depression is a mood disorder characterized by mild to severe anxiety , depressive mood , and 
 feelings of failure and worthlessness1 . 
 due to the recent economic crisis and the rapid social changes that promote nuclear family 
 households and excessive individualism ,
depression is strongly linked to suicide attempts , 
 making it a significant threat to public health2 .
the world health organization has estimated that , by 2020 , 
 depression will be the second most prevalent health concern worldwide after heart disease 
 and will be the second highest contributor to societal economic burden2 .
this steady increase in the global prevalence of depression 
 affects individuals and society as a whole . as a result
, numerous studies are being 
 conducted to identify the causes of depression and to develop effective treatments .
regular physical 
 activity has been shown to effectively decrease levels of anxiety and depressive mood ; it 
 can also enhance work performance3 and 
 improve various cognitive processes and performance by stimulating the central nervous 
 system4 .
furthermore , studies have shown 
 that physical activity reduces stress and decreases depressive symptoms among individuals of 
 all ages5 .
given the evidence for the 
 positive impact of physical activity on mental health , a lifestyle that incorporates regular 
 exercise and continuous physical activity is recommended for individuals of all ages . however , most previous studies of physical activity and depression have been limited by 
 their sole focus on a specific age group and/or gender , such as adolescents , married women , 
 or the elderly6,7,8,9,10,11 .
although many studies have investigated the prevalence of 
 depression in these particular age groups and have identified influencing factors , none has 
 examined the depressive symptoms in the general adult population of south korea12 .
therefore , using 2011 korea health panel 
 ( khp ) data , which represents national - level data , we analyzed the relationship between the 
 intensity of physical activity and symptoms of depression among individuals 20  years of age 
 or older in order to establish a baseline to be used in developing physical activity 
 programs that can improve depressive symptoms .
we analyzed data from the khp , which is conducted by the korea institute for health and 
 social affairs and the national health insurance corporation to investigate patterns of 
 health service use and how certain factors affect the use and cost of healthcare services . 

this survey also provides raw data that can be used for in - depth analyses .
the survey data 
 are collected annually via self - report questionnaires and in - person interviews , and the 
 first survey commenced on 1 april 2008 .
we used and analyzed the data available from the 
 2011 khp , which collected information from a total of 17,035 individuals , among who 12,946 
 were aged 20  years or over .
we excluded individuals with missing data ; therefore , a final 
 sample population of 12,350 participants was selected for this study .
this study was 
 approved by the institutional review board of the catholic university of korea with a waiver 
 for informed consent because the data were obtained from a public database 
 ( mc14eisi0091 ) .
participants were divided into two groups based upon responses to the question ,  during the 
 past year , have you felt sad or hopeless for more than two weeks to the extent that it 
 affected your daily activities ?  we selected potential factors that could influence 
 depressive symptoms , including physical activity , socio - demographic and economic 
 characteristics , and health behavior .
the intensity of the physical activity was categorized 
 as vigorous , moderate , and walking .
examples of vigorous - intensity physical activities were 
 running , hiking , playing soccer or basketball , jumping rope , playing singles tennis or 
 squash , swimming , bicycling at a fast pace , lifting heavy objects , and walking at a very 
 brisk pace .
moderate - intensity physical activities included playing volleyball , badminton , 
 table tennis , doubles tennis , calisthenics , moving light objects , swimming or bicycling ( at 
 a slow pace ) , and brisk walking . walking slowly or at a moderate pace for the use of public 
 transportation and brisk walking were included in the walking category .
we used the guidelines suggested by kim13 to divide the participants into exercising and non - exercising 
 categories based on the number of days and hours in which they took part in physical 
 activity .
the intensity of the physical activity was based on the physical activity 
 recommendations of the centers for disease control and prevention and the american college 
 of sports medicine , and these activities were categorized as follows : those who perform 
 vigorous - intensity activity for a minimum of 20 minutes at least three days each week ; those 
 who perform moderate - intensity physical activity for a minimum of 30 minutes at least five 
 days each week ; and those who walk for a minimum of 30 minutes for at least five days 
 weekly .
individuals who did not exercise regularly were placed into the non - exercising 
 group .
socioeconomic characteristics used in this study included gender , age , marital 
 status , area of residence , and education level .
age was divided into four categories : 
 2034  years , 3549  years , 5064  years , and over 65  years .
marital status was categorized as 
 single , married , or divorced / separated / widowed .
the participant areas of residence were 
 divided into either urban , including seoul and other metropolitan cities , or 
 non - metropolitan cities .
educational level was classified into four categories : completed 
 elementary school or less , completed middle school , high school diploma , and completed 
 four - year college or higher .
the respondents economic status was assessed by household income , health insurance status , 
 and employment status .
the annual household income is the combined gross income of all the 
 members of a household .
we used the 2011 average exchange rate ( 1 usd=1,127.4 korean won ) to 
 convert korean won to us dollars .
the characteristics of health behaviors , including cigarette smoking , alcohol consumption , 
 chronic disease status , and eating habits ( e.g. eating at regular times ) , were examined . 

