This study uncovered a depression prevalence rate of 26.9% among secondary school students. This discovery indicates a lower prevalence of depression when compared to a study conducted by Normala et al., which involved 1800 Malaysian secondary school adolescents aged 13 to 17 years and reported a depression prevalence of 32.7% among adolescents. It is noteworthy that Normala's study used the same depression assessment tool, the PHQ-9, but was conducted on a more limited scale, focusing on 10 out of 37 randomly selected secondary schools in the Hulu Langat district of Selangor [21]. When compared to NHMS 2019, the prevalence in the current study was higher. However, it is essential to note that NHMS 2019 is a national household survey focusing on the entire Malaysian population, not specifically on adolescents [5]. Furthermore, when making a comparison between the prevalence of depression among Malaysian adolescents and Japanese adolescents, it's evident that Malaysia had a notably higher rate, standing at 26.9% compared to Japan's 11.1% [22]. Conversely, the results of our current study aligned with those from Bangladesh, where the prevalence of depression among adolescents was also reported to be 26.5% [23]. It's important to note that both the studies conducted in Japan and Bangladesh utilized the same assessment tool as the one employed in our current investigation, which is the PHQ-9. In conclusion, the variation in depression prevalence can be attributed to a multitude of factors, including demographic characteristics, methodological disparities, as well as cultural, geographical, socioeconomic, and socio-political influences. Understanding these factors is essential for designing effective health policies and programmes that can enhance the health and well-being of adolescents in diverse countries.

The outcomes derived from the multiple logistic regression analysis unveiled that female adolescent exhibited a higher susceptibility to encountering depression, aligning with findings from prior research studies [2, 24]. Conversely, earlier research has posited that the elevated prevalence of depression among adolescents might be associated with factors like inadequate parental care, where some parents exhibit stricter behaviours towards female adolescents when compared to their male counterparts. Additionally, there exists a prevalent notion of diminished expectations concerning the capabilities and accomplishments of female adolescents in comparison to males [25]. Moreover, gender and cultural backgrounds can play a role in these disparities. It has been observed that male adolescents often attempt to endure depression symptoms unless they reach an extremely severe state, whereas females tend to express their symptoms at earlier stages [26]. These multifaceted elements collectively contribute to the divergent experiences of depression between female and male adolescents.

In terms of ethnicity, our study revealed that Malays, Other Bumiputeras, and Others were more prone to experiencing depression compared to Indians. This finding contrasts with a previous study that had indicated a higher susceptibility to depression among Chinese and Indian individuals [2]. However, due to limited survey data, we were unable to conduct a more comprehensive examination of the impact of these factors.

Furthermore, our current study has illuminated that older adolescents are at a higher risk of experiencing depression, a trend consistent with the findings from a study conducted by Xu H et al. [27]. This implies that as adolescents progress in age, they confront an increased vulnerability to developing depressive symptoms. Several contributing factors may underlie this pattern, including the physical, emotional, and social transformations experienced during adolescence, amplified academic pressures, evolving family dynamics, and heightened emotional awareness among older teenagers [28,29,30]. A comprehensive understanding of these intricacies is essential for addressing the mental well-being of older adolescents.

Recent research findings have hinted at a possible connection between less healthy eating habits and depression [10, 11]. By shedding light on these modifiable factors, we can deepen our understanding and empower public health authorities, healthcare professionals, and individuals to more effectively address and alleviate depression.

The results of this study have unveiled a noteworthy correlation between unhealthy dietary behaviours, especially the consumption of fast food, and the presence of depression. These findings are in line with previous research, including prospective studies involving Iranian [31] and Korean adolescents [32]. The link between fast food consumption and depression can be attributed to several factors. Firstly, fast foods tend to be deficient in essential nutrients and are often characterized by elevated levels of saturated fats, refined sugars, and additives. This subpar nutritional profile may impact the normal functioning of the brain, particularly its role in neurotransmitter synthesis, including serotonin—an important neurotransmitter closely tied to mood regulation. Serotonin, often referred to as the "feel-good" neurotransmitter, plays a pivotal role in controlling mood, emotions, and overall well-being [33,34,35]. A deficiency in serotonin levels has been associated with the emergence of depressive symptoms. The inadequacy of key nutrients in fast foods, which are essential for supporting serotonin production, could disrupt the delicate balance of neurotransmitters in the brain. This disruption might contribute to negative mood states and potentially elevate the risk of depression [36, 37]. In essence, the observed association between fast food consumption and depression in this study underscores the significant impact of the nutritional quality of one's diet on mental well-being. This highlights the importance of making informed dietary choices to promote both physical and mental health.

