‑1 Change in children's BMI classification between age 6 and age 10

6.4. Demographic and socio-economic factors

Amongst the demographic and socio-economic factors considered – the child's sex, ethnicity, household income, parental education, socio-economic classification and area deprivation – none were associated with a move out of overweight and only area deprivation showed any statistically significant relationship with the proportion of children who moved into overweight.

As shown in Figure 6‑2, 17% of children living in areas in the most deprived quintile moved into overweight compared with 11% of children living in areas in the least deprived quintile.

Figure 6‑2 Change in children's BMI classification between age 6 and age 10 by area deprivation

6.5. Other factors

Different factors were found to be associated with moving into and moving out of overweight.

6.5.1. Moving into overweight

In terms of moving into overweight, statistically significant differences were found according to maternal BMI, how regularly the child ate breakfast, average weekly screen time and whether or not the child had a TV in his/her bedroom. The results are summarised in Table 6‑1.

Table 6‑1 Percent of children who move into overweight between ages 6 and 10 by selected characteristics

		Move into overweight	Unweighted bases
Maternal BMI
Less than 18.5	%	9	19
18.5 to less than 25	%	11	1040
25 to less than 30	%	14	736
30 to less than 40	%	20	489
40 or over	%	18	62
How often does child eat breakfast
Never	%	15	44
Occasionally	%	21	176
Quite often	%	23	187
Always	%	13	2250
Average weekly screen time
Missing	%	32	34
Less than 14 hours	%	10	864
14 - <21 hours	%	15	942
21 - <28 hours	%	16	532
28 hours or more	%	18	285
Whether child has TV in bedroom
Yes	%	16	1582
No	%	11	1075

Children whose mothers were overweight or obese were more likely to move into overweight than those whose mothers were a healthy weight. Twenty percent of children whose mother's BMI was between 30 and 40 moved into overweight compared with 11% of those whose mother's BMI was between 18.5 and 25. Those who only occasionally or quite often ate breakfast were more likely to move into overweight than those who always ate breakfast (21% and 23% compared with 13% respectively). Higher weekly screen time and having a TV in the child's bedroom were both associated with a move into overweight. For example, 16% of children with a TV in their bedroom moved into overweight compared with 11% of those who did not have a TV in their bedroom.

6.5.2. Moving out of overweight

Birth weight, frequency of eating unhealthy snacks and having a TV in the child's bedroom were all associated with moving out of overweight. A summary of the results is shown in Table 6‑2.

Table 6‑2 Percent of children who move out of overweight between ages 6 and 10 by selected characteristics

		Move out of overweight	Unweighted bases
Birth weight
< 2.5 kg (low birth weight)	%	4	152
2.5 - <3kg	%	3	364
3 - <3.5 kg	%	5	963
3.5 - <4 kg	%	6	811
4 - <4.5 kg	%	8	306
4.5 kg or above	%	3	59
Frequency of eating unhealthy snacks
Up to once a day	%	10	606
More than once a day, up to twice a day	%	4	966
More than twice a day, up to three times a day	%	4	596
More than three times a day	%	4	489
Whether child has TV in bedroom
Yes	%	5	1582
No	%	7	1075

Children with heavier birth weights were more likely to move out of overweight than those who were lighter at birth. Eight percent of children who weighed between 4kg and 4.5kg moved out of overweight compared with 3% of children who weighed between 2.5kg and 3kg. Snacking on unhealthy items less frequently was associated with moving out of overweight. Ten percent of children who ate unhealthy snacks up to once a day moved out of overweight compared with just 4% of those who consumed unhealthy snacks more often. Children who did not have a TV in their bedroom were more likely to move out of overweight than those who did (7% compared with 5%). This was the only variable that was associated with both moving into and out of overweight.

6.6. Multivariable analysis of factors predicting moving into and out of overweight

As noted earlier, given the considerable socio-economic differences both in levels of overweight and obesity and in other factors related to overweight and obesity, it is necessary to control for these underlying relationships in order to obtain some insight into those factors which are independently associated with moves into and out of overweight.

As previously, multivariable logistic regression was undertaken to explore which factors identified in the initial analysis remain statistically significantly associated with move into or out of overweight after controlling for other variations.

