Introduction

Population weight gain in Thailand, as elsewhere, poses a looming health and social problem. Thailand has one of the highest frequencies of overweight and obesity in the Asian region [1]; recent Thai National Health Surveys show the age-standardized prevalence of adult obesity (BMI ≥25, Asian cutoff) increased from 25.6% in 1997 to 30.3% in 2004 [2] and 34.7% in 2009, with a substantial female majority [3]. As a result, diabetes, cardiovascular and other diseases are expected to surge [4] accompanied by economic and social costs and health care system challenges [5].

Population weight increase is a complex, multi-factorial problem attributed to socio-economic growth, urbanization, sedentary lifestyles and dietary change [6]. Reversing the structural determinants of weight gain remains a long-term goal but in the interim, targeting reductions in specific calorie-dense foods and beverages for which good replacements exist may be beneficial. One source of ‘useless’ calories is Sugar Sweetened Beverages (SSBs) including carbonated sweet beverages or soda which promote weight gain through their high sugar content, and a low induction of satiety [7], [8].

Thailand is now a major producer of sugar which Thais have consumed in increasing quantities over the last few decades. Between 1969 and 2003 the estimated intake of kilocalories in Thailand increased from 2110 to 2400 [9], and between 1983–2009 sugar consumption almost tripled from 12.7 to 31.2 kilograms per person per year [10]. Consumption of SSBs, which contain high fructose corn syrup and/or sucrose, has played a substantial part in this increase in Thailand. The most recent National Health Examination Survey in Thailand (2009) reported that over 30% of adolescents aged 6–14 years consumed SSBs almost every day or more often, as did 16% of those aged over 15; relative increases of 50% and 100% since the previous survey in 2003 [3].

While the evidence on the causal relationship between SSBs and weight gain in adults is firming [11], much of it is unhelpful for assessing causality (many studies being too small, too brief or at risk of reverse causality) so it has been challenging to grasp the true nature of the association. A very recent WHO-sponsored review by Te Morenga et al, [12] restricted to the few relevant prospective cohort studies of reasonable duration and randomised trials (RCTs) of adequate design, concludes that SSBs are likely determinants of adult weight gain, despite the still relatively limited data. One prospective study [13] has linked SSB use to modest weight gain in an older Asian population, but the relationship was not examined in detail. In this paper, we add to the available data in general, and substantially expand knowledge in the Asian context, via investigating level of consumption of SSBs in a large national cohort of Thai adults in 2005 and 2009 and their contribution to weight gain over this time.

Methods

Results

Respondents to both surveys were broadly representative of the Thai population on socio-economic, demographic (other than age) and ethnic characteristics. Of the 59 283 who returned questionnaires in 2005 and 2009, 54.8% were females. The median age was 30; 51.5% were urban residents, and the median annual income was $US 2550.

Patterns in SSB consumption

Exposure to SSBs and weight, BMI and obesity levels in 2005 and 2009 are broken down by sex in Table 1, demonstrating that most people consumed SSBs 3 times a month or less (55% in 2005, 65% in 2009), and only a minority drank them every day (7% in 2005, 5% in 2009; Table 1). The calculation of the sample's mean weight change was based on an average of individual weight change calculations. Overall, mean weight increased 1.9 kg (SD 4.3); from 52 kg to 54.2 kg for females (an increase of 1.8 kg) and from 65 kg to 67.1 kg for males, (an increase of 2.0 kg) with a corresponding increase in the prevalence of female obesity from 10.4% to 15. 6%; male obesity from 23.9% to 29.9%; and for both 16.5% to 22.1% (Table 1).

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Table 1. SSB consumption and body size measures in 2005 and 2009 (n = 59,283).

https://doi.org/10.1371/journal.pone.0095309.t001

A heavier SSB consumption pattern of at least three times a week was more frequent in males, in younger persons, in those living in Bangkok or in those who had lived in urban areas for longer, and in those who had less education, were single, or who smoked or drank alcohol (Table 2). These socio-demographic patterns of SSB consumption were similar in 2005 and in 2009 (Table 2).

