INTRODUCTION
A plethora of studies have already investigated the association between general obesity and depression. However, a previous meta-analysis has found that the association between abdominal obesity and depression is stronger than between general obesity and depression [1]. Abdominal obesity in such studies was defined by waist circumference (WC) [2]. Several ACCESSstudies have reported the association between abdominal obesity and depression in South Korea. One of the studies found that women aged between 44 and 56 with abdominal obesity showed a decreased risk of having depressive symptoms [2]. This is called the “jolly fat” effect which posits that there is a negative association between weight and depressive symptoms [3]. This effect pertains more to older age adults as their functional limitations and other health conditions can result in changes in mood and weight [4]. In support of this, another study reported that women with general obesity and abdominal obesity, measured by body mass index (BMI) and WC respectively, were less likely to have depressive symptoms in low-stress conditions [5]. There have been a few studies that investigated this association further with the Asian population. Previous research examined the gender difference in the association with a sample over 45 years old [6]. The results indicated that males with abdominal obesity, measured by WC, were less likely to suffer from depressive symptoms than those without abdominal obesity. Such an association, however, was not present among female participants. Contradictory findings have emerged regarding the relationship between obesity and depression, challenging the “jolly fat” hypothesis. For instance, a previous study focusing on Korean women found that the prevalence of depression was higher among those with abdominal obesity compared to those in the normal weight group, with the study cohort having an average age of 49.9 years [7]. Additionally, a meta-analysis indicated that the positive association between BMI and depression is more consistent among females [8]. Despite these findings, there is a scarcity of research specifically examining the association between obesity and depression within a broader Korean population cohort, encompassing a wider age range and both sexes. This paper seeks to clarify the direction of the association between abdominal obesity and the risk of depression in the Korean population.
This study is also interested in examining the association by using WC to measure abdominal obesity. One of the few previous studies that investigated the relationship between obesity and depression in adults used a general bodyweight status defined by BMI rather than abdominal obesity defined by WC [9]. Research, however, has found that abdominal obesity is not proportional to overweight defined by BMI [10]. BMI has been criticized for not accurately measuring the degree of regional adiposity or the accumulation of fat in certain areas [11]. Hence, it has been suggested that WC is a more accurate measure of abdominal obesity which illustrates the abdominal visceral adiposity. It has also been found that visceral fat that is present in abdominal obesity produces a larger amount of cytokines than subcutaneous fat [12,13]. A high level of cytokines has been found in both visceral obesity and in depression, which emphasizes the use of WC for a more accurate measurement of obesity. Therefore, this current study used WC to measure abdominal obesity and examined its association with the risk of depression.
Finally, in this study, we aimed to investigate the association between abdominal obesity and the risk for depression between sexes and evaluate any sex differences using a nationwide cohort study in South Korea. A previous study has found a difference in obesity rate between sexes with a higher obesity rate in women in low- and middle-income countries than in men [14]. A meta-analysis has also found that the association between BMI and risk of depression depends on gender [8]. Since it has also been found that biological diathesis factors can predispose women to have a greater risk of developing depression than men, it is important to evaluate the association between abdominal obesity and any existing sex differences [15]. Based on these findings, we hypothesized that abdominal obesity and the risk for depression are positively associated and that this association is stronger for women than for men.
METHODS
Data collection
This study used a dataset from the National Health Insurance Sharing Service (NHISS) of South Korea [16]. NHISS is a public insurance institution in South Korea where nearly 97% of the population is enrolled. NHISS contains various datasets including admission, emergency room visits, ambulatory care visits, pharmaceutical services, health checkups, and death. During the health checkup, people are primarily screened for smoking, drinking, exercise, BMI, and cholesterol, and diagnostic history such as diabetes, hypertension, and hyperlipidemia was extracted from the claims data. Mental health conditions such as depression were also screened along [17]. The dataset includes health examination data of those who underwent a medical examination in 2009 and their claims data from 2002 to 2018. A total of 10,585,844 people underwent a health checkup from January 1st, 2009 to December 31st, 2009. Furthermore, the NHISS data are anonymized to protect confidentiality [18]. The study protocol was approved by the Institutional Review Board of the Samsung Medical Center (IRB: SMC 2020-08-102). Informed consent was waived since this research did not involve any identifiable private information of study patients.
Abdominal obesity (WC)
Abdominal obesity was measured by WC and was further divided into six levels: ≤79, 80-84, 85-89, 90-94, 95-99, ≥100 for males and ≤74, 75-79, 80-84, 85-89, 90-94, and ≥95 for females (all in cm). According to the Korean Society of the Study of Obesity, they proposed that the cut-off value for abdominal obesity is 90 cm for males and 85 cm for females [19].