The current study has unveiled a significant positive association between the intake of carbonated soft drinks and the presence of depression. This discovery aligns with prior research, including a study involving Norwegian adolescents that also identified a connection between soft drink consumption and mental health concerns [38]. Moreover, multiple earlier studies have investigated the cross-sectional relationship between high carbonated soft drink consumption and depression or depressive symptoms [39,40,41,42]. It's essential to acknowledge that sugar-sweetened beverages, such as carbonated soft drinks, fall under the category of beverages containing caloric sweeteners. Previous research has indicated a link between excessive consumption of sugary beverages and an elevated risk of depression [7, 31]. To shed light on a potential biological mechanism underlying this association, researchers have put forth a hypothesis. Refined sugar, often abundant in sweetened beverages, has been shown to inhibit the growth hormone known as brain-derived neurotrophic factor (BDNF). A decrease in the normal functioning of BDNF has been associated with depression [43, 44]. In simpler terms, excessive refined sugar intake, as commonly found in sugary beverages like carbonated soft drinks, may have an adverse effect on the production and function of BDNF, potentially contributing to the onset or worsening of depressive symptoms. In the light of these findings, there is a compelling case for implementing policies aimed at restricting the availability of sugary beverages within educational institutions, such as schools, as well as in community and recreational centre. Concurrently, promoting the consumption of water as an alternative beverage choice could offer substantial benefits to the overall health and well-being of adolescents.

Contrary to the commonly held belief that unhealthy eating habits lead to depression, previous research offers an alternative perspective, suggesting that depressive symptoms can play a role in influencing unhealthy eating behaviours. Earlier studies have found that adolescents experiencing symptoms of depression encounter greater difficulties in maintaining a healthy diet [13]. Additionally, an interesting observation is that adolescent girls may resort to snacks and junk food as a way of coping with their depressive symptoms [45]. A comprehensive study revealed a connection between poorer psychological health and a tendency to consume soda, sweetened drinks, French fries, and fast foods [46]. The emergence of depressive symptoms might steer individuals towards less healthy dietary choices, with emotional eating behaviour playing a crucial role as food becomes a means of regulating negative emotions [47]. Research explains that the consumption of sweet and fatty foods induces positive feelings and alleviates stress by affecting certain neurotransmitters like dopaminergic, along with enhancing the function of the serotonergic system [48]. As a result, individuals dealing with depression may show a preference for consuming foods high in sugar or fat, especially when facing perceived stress [48]. This intricate interplay highlights the complex relationship between mental health and dietary habits, emphasizing the need for a nuanced understanding when addressing these dynamics.

Similarly, our study has revealed a significant association between the adoption of healthy dietary habits, specifically the consumption of fruits, and the presence of depression. These findings align with previous research outcomes [15, 32]. This observed phenomenon may be attributed to the nutritional richness of fruits, as they are known for their diverse and abundant nutritional content. They serve as plentiful sources of dietary fibre, essential vitamins, and minerals [49], and they are also packed with dietary antioxidants [50]. These components collectively contribute to the nutritional value of fruits. Firstly, dietary fibre, which is abundant in fruits, plays a crucial role in maintaining digestive health. It aids in regulating blood sugar levels and ensures a consistent energy supply throughout the day, reducing the likelihood of energy fluctuations and mood swings. Additionally, dietary fibre supports a healthy gut microbiome, and emerging research suggests that the connection between the gut and the brain could have a significant impact on mood regulation [51]. Secondly, fruits are rich in essential vitamins and minerals that are fundamental for overall well-being. Some of these nutrients, such as B vitamins like folate and minerals like magnesium, have been associated with mood regulation. For instance, deficiencies in folate have been linked to an increased risk of depression. The presence of these nutrients in fruits may contribute to a more balanced and stable mood [52,53,54]. Lastly, the dietary antioxidants found in fruits are renowned for their ability to combat oxidative stress and inflammation in the body. Oxidative stress and inflammation have been implicated in the development of mood disorders, including depression [54]. Therefore, the antioxidant-rich nature of fruits may help mitigate these factors and exert a protective effect on mental well-being. In summary, the observed negative correlation between fruit consumption and depression in our study suggests that the nutritional richness of fruits, encompassing dietary fibre, essential vitamins, minerals, and dietary antioxidants, may collectively contribute to their potential in alleviating depressive symptoms. This underscores the significance of incorporating a variety of fruits into one's diet not only to promote physical health but also to support mental well-being. However, it's worth noting that in our study, we did not find a significant link between adolescents' vegetable consumption and depression, despite previous research findings suggesting such a connection [15, 16, 32].

Similar to any scientific inquiry, the current study presents both strengths and limitations that necessitate consideration. One of its notable strengths lies in the comprehensive and ethnically diverse composition of the study's participant pool. In essence, the large and ethnically varied sample size equips the study with the capability to detect meaningful relationships and associations within the data, thereby elevating the reliability of the findings. Furthermore, this study distinguishes itself by focusing on a population-based sample rather than a clinical one. This distinction holds particular significance because it broadens the study's sphere of relevance. By examining depressive symptoms and dietary behaviours within a broader, community-based sample of adolescents, the findings are more likely to have practical applicability in the real world. In other words, the insights gleaned from this research extend beyond a specific clinical context and can be more broadly applied to a larger population of adolescents.

However, it's essential to acknowledge a significant limitation inherent in the study's design, namely, its cross-sectional nature. A cross-sectional study collects data from participants at a single point in time, offering a snapshot of information. While cross-sectional studies are valuable for identifying associations and trends within a population, they are incapable of establishing causality. In other words, the study can reveal that certain dietary behaviours are linked to depressive symptoms, but it cannot definitively ascertain whether one directly causes the other. Additionally, all the information in the study was self-reported, introducing the possibility of information bias and recall bias that may impact the conclusions.