Study sample

Student self-report data from the Health Behaviour in School-aged Children (HBSC) survey and the School Health Research Network (SHRN) surveys in Wales, from 1998 to 2017 were used. Surveys were conducted approximately every two years from 1998 to 2017. Data are appended over the years to create a repeated cross-sectional dataset as in previous studies [27]; Data on SSBs were available from 1998–2017, and EDs from 2013–2017. The HBSC survey, a collaborative cross-national survey, is administered every four years and currently involves 50 countries and regions across Europe and North America. The SHRN survey is administered every two years and is based on the HBSC survey allowing integration of the two surveys every four years. Over-time the SHRN survey sample size has grown due to the increasing number of schools in Wales agreeing to conduct the survey. Details on study sampling strategies and procedures can be accessed elsewhere [28, 29].

Sociodemographic characteristics

Gender (response options: ‘Boy’ and ‘Girl’) and school year were reported in all survey years. School year and corresponding age groups were: Year 7 (age 11–12), Year 8 (age 12–13), Year 9 (age 13–14), Year 10 (age 14–15) and Year 11 (age 15–16). An indicator of socioeconomic status (SES) was available from 2002, using the Family Affluence Score (FAS) [30, 31] which comprised measures of: car and computer ownership, bedroom occupancy and family holidays. From 2013 onwards, two additional measures (dishwasher and bathroom ownership) were included [32]. Scores for each of the four/six survey items were summed for a total score, whereby a higher score indicated greater affluence. This score was split at the median in each survey year to achieve ‘low’ and ‘high’ SES.

Definitions of outcome variables

Time trends

Time trend analysis results are shown in Figs 1 and 2. The proportion of students consuming SSBs daily decreased steeply from 2000 to 2006, appearing to plateau from 2009, dropping from 57% to 18% across the time series in 2017. Similarly, the proportion reporting never or less than weekly consumption increased four-fold from 7% in 1998 to 29% in 2017. Weekly SSB consumption increased steadily since 2000 and has remained constant since 2006 at 49% to 52% in 2017. More detailed analysis showed that the ‘Once a week’ and ‘2–4 times’ a week were mostly attributable to the increase (See S13 Table). Regression analyses indicate that relative to the highest consumption category (i.e. daily consumption), consumption of SSBs never or less than weekly increased significantly over time (RRR 1.08, p<0.05, CI 1.08–1.09), while weekly consumption also increased compared to daily consumption (RRR 1.05, p<0.05, CI 1.04–1.05). Hence, findings indicate an overall trend toward declining SSB consumption over time, indicated by increasing movement of the population toward lower consumption categories.

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Fig 1. Reported SSB consumption between 1998 and 2017.

https://doi.org/10.1371/journal.pone.0248847.g001

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Fig 2. Reported ED consumption between 2013 and 2017.

https://doi.org/10.1371/journal.pone.0248847.g002

Since 2013, daily ED consumption has remained stable at 6%. Weekly ED consumption has steadily decreased from 23% in 2013 to 15% in 2017. Conversely, a steady increase in reports of never or less than weekly consumption is estimated, 71% in 2013 vs. 79% in 2017. Upon further inspection, this steady increase reflects the increase in reports of ‘Never’ consumption over the years (see S15 Table). Regression analyses indicated that relative to daily consumption, reports of never or less than weekly remained unchanged over time relative to daily consumption (RRR 1.00, p<0.05, CI 0.98–1.02) although weekly consumption decreased over time relative to daily consumption (RRR 0.89, p<0.05, CI 0.87–0.91); note that when year was treated as categorical, RRR’s showed an increase for never and less than weekly consumption (2015 RRR = 1.05, 2017 RRR = 1.03), and a decrease for weekly consumption (2015 RRR = 0.92, 2017 RRR = 0.68). Hence, while very regular consumption remains stable, the proportion of adolescents consuming EDs has fallen.

SSB consumption and demographics

School year.