regardless of the 
 amount of cigarette or alcohol consumption , current smokers and/or alcohol drinkers were 
 designated as cigarette smokers and/or alcohol drinkers , respectively .
individuals with at 
 least one chronic disease diagnosed by a physician and lasting for longer than three months 
 were considered to have a chronic disease ; if there had been no diagnosis , they were 
 considered to be free of chronic diseases .
respondents were also asked if they ate meals at 
 regular times in order to assess their eating habits .
we performed a frequency analysis and a descriptive statistical analysis to calculate the 
 frequency , percentile , mean , and standard deviation of the variables , including physical 
 activity , socio - demographic and economic characteristics and health behavior .
separate 
 univariable logistic regression analyses for each variable and for the intensity of physical 
 activity were performed to examine their associations with depression .
after controlling for 
 confounding variables , a multivariable logistic regression analysis was conducted to 
 identify the association between depression and the intensity of physical activity . 

statistical analysis was performed on the survey data using the ibm statistical package for 
 the social sciences ( spss ) software ( version 21 ; spss inc . ,
table 1table 1.general characteristics of the study population , and results of simple logistic 
 regression ( n = 12,350)variablesn ( % ) /mean  sdor(min , max)genderfemale6,581 ( 53.3)1.81male5,769 ( 46.7)refage ( years)20342,290 ( 18.5)ref35493,996 ( 32.4)0.9450643,250 ( 26.3)0.58652,814 ( 22.8)0.59marital statussingle1,952 ( 15.8)refmarried8,889 ( 72.0)0.88divorced / separated / widowed1,509 ( 12.2)0.46regionrural6,882 ( 55.7)refurban5,468 ( 44.3)1.11educationless than elementary school2,697 ( 21.8)refmiddle school1,412 ( 11.4)1.09high school3,944 ( 31.9)1.63university4,297 ( 34.8)2.14annual household income ( unit : usd)35,531  24,329 ( 80 , 222,013)1.00health insurancemedical aid604 ( 4.9)refnational health insurance11,746 ( 95.1)3.68economic activityno4,803 ( 38.9)refyes7,547 ( 61.1)1.65smokingno9,568 ( 77.5)0.76yes2,782 ( 22.5)refdrinkingno3,699 ( 30.0)0.62yes8,651 ( 70.0)refchronic diseaseno4,725 ( 38.3)2.16yes7,625 ( 61.7)refregular mealsno2,651 ( 21.5)refyes9,699 ( 78.5)1.65vigorous physical activityno10,833 ( 87.7)refyes1,517 ( 12.3)1.90moderate physical activityno11,009 ( 89.1)refyes1,341 ( 10.9)1.19walkingno9,570 ( 77.5)refyes2,780 ( 22.5)0.88sd : standard deviation ; or : odds ratio ; ref : reference . p<0.05 ; 
 p<0.01 ; p<0.001 shows the distribution of the subjects general characteristics and the 
 relationships between these characteristics and depressive symptoms as analyzed by 
 univariable logistic regression .
gender , age , marital status , education , annual household 
 income , health insurance type , economic activity , smoking , chronic disease , eating habits , 
 and rigorous physical activity were significantly associated with depressive symptoms . 

participants were distributed as follows : 12.3% performed regular vigorous physical 
 activity , 10.9% took part in regular moderate physical exercise , and 22.5% walked regularly . 

individuals participating in vigorous physical activity were 1.90 or less likely to be 
 depressed ( p<0.001 ) than those who were not engaged in any physical activity , while there 
 was no significant difference between individuals participating in moderate physical 
 activity or walking and individuals who were not engaged in any physical activity ( table 1 ) .
p<0.05 ; 
 p<0.01 ; p<0.001 we performed logistic regression analysis after adjusting for all confounding factors in 
 order to identify the relationships between the intensity of physical activity and 
 depressive symptoms .
there were significant associations between depressive symptoms and 
 gender ( p<0.001 ) , annual household income ( p<0.001 ) , health insurance type 
 ( p<0.001 ) , employment status ( p=0.016 ) , chronic disease ( p<0.001 ) , regular eating 
 pattern ( p<0.001 ) , and vigorous physical activity ( p=0.004 ) .
participants who had higher household incomes were 1.00 or more 
 likely to have less depressive symptoms .
respondents who were insured by the national health 
 insurance were 2.34 or more likely to not have depressive symptoms than those with medical 
 aid .
the unemployed had a 1.21 or greater risk of being depressed than those who held jobs . 