6.6.1. Moving into overweight

Given the small number of variables showing statistically significant differences in the initial analysis, all variables – area deprivation, maternal BMI, how regularly the child ate breakfast, weekly screen time, whether child has a TV in his/her bedroom, and whether child gets recommended amount of sleep - were entered into a single model. The outcome variable indicated whether the child had moved into overweight/obesity from being healthy or underweight between age 6 and age 10 with the reference category being children who were healthy or underweight at age 6 and had remained so at age 10. Children who were already overweight/obese at age 6 were excluded from the analysis.

A summary of the results is provided in Table 6‑3. Of the various variables considered, two – maternal BMI and weekly screen time - remained statistically significant after controlling for differences in the other variables whilst a further two – TV in child's bedroom and breakfast habits – narrowly missed statistical significance^[24] but appear to still be of some importance. The findings indicate that, after controlling for other differences between them, compared with children whose mothers were healthy or underweight, those whose mother was obese were more likely to move into overweight between ages 6 and 10. The odds of moving into overweight for children whose mother was obese were 2.5 times those for children whose mother was a healthy weight or underweight. Similarly, compared with children who were reported as having less than 14 hours of screen time per week, those with longer durations of screen time were more likely to move into overweight. The results also tentatively suggest that having a TV in their bedroom and eating breakfast only occasionally or quite often, as opposed to always – whilst narrowly missing statistically significance – may nevertheless also have some association with a move into overweight, after controlling for other differences between the children, including the level of area deprivation.

Table 6‑3 Factors predicting a move into overweight between age 6 and age 10 – summary results^[25]

	Sig.	Odds ratio
Maternal BMI (ref: less than 25)	< .001
Missing		1.939
25 to less than 30		1.338
30 and over		2.512
Average weekly screen time (ref: less than 14 hours)	< .01
Missing		4.743
14 - <21 hours		1.536
21 - <28 hours		1.800
28 hours or more		1.773
Whether child has TV in bedroom (ref: No)	NS (0.06)
Yes		1.282
How often child eats breakfast (ref: always)	NS (0.06)
Never		0.874
Occasionally		1.584
Quite often		1.676
Area deprivation – quintiles (ref: 1 - least deprived 20% of areas)	NS
Whether child gets recommended amount of sleep	NS

Pseudo R squared = 0.08

6.6.2. Moving out of overweight

Again, because only a small number of variables showed statistically significant relationships in the initial analysis, all variables – frequency of unhealthy snacks, whether the child had a TV in his/her bedroom and birth weight - were entered into a single model. To control for differences in these variables by social background characteristics, each of the social background variables was also added – household income, parental education, socio-economic classification and area deprivation^[26]. The outcome variable indicated whether the child had moved out of overweight/obesity between age 6 and age 10 with the reference category being children who were overweight/obese at age 6 and who had remained so at age 10. Children who were a healthy weight or underweight at age 6 were excluded from the analysis.

The results are summarised in Table 6‑4. They indicate that, after controlling for other differences between them, compared with children in highest income group, those in lower income groups were more likely to move out of overweight between ages 6 and 10. Area deprivation was not associated with a move out of overweight.

Children who ate unhealthy snacks more often and those with a TV in their bedroom were less likely to move out of overweight. The relationship with unhealthy snacks was not linear which may reflect the accuracy of the measure. Nevertheless, the odds of children who had an unhealthy snack (sweets, chocolate or crisps) once or more a day of moving out of overweight were around 60% lower than those of children who had such a snack less often. Similarly, the odds of children with a TV in their bedroom moving out of overweight were 40% lower than those of children without a TV in their bedroom.

Table 6‑4 Factors predicting a move out of overweight between age 6 and age 10 – summary results^[27]

	Sig.	Odds ratio
Household annual equivalised income (ref: Top quintile >=£39,216)	< .05
Missing		1.424
Bottom Quintile (<£13,450)		1.061
2nd Quintile (>=£13,451 <£22,827)		2.423
3rd Quintile (>=£22,827< £29,375)		1.375
4th Quintile (>=£29,375< £39,216)		1.257
Frequency of unhealthy snacks (ref: up to once a day)	< .01
Once or twice a day		.427
Three times a day		.390
Four or more times a day		.493
Whether child has TV in bedroom (ref: No)	< .05
Yes		.598
Area deprivation – quintiles (ref: 1 - least deprived 20% of areas)	NS (0.09)
2		.552
3		.663
4		.665
5 Most deprived		.295
Highest parental educational qualification (ref: degree level)	NS
Socio-economic classification (ref: Managerial and professional)	NS
Birth weight (ref: 3 to 3.499kg)	NS