The findings demonstrate that there was a significant increase in the prevalence of overweight/obesity, inadequate intake of fruits and vegetables, physical inactivity, and sedentary behaviour during leisure time, parental tobacco use, physical harm and bullying victimization, loneliness and worry-induced sleep disturbance, ever sex, and suboptimal hand hygiene after using the restroom, among both sexes across three GSHS in 2008, 2015, and 2021 in Thailand. There was also a significant decrease in physical attacks. Among boys, a significant increase of inadequate tooth brushing, and suboptimal hand hygiene (not always using soap to wash hands), and a decrease in smoking quit attempts, physical fighting, and two or more sexual partners, and among girls a decrease in non-condom use was found. Furthermore, among girls, a significant increase in the prevalence of tobacco (cigarettes and other) use, current other tobacco use, current alcohol use, lifetime drunkenness, trouble from alcohol use, lifetime drug use, suicidal ideation and suicide plan was found.
Overweight/obesity, physical inactivity, sedentary behaviour, inadequate fruit and vegetable consumption increased from 2008 to 2021 among adolescents in Thailand. An increase of overweight/obesity among adolescents has been observed globally [29], regionally [30, 31] and in Thailand [32] previously. Among teenagers, the prevalence of sedentary behaviour was roughly twice as high as global estimates (30%), there was a marginally higher prevalence of physical inactivity than the global average of 85% [33], and the proportion of active commuting to school (≥ 3 days/week) (26.6% in 2021) was much lower than global estimates (40.3%) [19]. The increase in childhood obesity, as also found in a trend study in the United Arab Emirates (UAE) [34], may be due to rapid socioeconomic and nutritional transition [35] (greater consumption of processed, sugar-sweetened soft drinks, greater sedentary behaviour and physical inactivity, and lower intake of fruit and vegetables) [36–38]. The enormous increase in sedentary behaviour as also found in a trend study in the UAE [34], may be linked to teens in Thailand using mobile phones and the internet more frequently [39, 40]. In 2021, the percentage of boys and girls experiencing food insecurity was less than 4.0%. It is possible that the school meal programme in Thailand contributed to this decrease in food insecurity [41].
Not among boys, but among girls there was a notable rise in current tobacco use (from 1.3–5.0%) and cigarette use (from 2.2% in 2008 to 6.5% in 2021). Among girls (13–15 years), also slight increases in current cigarette use (from 5.2% in 2009 to 6.5% in 2015) and current other tobacco use (from 1.3% in 2009 to 3.8% in 2015) were found in Thailand based on the Global Youth Tobacco Survey [6, 42]; similar trend results were also found among adolescent girls in Argentina [43]. Parental tobacco use increased in both sexes, passive smoking remained unchanged in both sexes over time, and smoking quit attempts decreased among boys and insignificantly among girls. Among 13-15-year-old school adolescents in Thailand e-cigarette use increased from 3.3% in 2015 to 8.1% in 2021 [44], and among 11-16-year-old school adolescents current e-cigarette use in 2019 was 5.9% among boys and 1.3% among girls [45] and in 2015 based on the Global Youth Tobacco survey, 4.7% of boys, and 1.9% of girls currently used electronic cigarettes [6]. We observed a triple increase of current alcohol use among girls to 28.6% (even higher than in boys 26.6%), which is higher than among adolescents (12–15 years) from 57 lower resourced countries (25.0%) [46]. Similarly, other alcohol use indicators, such as history of drunkenness (27.8%), significantly increased among girls, higher than in the 57-country study among both sexes (17.9%) [46]. It's probable that Thailand's alcohol-marketing tactics support these patterns; for example, children and teenagers reported being more exposed to alcohol advertisements on social media than adults were, and higher exposure to alcohol advertising increased the liking in alcohol advertising, which in turn was associated with alcohol use [47]. However, as part of an overall effort to control alcohol, Thailand's 2008 Alcoholic Beverages Control Act raised the minimum purchase age to 20 years old in order to limit the number of new drinkers [48], with apparently having little deterring effect in alcohol purchasing pattern. The most common source of obtaining alcoholic beverages was in the Thailand 2021 GSHS (aged 11–16 years) through purchase in a store, and to a lower extent from friends and from family (analysis not shown). The rising rates of alcohol consumption and abuse among teenagers in Thailand over time necessitate greater efforts to develop strategies that promote alcohol abstinence and lower alcohol misuse among this demographic [7]. Lifetime drug use was higher in boys (11.1% in 2008) than girls (1.3% in 2008), and remained unchanged among boys but significantly increased in girls to 3.0% in 2021. It's possible that the study's findings about the decline in peer support and parental support among girls contributed to the rise in substance use overall and among girls specifically.