Outcome (depression)
The study measured depression as the main outcome. Participants were diagnosed with depressive disorder during the follow-up period based on the International Statistical Classification of Disease and Related Health Problems 10th revision (ICD-10). Newly onset depression was defined as ICD-10 CM codes for depression; major depressive disorder with single episode for F32.0-32.9 and major depressive disorder with recurrent episodes for F 33.0-33.9.
Definitions of covariates
Demographic variables included age, sex, income level as divided under 20% or above, current smoking status, amount of alcohol consumption, and physical activity. Three levels of alcohol consumption were identified which 0 g of alcohol per day were non-drinkers, <30 g per day was mild drinkers and ≥30 g of alcohol per day was heavy drinkers [20]. Other factors that were included in the standardized health examination data included height, weight, WC, fasting glucose, blood pressure (systolic and diastolic), total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, triglycerides, and creatinine level.
Statistical analyses
Before conducting a statistical analysis, any missing data and those who were previously diagnosed with depression (ICD-10 codes: F32, F33) before the index year were excluded from the study. We presented continuous variables as mean± standard deviation (SD) and categorical variables as numbers and percentages. Independent t-tests and chi-square tests were used to compare the differences in factors between groups. Cox proportional-hazard regression analyses were used to examine the association between abdominal obesity and the likelihood of depression. Model 1 was non-adjusted; model 2 was adjusted for age and sex; model 3 was adjusted for age, sex, smoking, drinker level, regular exercise, low-income status, baseline diabetes, hypertension, and dyslipidemia. Statistical analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA), and a p-value <0.05 indicated significance.
RESULTS
Demographics
Among 9,041,751 participants, 1,376,279 were diagnosed with depression. Table 1 shows the baseline characteristics of the study participants. The lowest WC level (<80 cm for males and <75 cm for females) had the greatest number of participants (n=3,381,945). Meanwhile, the highest level of WC (≥100 cm for males and ≥95 cm for females) had the smallest number of participants. The mean age was youngest for the group with the lowest WC (M=41.6, SD=13.3 years) and the oldest for the group in the 5th level of WC (M=51.5, SD=14.2 years). Regarding smoking levels, participants in the second WC level were more likely to be current smokers while participants in the fourth WC level were more likely to be exsmokers. Finally, there were more non-smokers in the first level of the WC.
In terms of alcohol consumption, there were both more non-alcohol drinkers and heavy drinkers with the highest level of WC. Finally, there were more mild drinkers in the second level of the WC. Participants in the second level of WC showed the highest level of regular exercise.
The association of abdominal obesity with risk of depression by sex
The average follow-up length was 7.6 years (SD=1.9). Overall, 1,376,279 subjects were newly diagnosed with depression during the follow-up period, and the incidence rate was 20.0 per 1,000 person-years. Overall, it was found that the higher one’s WC is, the higher the risk of depression with a longer follow-up period. The group with the highest level of WC indicated the highest risk of depression (Figure 1). Males in the fourth, fifth, and sixth levels of WC all showed an increased risk for depression (hazard ratio [HR]=1.02, 95% confidence interval [CI]: 1.01-1.03 for the fourth level; HR=1.05, 95% CI: 1.04-1.07 for the fifth level; HR=1.09, 95% CI: 1.07-1.11 for the sixth level in Model 3) (Table 2). Meanwhile, the male participants in the second level of WC did not show an elevated risk (HR=1.00, 95% CI: 0.99-1.01 in Model 3) for depression. Males in the first level of WC showed an increased risk (HR=1.03, 95% CI: 1.02-1.04 in Model 2) for depression only when controlled for age and sex.
For females, participants in the fourth, fifth, and sixth levels of WC showed an increased risk for depression (HR=1.01, 95% CI: 1.00-1.02 for the fourth level; HR=1.03, 95% CI: 1.02- 1.04 for the fifth level; HR=1.03, 95% CI: 1.02-1.05 for the sixth level in Model 3) (Table 2). However, female participants in the first and the second level of WC showed a decreased risk for depression (HR=0.93, 95% CI: 0.92-0.93 for the first level; HR=0.97, 95% CI: 0.97-0.98 for the second level in Model 3).
The association of abdominal obesity with risk of depression by age group
The age was divided into three groups for both sexes: 20- 39, 40-64, and older than 65. Males who are older than 40 showed an elevated risk for depression with higher WC levels (HR=1.09, 95% CI: 1.07-1.11 for age 40-64; HR=1.08, 95% CI=1.05-1.11 for age older than 65) than males who are aged between 20 and 39 (HR=1.05, 95% CI: 1.02-1.09). Older males who are underweight had a higher risk for depression (HR=1.03, 95% CI: 1.02-1.04 for age 40-64; HR=1.04, 95% CI: 1.02-1.06 for ages older than 65) than younger males (HR=1.00, 95% CI: 0.99-1.02) (Table 3).