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Table 2. Characteristics of those consuming 3 or more SSBs per week and weight change between 2005 and 2009.

https://doi.org/10.1371/journal.pone.0095309.t002

Just under half the cohort (37% to 46%) continued to maintain the same level of SSB consumption in the period between the two surveys, although overall, SSB consumption declined. The proportion of persons consuming SSBs three or more times per week dropped from 23% in 2005 to 16% in 2009 (Table 2). This decline is not an artefact of the cohort ageing. Figure 1a, which compares SSB consumption in the two surveys for persons of the same age, illustrates clearly that consumption has declined by about the same amount in all ages and in both sexes.

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Figure 1. SSB and age and age-weight distribution in 2005 and 2009.

https://doi.org/10.1371/journal.pone.0095309.g001

SSB consumption in 2005 and weight change 2005–2009

Figure 1b compares weights in the two surveys for persons of the same age. For both sexes mean weight at every age was higher in 2009 than in 2005 indicating that weight was increasing in the population as a whole. Females, older persons, high school graduates and single persons showed less increase in weight than their comparators (Table 2).

SSB consumption in 2005 was strongly predictive of weight change between the 2005 and 2009 surveys, with increasing consumption frequency statistically significantly associated with greater weight gain in all age groups and in both sexes. Effect modification was modest or absent for most other variables (Table 3). These associations remained after adjusting for baseline socio-demographic factors, smoking and drinking, physical activity (leisuretime, physical activity, screentime and housework/gardening) and consumption of energy-dense foods (deep fried foods and Western-style fast foods).

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Table 3. SSB consumption in 2005 and weight change between 2005 and 2009, by age and sex.

https://doi.org/10.1371/journal.pone.0095309.t003

SSB consumption in 2005 was the strongest predictor of future weight gain among the physical activity and energy-dense diet variables available in our survey (Table 4). In the model, persons who consumed SSBs at least once a day gained 0.5 kg more than those who consumed SSBs less than once a month, slightly less than the overall unadjusted effect of 0.8 kgs (2.4 kgs once a day to 1.6 kgs for less than once a month) (Table 4).

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Table 4. SSB consumption in 2005 and weight change between 2005 and 2009.

https://doi.org/10.1371/journal.pone.0095309.t004

The associations between SSB consumption in 2005 and weight change persisted even when physical activity factors and energy-dense diet as measured in 2009 were included in the model (data not shown). In Table 4 the regression coefficients represent weight change in a given category relative to the reference category of that variable. The only other relevant variable in the model that showed a significant trend was western style fast food consumption in 2005 (p 0.007). The estimated weight gain for the average person in the sample was 1.9 kg (95%C I 1.95–1.96).

Change in SSB consumption between 2005 and 2009 and weight change 2005–2009

Only 20% of persons reported an increased frequency in SSB consumption while 38% reported a decreased frequency between the two surveys and most (42%) reported the same frequency of SSB consumption. Males, older persons, university graduates, smokers and alcohol drinkers were more likely to report no change in SSB consumption frequency (Table 2). Figure 2 shows the weight changes (kgs on the Y axis) estimated for the five combinations of SSB consumption which range from less than once a month to once a day (X axis) at the two time points controlling for socio-demographic characteristics and lifestyle factors (smoking, drinking, physical activity and energy-dense diet in 2005 and 2009). Confidence intervals, shown as vertical grey lines, are narrow for some subgroups due to the large study size and are therefore difficult to see. SSB consumption in both 2005 and in 2009 were statistically significant factors in the model (LR test comparing model with consumption in 2005 and 2009 vs model omitting 2005 consumption: p<0.0001). Overall the average person in the sample gained 1.9 kg (mean SD 4.3). In persons who maintained the same frequency of consumption at the two surveys, weight gain increased steadily (<once/month: 1.5 kg, 1–3/month: 2.0 kg, 1–2/week: 2.1 kg, 3–6/week: 2.4 kg, daily+: 2.8 kg). Due to this study's large sample size, confidence intervals are only operational at the second decimal point rendering them unnecessary. The figure also shows the weight change that occurred for each consumption category as people's consumption varied between 2005 and 2009. For example, people who drank once to 3 times a month in 2005 and increased their consumption to once a day in 2009 increased their weight by almost 3 kgs. In contrast, if their consumption dropped to less than once per month their weight increase was smaller (1.5 kgs). The 20% of men and women who increased their consumption showed consistently greater weight gains compared to persons who maintained the same level of consumption while those who decreased their intake showed consistently less weight gain (Figure 2). The excess average weight gain for those who increased their consumption of SSBs compared to those who maintained it was 0.3 kgs, while persons who decreased their consumption gained 0.2 kgs less than the stable group.

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Figure 2. Estimated weight gain by SSB consumption frequency in 2005 and 2009.

Estimates were derived from a regression model adjusting for socio-demographic characteristics and lifestyle factors (smoking, drinking, physical activity and energy-dense diet).

https://doi.org/10.1371/journal.pone.0095309.g002

Discussion

This large, prospective cohort study of Thai adults provides detailed information on patterns of association between drinking SSBs and subsequent weight gain in Asia. More frequent SSB consumption in 2005 was associated with weight gain between 2005 and 2009 among men and women of all ages, independent of other risk factors. The weight of men who drank SSBs at least three times a week in 2005 increased on average by 2.0 kgs between 2005 and 2009 while the weight of women who consumed in a similar fashion increased by 1.8 kgs. Persons who drank SSBs at least once a day over the 4 years gained on average 0.5 kg more than those who consumed it less than once a month. Weight gain was accelerated in those who increased their intake during follow-up and slowed among those who lowered it. Men, those under 25, urban dwellers, Bangkok residents and those with lower levels of education were likely to consume SSBs more than three times a week. They were also more likely to smoke and drink suggesting that they did not adopt health promotion messages in general.

A current, definitive WHO-sponsored review with a strong public health focus by Te Morenga [12] presents results from prospective cohorts (minimum 1-year follow-up) and RCTs (minimum duration 2 weeks) which assess relations between sugar intake (or removal in RCTs) and measures of body fatness. The subsets which address our central question regarding SSB intake in adults were relatively small (6 papers reporting 7 prospective cohort studies of SSBs with 2–30 years of follow-up; 5 RCTs (lasting 2.5 to 10 months) studying reduced sugar intake, 10 (8 running for less than 8 weeks) adding sugars to the diet). The analyses of cohort data by Te Morenga, (details and figures on the Web, [12]) show fairly consistent positive associations of higher body weights with baseline SSB intake and with increases in SSBs. There is no summary overall estimate to compare directly with ours. The trial data [12] show short-term reductions in sugar intake led to significantly reduced weight overall (−0,8 kg, 95% CI −1.2 to −0.4); trials with added sugars (4 as SSBs) yielded an almost identical increase (0.72, 95% CI 0.3 to 1.2). The 2 longer-term (>8 weeks) trials of added sugars reported much larger increases, 2.7 kg, 95% CI 1.7 to 3.8. The 5 cohort studies in children included in the meta-analysis all showed increases in fatness over varied time periods among those consuming higher amounts of SSBs, OR  = 1.6 (1.3–1.8) (34). It has been harder to reproduce the weight benefits of reducing sugars in RCTs in children, with poor compliance being a major challenge. Where this has been met in the longer term (a year or more), results are solidly suggestive of benefit [17], [18], [19], [20].