Older students were less likely to consume SSBs never or less than weekly, compared to daily consumption; Year 9s and 10s were the least likely to report SSB consumption as never or less then weekly (RRR 0.70, p<0.05, CI 0.67–0.73/0.74). Year 8s had the highest likelihood of consuming SSB’s never or less than weekly (RRR 0.86, p<0.05, CI 0.82–0.90). For weekly SSB consumption, a similar pattern was observed but Year 10’s and 11’s had the highest consumption risk (RRR 0.79, p<0.05, CI 0.76–0.83) and Year 8’s had the lowest (RRR 0.95, p<0.05, CI 0.91–0.99). For the full model, see Table 2. Over time, 21% of Year 7’s consumed SSBs daily, compared to 24% of Year 9’s, and 25% of Year 11’s (S6 to S10 Tables).

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Table 2. Multinomial regression of SSB consumption and sociodemographic characteristics with daily use as the reference category (n = 157,564).

https://doi.org/10.1371/journal.pone.0248847.t002

ED consumption and demographics

Gender.

When comparing ED consumption to daily consumption, girls were more likely to report consumption as never or less than weekly compared to boys (RRR 1.85, p<0.05, CI 1.77–1.95), however this was not apparent for weekly consumption reports (RRR 1.00, p = 0.91, CI 0.94–1.05) (Table 3). Over time, 7% of boys consumed EDs daily compared to 4% of girls; with little change across years (see S3 Table).

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Table 3. Multinomial regression of ED consumption and sociodemographic characteristics with daily use as the reference category (n = 135,712).

https://doi.org/10.1371/journal.pone.0248847.t003

Demographic patterning of SSB and ED consumption overtime

The association between SES and SSB consumption changed over time. Where models used the reference category of ‘daily consumption’, lower socioeconomic groups were less likely to respond ‘never or less than weekly’ in SSB consumption compared to higher socioeconomic groups (RRR 0.91, p<0.05, CI 0.88–0.95). Descriptive data (S4 and S5 Tables) indicate that in 2002, consumption was very similar for higher and lower SES groups (e.g. 37% daily consumption for both groups), and while consumption has fallen for both groups, it has done so fastest in higher SES groups, leading to increased inequality (i.e. in 2017, 21% of young people from poorer families report daily consumption vs 16% of those from more affluent families). Regression models show that whilst both groups increased their reports of ‘never or less than weekly’ SSB consumption, the rate of increase was slower for low socioeconomic groups, indicating greater movement toward non-consumption in more affluent groups (S1 Fig). Likewise, lower socioeconomic groups were less likely to consume SSBs weekly compared to high socioeconomic groups (RRR 0.92, p<0.05, CI 0.89–0.95). S2 Fig shows that the socioeconomic pattern has changed over time in this model. In 2002, lower socioeconomic groups were more likely to consume SSBs weekly, however by 2004 this reversed with higher socioeconomic groups being more likely to using them weekly (compared to daily). As a result, the gap between low and high socioeconomic groups has widened, with lower socioeconomic groups being more likely to consume SSB’s daily compared to weekly. This trend has changed in most recent years, with weekly consumption decreasing since 2015 for both socioeconomic groups.

The absence of time varying effects via other characteristics such as gender, school year and SES suggests that individual characteristics of SSB consumers have remained relatively stable over time, with RRR’s being 1.00–1.01; likewise, for EDs consumption, with wider confidence intervals observed. For interaction estimates see S12 Table.