Consistent with some earlier trend studies [34, 49], injuries and bullied increased among both sexes; however, this survey demonstrated that being physically assaulted and engaging in physical fighting decreased over time, as also found in trend studies in Lebanon, Morocco and UAE [34, 49, 50]. One possible explanation for the decrease in interpersonal violence, is the unchanged substance use among boys in this study, and the increase of lack of peer support among females increased bullying victimisation among females. The prevalence of physical injury in this study in 2021 was more than ten percent higher than among teens in four countries in Southeast Asia (boys: 50.3% and girls: 34.3%) [12], and this study's prevalence of bullying victimization (27.5 percent for girls and 33.9% for boys) was comparable to a study conducted among teenagers in five ASEAN countries [13].
Regarding sexual behaviour, among both sexes, ever sex increased significantly from 2008 to 2021, over time, the number of sexual partners fell significantly among boys but not girls, and both boys and girls used fewer condoms. Comprehensive sexual education has been provided in most schools in Thailand, which may have reduced the number of high-risk sexual behaviours [51]. In contrast to research conducted on teenagers in four Southeast Asian nations, the prevalence of ever having had sex (19.8% in 2021) was higher (8.5%), the prevalence of non-condom use (20.8% in 2021) was lower (46.9%) and the proportion of having had multiple sexual partners (35.8% in 2021) was lower (59.7%) than in the four ASEAN nations [15].
All four poor hygiene behaviours increased among increased among both sexes in this study. Compared to studies among adolescents in six Southeast Asian countries, the prevalence of inadequate tooth brushing (17.0% in 2021) was similar (17.1%), sub-optimal hand washing behaviour (70.9% prior to eating, 34.1% after restroom use, and 71.4% using soap) was higher than in the six countries in Southeast Asia (44.8% prior to eating, 31.9% after restroom use, and 55.8% using soap) [16].
From 2008 to 2021, there was a significant increase in three mental health indicators (loneliness, worry-induced sleep disturbance, and no close friends) and all five mental health indicators (suicidal ideation, worry-induced sleep disturbance, suicide plan, and no close friends) among boys as well as in girls. According to this study, there may be a connection between the rise in sedentary behaviour and physical inactivity as well as the notable decline in parental support (connection and supervision) and/or peer support and poor mental health indicators.
From 2008 to 2021, there was a significant increase, among girls in all five mental health indicators (sleep problems, suicidal ideation and plan, being lonely, and no close friends) and among boys three mental health indicators (sleep problems, lonely and no close friends). One possible factor contributing to these increases in poor mental health indicators may be related to the significant decline of peer and/or parental support (supervision and connectedness) and increase in physical inactivity and sedentary behaviour as shown in this study. The prevalence of suicidal ideation (12.1% in boys and 24.0% in girls in 2021) in this study was higher than in a study among adolescents in seven ASEAN countries (9.3% in boys and 15.1% in girls) [52].
In terms of protective factors, school attendance was higher in girls than in boys and stayed that way over time, but peer support declined in girls but not in boys. In this study, parental supervision and connectedness significantly declined over time for both boys and girls, which was also found among adolescent in Argentina [43]. This decline in parental support and decrease in parent-adolescent quality communication has been attributed to rapid socio-economic changes in Thailand [53]. A study conducted among Thai school-aged adolescents revealed a correlation between reduced suicidal ideation and depression and higher levels of parental support [54]. Overall, compared to adolescents in six Asian countries, the prevalence of parental support (supervision 16.9%, connectedness 30.3%, and bonding 42.9%) in this analysis was lower for parental supervision (25.9%) and parental connectedness (30.3%), but not for bonding (36.6%) [55].
Study limitations
The lower secondary school net enrolment rate in Thailand was 94.7% in 2008, 87.1% in 2015, and 95.3% in 2021 [56]. This suggests that some teenagers who did not attend school were left out of the analysis of this study conducted in Thailand. Some GSHS study variables that were only evaluated in one or two waves of the Thailand GSHS, like soft drink consumption, fast food consumption, and suicide attempts, were left out of this paper. Because of the cross-sectional study design, causal inferences are not possible. In spite of the possibility of bias, the GSHS anonymously gathered self-report data that may have reported accurate information, particularly on delicate subjects.