In the only other large cohort study of Asian adults (older Chinese Singaporeans), participants who consumed SSBs 2 to 3 times a week had a significant increase in weight (0.53 kg over 5.9 years) compared with those who drank less than monthly [13], quite similar to our estimate; no further details were reported. Added to the above findings, our results strengthen and extend the evidence, for Asia in particular as well as in the wider sphere, that drinking SSBs in even moderate amounts leads in the mid-to-longer term to increases in weight; that increasing intake increases this effect; and perhaps most importantly, that diminishing it reduces weight gain. There are good reasons for accepting these observations as reflecting the causal link proposed by Hu (11). While it is difficult to put an exact figure on the size of the obesogenic effect of drinking SSBs, it is clearly substantial and continuing over time; there is high consistency in the effect in both cohort studies and RCTs (where confounding by other dietary factors is avoided), including lowered risk with decreased consumption in trials and in our cohort and others [21], [22]; associations are greater at higher intakes; directionality of effect is clear. There is an obvious mechanism of adding ‘empty’ calories with limited effect on satiety [7] with no compensatory reduction in overall energy intake in contrast to the ingestion of solid sugars which induce a compensatory reduction in energy intake [7]. Thus targeting SSB consumption as an accessible lever in public health campaigns to control weight gain in both children and adults, in Thailand and elsewhere, makes good sense [11]. Of course simply identifying the lever is insufficient in itself, but these products stand out from many other energy-providing components of our diets as being more readily substitutable, and thus potentially more amenable to control through a mix of personal and social interventions.

In light of the evidence demonstrating the public health benefits of reducing SSB consumption it is heartening to note that in contrast to increasing SSB consumption in Thailand [3] and internationally, the proportion of the Thai cohort consuming SSBs three or more times a week declined from 23% in 2005, to 16% in 2009 (independent of age and sex) with only 20% reported increased frequency of consumption. Thailand has Asia's largest per capita consumption of carbonated beverages at 39.2 litres [23]. A business report observed that the SSB market has maintained a 6% growth even while sales of bottled water, fruit/vegetable juice and other drinks [24] increased. Thailand has campaigned since 2002 to reduce children's sugar consumption [25] and is raising awareness among adults of the health risks of sugar and SSBs [26]; a message that may be more likely to be picked up by TCS members than the less educated general population.

Generally, in western countries SSB consumption has increased only slightly over the last several decades but appears now to be slowing. Recent business reports show that Australia's consumption of soda has declined slightly since 2005 [27], recent increases in SSBs in the UK were a modest 0.7% [28] decreases are occurring in the US [29] with a Wall Street Journal expressed concern about declining US soda sales [30]. However, consumption of other types of SSBs such as sports and energy drinks may have increased among adolescents [31].

One limitation of this study was that we only recorded self-reported frequency of consumption so it is not possible to estimate the contribution of SSBs to total energy intake as an explanation for the observed association. However, using a recent estimate of average Thai daily energy consumption as 3100 kcal per person [32], and a 12 oz can of soda as a serve (150 kcal see [8]), we estimate SSBs contributed 4.83% of the daily energy consumption to those who drank them daily.

A second limitation is that our survey question used in 2005 was “How often do you consume soda?” This question was not part of a validated food frequency questionnaire and it is less inclusive than a question about SSBs more broadly; it does not capture consumption of sweetened teas, flavoured milk and sports drinks contributing to an under-estimation of sugar consumption from SSBs. Nor did we specifically ask about artificially sweetened or diet drinks; however, these are known to make up only about 1 to 3% of the Thai SSB market [33], [34]. It is likely that SSB intake is, if anything, under-reported overall due to the use of this single question. Self-reported weight is known to be quite accurate in our cohort [16], so overall it is unlikely that the relations we observed were exaggerated by measurement error; if anything, the exposure misclassification will have had the opposite effect.

A third limitation is loss to follow-up although the rate (32% = 27 851/87 134) was reasonable for a current large-scale observational study. There were some minor differences between respondents and non-respondents. We have no direct measure of whether non-response could be related to the outcome of interest (i.e. weight change) [35], [36], but the prevalence of obesity among respondents at baseline (17%) and non-respondents (14%) was similar. The main reason for non-response was loss of follow up contact with younger and more mobile cohort members; 52% of respondents were aged 30 or younger compared with 72% of non-respondents; and daily SSB consumption was somewhat lower among respondents than non-respondents (23% to 30%). As well as age there are a number of other covariates which were related to some degree to both non-response and outcome (marital status, urbanisation type, location of residence, education, smoking and fried food consumption); their inclusion in the regression model will have further mitigated the potential for selection bias to affect results materially. The possibility of residual confounding by other dietary components has to be considered given the limitations in our dietary data and the mild confounding of the unadjusted estimates indicated by our multivariable results. This will of course have been offset at least in part by the exposure misclassification.