Overall consumption trends

Almost 80 years since the first calls for a reduction in SSBs [5, 15] our findings provide an encouraging outlook on trends in SSB consumption among adolescents. We found that consumption reports since 1998 indicate a positive shift for daily SSB consumption with approximately 40% fewer adolescents reporting daily consumption in 2017 compared to 2000. A noticeable upward trend was observed for the number reporting never or less than weekly SSB consumption. ED consumption was not measured prior to 2013, but showed small decreases over time with one in four young people using EDs at least weekly in 2013. Recent findings in Denmark also displayed a decrease in daily SSB consumption between 2002 and 2018, albeit lower prevalence rates were observed in 2018 among Danish adolescents at 6.4% [24]. Compared to other HBSC countries, daily SSB consumption rates are somewhat lower than Malta, Belgium and Bulgaria at 34–37% [25]. The overall ED consumption rates for Wales are comparable to findings among Canadian adolescents [33] while lower rates were previously reported among a Korean population (11.4%) [34]. Comparable weekly SSB consumption rates were recently reported among Australians aged 15 or older [35]. While the sampling of differing populations, data collection tools and analyses makes direct comparisons of prevalence difficult, present findings provide the first insights into consumption trends of young people in Wales.

A number of environmental and policy changes may have contributed to the observed reduction in daily SSB consumption between 2000 and 2009. In 1996, the UK was reported to have one of the highest proportions of overall food advertisements worldwide, with 79% of adverts devoted to sweet or high fat foods. Since, more stringent advertising guidelines have been introduced to reduce young people’s exposure to advertising of unhealthy food products. Between 2001 and 2007, a series of school food policies were also introduced across Wales in an attempt to improve the nutritional standards of food and drink provided in schools [36, 37]. Despite revealing a substantial decrease in daily SSB consumption rates, 52% of adolescents continue to consume SSBs on a weekly basis and 6% still consume EDs daily. As such, further political action is required to maintain downward trajectories, notwithstanding any impacts which may have since occurred because of the 2018 SIDL. Furthermore, as the global ED market is forecast to reach a net worth in excess of $84 billion by 2025 (projected 7% increase in sales) [12], it is of public importance that consumption trends among young people continue to be monitored.

Socioeconomic patterning in consumption

We found clear patterning of SSB and ED consumption according to SES, observing higher consumption rates among young people from lower socioeconomic groups. These findings echo those of wider studies concerning adolescent SSB [25, 38, 39] and ED [38–40] consumption rates. With a lack of current legislation, the present findings are a potential reflection of the current marketing and availability landscape, with EDs being as affordable as SSBs [40] and some marketing trends disproportionately aimed at minority youth consumers [41]. A rapid UK-based review highlighted that ‘own brand’ EDs are often available at a cheaper price than water with young people preferring cheaper, less well-known varieties [42].

Our time trends analyses indicate that inequalities in SSB consumption have increased over time. While in 2002 there was no socioeconomic difference in SSB consumption, and declines have been observed for both groups, these have been faster among children from more affluent families. Hence, whilst actions to date may have led to an improvement in SSB consumption at the population level, actions may have also inadvertently contributed towards growing inequality. A key rationale for introducing a SDIL in 2018 was the expected equitable impact on population health (observing greater health gains in those with the worst health problems). With earlier UK-based models projecting a potential widening of inequalities due to the SDIL however [43], unearthing the impacts of the SIDL among young people is vital as are urgent policies aimed at reducing inequalities.

Present findings are in line with the current obesity landscape, which also reveals a persistent widening of inequalities, as the gap between child obesity prevalence in the most and least deprived areas of Wales continues to broaden [44]. It is widely accepted that considerable effort will be required to halt the growing inequalities in obesity rates as child poverty is likely to increase and in turn inequalities will persist or worsen. As part of a nationwide strategy [20], the Welsh Government seeks to reduce the impact of ill health and inequality, which includes a reduction in the diet inequality gap between the most and least deprived communities. Present findings can inform the divergent trends noted among obesity in young people, yet at present, actions towards a UK-wide legislation for ED sales appear to have become stagnant [18] and the evaluation of the 2018 Sugar tax is ongoing [22].

Our findings have important implications for practitioners and policy makers alike, demonstrating how secular consumption trends are disparate between socioeconomic groups, an area which is pivotal for the introduction or modification of responsive interventions. Pinpointing the underlying factors which contribute to such socioeconomic differences is key to ensuring policy interventions facilitate healthy food choices for all population groups [23]. Future work will look to examine any differences in consumption rates across socioeconomic gradients in light of the introduction of the soft drinks levy in 2018.