Females who are aged between 40 and 64 showed the highest risk of depression with higher WC (HR=1.08, 95% CI: 1.06-1.10) than younger and older females (HR=1.06, 95% CI: 1.00-1.13 for age 20-39; HR=1.01, 95% CI: 0.99-1.03).
DISCUSSION
In this study, we investigated the association between abdominal obesity and the risk for depression by sex and age group in a nationwide cohort study in South Korea. To our knowledge, this is the first study that demonstrated the association between abdominal obesity and the risk of depression by sex with a wide range of ages.
The results demonstrated four major findings. First, in general, there was a positive association between abdominal obesity and the risk of depression. Such positive association was more present in males than in females. Underweight males showed an elevated risk for depression whereas underweight females showed a decreased risk for depression. Finally, the risk of depression increases with the WC and longer followup periods. Our hypothesis was supported for females and partially supported for males.
We further examined the association between abdominal obesity and the risk of depression by looking into sex differences. While both sexes showed a positive association, males showed a stronger association than females. Moreover, only males showed an increased risk for depression when underweight. A possible explanation for this difference in sex could be attributed to weight stigma consciousness which posits how strongly one is aware that they are discriminated against based on their weight. A previous study has found that weight stigma consciousness impacted binge eating among males only [21]. This association was significantly mediated by depression more for males than for females. Therefore, abdominal obesity among males could be associated with their weight stigma consciousness along with their level of depression and their binge eating habits.
This sex difference could be due to ideal body image. Men tend to idealize muscular bodies and therefore their perception of being underweight leads to dissatisfaction [22]. This condition was associated with depression symptoms among males [23]. In contrast, women idealized a lean body and their perception of a larger body, leading to dissatisfaction and associated with depression symptoms. Hence, the difference in the perception of body images could explain why underweight males are more likely to have an increased risk of depression than underweight women.
The results of our study do not support the previous findings regarding the jolly fat hypothesis. It partially contradicts several previous studies [2,6,24]. Previous research examined the association with a sample over 45 years old [6]. The results of the previous study indicated that males with abdominal obesity, measured by WC, were less likely to suffer from depressive symptoms than those without abdominal obesity. However, such an association was not present among female participants. These findings were partially corroborated by another study in China with elderly aged over 60 [24]. They found that the elderly with general obesity and abdominal obesity are less likely to have depression symptoms regardless of sex. However, the findings from this current paper contradict these findings. In general, the findings indicated that there is a positive association between abdominal obesity (WC) and the risk of depression in both males and females.
There are several plausible reasons underlying these inconsistent results between previous studies and this current study. First, the previous study used BMI to measure obesity [9]. Since this current study used waist circumference to focus on abdominal obesity, it is possible that these two different measurements measured different aspects of weight. It has been found that health consequences due to obesity could be underestimated by BMI [25]. BMI does not accurately indicate body fat and therefore, the inverse association between obesity and the risk of depression from the previous study could be attributed to the different measurements from the current study.
Second, our data takes into consideration a wider age range of the population. The present study differed from previous studies by incorporating participants of younger age. We had participants whose age was between 20 and 39 whereas previous studies focused mainly on a population older than 40. As previously stated, the jolly fat hypothesis pertains more to older age adults [4]. Hence, the difference in the age range of the population could explain why the current findings refute the previous findings on the negative association between abdominal obesity and the risk of depression.
The study, however, still has limitations that call for future studies. First, the study could not control for an additional variable that influences depression. For instance, deterioration in cognitive abilities in the elderly and various stress levels can affect their mental health such as depression, which can also lead to poor eating habits [9]. Second, as this is a cross-sectional study, it precludes any conclusion on a causal relationship between abdominal obesity and diagnosis of depression. Moreover, although the measurement of WC is more accurate than BMI measurements, it is important to note that abdominal obesity is characterized by an accumulation of visceral fat [26]. As an alternative, computerized tomography scans can provide a more reliable and valid measurement of abdominal obesity.
Finally, the dataset we utilized has some limitations that may introduce potential biases. Waist circumference was measured manually by medical staff, which could lead to measurement variability. Additionally, data on alcohol consumption and exercise were self-reported, which may result in inaccuracies due to recall bias or social desirability bias. Furthermore, depression diagnoses may sometimes be influenced by factors unrelated to the actual presence of the condition, such as the need to prescribe medication. Therefore, the subjectivity inherent in these datasets warrants careful consideration.
Despite these limitations, there are strengths of the present study to put forward. First, to our knowledge, this is the first study to study the association between abdominal obesity and the risk of depression using WC. Moreover, incorporating younger participants in the study allowed us to elaborate on existing results in the research field. Finally, a comparison of the association between two sex was possible as the study included both of the Korean population.
In conclusion, this present study shows that in general, abdominal obesity is positively associated with an increase in the likelihood of depression. It is also important to note that underweight males are also at risk of having depression.
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