A final limitation is that cohort members are more educated and somewhat younger than the general Thai population [14]; however, associations and trends identified among the cohort are generally consistent over time and may be manifested among the wider Thai population in the near future. This study derives from a large sample size drawn from diverse economic, social and geographic backgrounds and is similar to the STOU student body from which it is drawn. It is only the second study known to us conducted among an Asian population vulnerable to the effects of weight gain, type 2 diabetes, metabolic syndrome and other diet-related chronic diseases. Adding to the importance of this study is the growing body of evidence linking SSB consumption directly with these conditions [7], [8] particularly in vulnerable Asian populations [13] and among Thais who are comparatively short statured [37] and more prone to metabolic challenge.

The decrease in SSB consumption in the Thai cohort may signal future directions for the Thai population, particularly in light of Thailand's campaign to reduce sugar consumption. Thailand, which is a model for progressive food and nutrition policy [38], [39], has already restricted advertising and sales of sugar products to children. Nevertheless, successful SSB-lowering trials in children and adults elsewhere offer examples for further large scale public health interventions in Thailand; for example combining different approaches to SSB reduction at the Thai province or district level for comparison. International trials [17], [19] also illustrate the need for continued public health efforts to maintain reduced SSB consumption and weight over time. Nevertheless, SSBs should not be the only focus of health promotion; their consumption often co-occurs with low physical activity levels and energy-dense diets which are independently associated with weight gain. Indeed not only should reductions be made in the population's energy consumption via diet but environmental changes to increase the populations' use of energy should be encouraged [40], [41].

Conclusion

Sugar consumption, particularly in SSBs, is now considered to be a major health threat comparable to smoking [38] and evidence suggests that removing SSBs from diets will have a positive impact on weight over time. They increasingly are a target for health promotion because they have no redeeming nutritional benefits and can be eliminated from the diet without ill effects. Within our Thai cohort we will continue to monitor SSB consumption and links with diabetes, metabolic syndrome and dental health and we will increase the precision of our capture of SSBs; we note the desirability of other longitudinal studies doing the same. As a key ‘transitional’ consumable, further study of the economic and socio-cultural trends related to SSB consumption in Thailand will help illuminate the country's health and nutrition transition.

Acknowledgments

We thank the staff at Sukhothai Thammathirat Open University (STOU) who assisted with student contact and the STOU students who are participating in the cohort study. We also thank Dr Bandit Thinkamrop and his team from Khon Kaen University for guiding us successfully through the complex data processing.

The Thai Cohort Study Team contributed to the development of the study and research instrument. It consists of: ^*Thailand: Jaruwan Chokhanapitak, Suttanit Hounthasarn, Suwanee Khamman, Daoruang Pandee, Suttinan Pangsap, Tippawan Prapamontol, Janya Puengson, Sam-ang Seubsman, Boonchai Somboonsook, Nintita Sripaiboonkij, Pathumvadee Somsamai, Duangkae Vilainerun, Wanee Wimonwattanaphan, Cha-aim Pachanee, Wimalin Rimpeekool, Tewarit Somkotra, Arunrat Tangmunkongvorakul, Benjawan Tawatsupa Australia: Chris Bain, Emily Banks, Cathy Banwell, Bruce Caldwell, Gordon Carmichael, Tarie Dellora, Jane Dixon, Sharon Friel, David Harley, Tord Kjellstrom, Lynette Lim, Anthony McMichael, Tanya Mark, Penny Haora, Adrian Sleigh, Lyndall Strazdins, Susan Jordan, Janneke Berecki-Gisolf, Rod McClure, Vasoontara Yiengprugsuwan.