individuals with irregular eating habits were 1.73 or more likely to be depressed than those 
 with regular eating patterns .
furthermore , only vigorous physical activity had a significant 
 association with depressive symptoms , which is consistent with the results of the unadjusted 
 univariable analysis , which showed a 1.49 or higher incidence of being depressed among 
 individuals who did not participate in vigorous physical activity than those who did ( table 2table 2.relationships between physical activity intensity and depressive symptoms 
 according to multiple logistic regression ( n = 12,350)variablesor95% cigenderfemale ( ref)male1.57***1.3121.889age ( years)2034 ( ref)35490.900.6731.18950640.740.5361.027 651.120.7811.617marital statussingle ( ref)married1.200.9031.586divorced / separated / widowed1.050.7521.473regionrural ( ref)urban1.030.8971.187educationelementary school or less ( ref)middle school0.940.7491.184high school1.070.8571.339university1.110.8481.451annual household income1.00***1.0001.001health insurancemedical aid ( ref)national health insurance2.34***1.8572.941economic activityno ( ref)yes1.21 * 1.0351.404smokingyes ( ref)no1.120.9031.376drinkingyes ( ref)no0.880.7501.020chronic diseaseyes ( ref)no1.75***1.4532.108regular mealsno ( ref)yes1.73***1.4822.028vigorous physical activityno ( ref)yes1.49**1.1371.943moderate physical activityno ( ref)yes1.020.8051.300walkingno ( ref)yes0.880.7531.039or : odds ratio ; ci : confidence interval ; ref : reference . p<0.05 ; 
 p<0.01 ; p<0.001 ) . or : odds ratio ; ci : confidence interval ; ref : reference . p<0.05 ; 
 p<0.01 ; p<0.001
in this study , we analyzed the four characteristics of physical activity level , 
 socio - demographic and economic characteristics , and health behavior in order to investigate 
 the relation between physical activity level and depressive symptoms .
of all the variables 
 we examined , gender , annual household income , health insurance type , employment status , 
 chronic disease status , regular eating habits , and vigorous physical activity showed 
 significant associations with depressive symptoms .
we found that vigorous physical activity was strongly associated with a decrease in 
 depressive symptoms , while moderate physical activity and walking had no significant 
 relationship with depressive symptoms .
previous studies have demonstrated that physical 
 activity can decrease stress by impacting hormone levels , improving mood , and increasing 
 energy , as well as reducing anxiety and anger14,15,16,17,18 ; 
 our findings are consistent with these reports , which suggests that a lack of physical 
 activity is strongly associated with depressive symptoms .
this study contradicts a previous study that concluded that vigorous exercise is negatively 
 associated with depression , while mild to moderate exercise decreases depressive 
 symptoms7 .
however , other studies are 
 consistent with our findings , indicating that vigorous exercise reduces the symptoms of 
 depression .
suggest that only vigorous exercise is significantly associated with 
 depression19 ; similarly , ledwidge found 
 that perform aerobic exercise , which was considered a vigorous physical activity in the 
 present study , is effective at alleviating the symptoms of depression20 , 21 .
moreover , 
 morgan22 , sime23 , and folkins and sime4 suggested that vigorous exercise , including jogging , swimming and 
 cycling , reduces depressive symptoms or anxiety , but mild physical activity , such as 
 walking , is not effective at improving mental health .
the differences in basic physical 
 strength and exercise intensity preference based on age group may explain these 
 findings24 .
furthermore , a personal 
 preference either for individual or group exercise can affect a person s psychological mood 
 after exercise , such as depression .
in addition to the intensity of the physical activity , this study also showed statistically 
 significant associations between depressive symptoms and other variables , including gender , 
 annual household income , health insurance status , employment status , chronic disease status , 
 and a regular eating pattern , which are consistent with most previous studies25,26,27,28,29,30,31,32,33,34 . 
 among these variables , annual household income , health insurance type , and employment status 
 are income - related variables .
they indicate that depressive symptoms are reported less often 
 by those with a higher income . according to ho et al .
, quality of life influences 
 depression , suggesting that income is closely associated with perceived quality of life26 . also , individuals with a chronic disease 
 are more likely to be depressed than those without chronic disease31,32,33,34,35,36,37,38 , implying a 
 strong association between depression and health - related quality of life39 . while most previous studies of physical activity and depression have focused on certain age 
 groups , we analyzed nationally representative khp data of adults aged 20  years and over .
our 
 findings are significant in that they indicate there are different correlations between 
 physical activity and depression depending on the intensity of the exercise .
further studies 
 will be needed to build upon our findings and analyze the relationship between physical 
 activity and depression in an adult population .
future studies should also include those 
 diagnosed with depression secondary to a physical illness , which would help identify the 
 individual factors that influence related variables .
 [ purpose ] the aim of this study was to examine the relationship between the intensity of 
 physical activity and symptoms of depression . [ subjects and methods ] we analyzed the 
 influence of different intensities of physical activity on depressive symptoms using the 
 data of 12,350 adults over the age of 20  years who had completed the 2011 korea health 
 panel . after controlling for confounding variables , 
 a multivariable logistic regression 
 analysis was conducted to identify the association between the intensity of physical 
 activity and depressive symptoms . 
 [ results ] the results showed that vigorous physical 
 activity had a significant effect on depressive symptoms . 
 the incidence of depressive 
 symptoms was 1.487 ( 95% ci 1.137 , 1.943 ) or higher among people who did not participate in 
 regular activity than it was among those who took part in regular intense physical 
 activity . 
 [ conclusion ] it appears that physical activity affects depressive symptoms , a 
 result which is in agreement with previous studies . 
 however , we also showed a difference 
 in influence according to activity intensity . 
 thus , the intensity of physical activity 
 should be considered when developing physical activity programs for improving depressive 
 symptoms . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION