Urban and rural Differences in the Prevalence of Gender and Age specific Obesity and related Health Behaviors in Korea

Obesity is one of the major predictors for heart disease and other chronic diseases including hyperlipidemia, hypertension, atherosclerosis and hyperinsulinemia (1). The prevalence of obesity is increasing rapidly worldwide (2). The principal causes of the increasing obesity problem have known to be sedentary lifestyles and a high-fat and energy rich diet.

Body mass index (BMI) is widely used in the adult population to define overweight and obesity although it is a less ideal method for measuring body fat. The cut off points of 25 kg/m² and 30 kg/m² have been recognized internationally as definitions of adult overweight and obesity, respectively (3). However, recent studies have shown that Asians have a lower BMI but a higher percentage of body fat than Caucasians at the same BMI (4, 5). Deurenberg et al. also suggested that the relationship between BMI and body fat percentage differs between ethnic groups (6). Therefore, increased health risks associated with obesity appear to occur at a lower BMI in Asians. Several Asian countries and the World Health Organization (WHO) expert panel proposed different cut-off values as overweight (BMI 23-24.9 kg/m²) and obesity (BMI ≥25 kg/m²) based on the morbidity of their own population (7-10).

Aging is associated with changes in body composition. Cross-sectional and longitudinal studies suggest an increase in body mass and adiposity with aging despite decreases in energy intake or considerable physical activity especially until the ages of 65-70 yr although the patterns seem to differ by gender (11-13). Those studies reported that men had a progressive body mass gain from early adulthood to late middle age while women had a continuous increase from early adulthood to postmenopause. Nationally representative cross-sectional data showed that the prevalence of obesity by BMI in men increased with aging until 35-45 yr of age, followed by plateau in Japan, but a continuous increase in United States until 65-75 yr of age (14, 15). On the contrary, women's BMI increased continuously as they increased in age for both countries. The BMI started to decrease after 65-75 yr of age in both men and women in those countries.

The economic growth in the Republic of Korea increased markedly over the past several decades. At the same time, Western lifestyles including their diet has been introduced rapidly into the country. In addition, industrialization and growth of the technology sector have led to a decrease in physical activity. As those lifestyles change, obesity has also increasingly become a social problem in Korea (16, 17). A study from the Indian population reported the prevalence of obesity by residential difference, which may reflect the degree of industrialization in a country, and suggested that those urban-rural differences would be due to the differences in lifestyle factors including physical activity and nutrition by residents (18). However, few studies focused on the distribution of BMI and trends of related health behaviors, especially based on the effect of gender, aging and residential areas in current representative Korean population although certain transition must exist cross-sectionally.

The purpose of this study was to discuss the distribution of BMI and obesity related health behaviors by gender, age and residence in Korea using the data obtained from the 1998 National Health and Nutrition Survey gathered by Ministry of Health and Welfare of Republic of Korea and Korea Institute for Health and Social Affairs in November 1998.

Data from the 1998 Korean National Health and Nutrition Survey were used in the study. The survey employed a stratified multistage probability sampling design. A total number of 12,283 Korean households with 39,060 household members participated in the Health Interview Survey. One out of three samples from the Health Interview Survey were selected to conduct a Health Examination and Nutrition Survey (19). Anthropometric data were available for 10, 880 participants aged 10-94 yr. Among those, adults aged 20-64 yr were selected to examine the differences in the trends of obesity and related health behaviors. Subjects over the age of 65 yr were excluded from this analysis because BMI started decreasing after 65 yr of age in Korean women. This may be considered as a reflection of selection survival in the population. Finally, 5,670 were used to analyze the data in this study.

Height and weight were measured based on standard procedures and BMI was calculated by dividing the weight in kilograms by the square of the height in meters.

Dietary intake was assessed by the single 24-hr dietary recall method. Experienced interviewers instructed respondents to recall and describe the foods and beverages consumed over the previous 24 hr. Energy, protein, fat and carbohydrate intake were calculated and reported, and they were also expressed as a percentage of total energy.

Adults aged 20-64 yr were further divided into three groups using a 15-yr age interval, 20-34, 35-49 and 50-64 yr. Residential areas were divided into three categories as big cities, small cities and rural areas. The average BMI and energy, protein, fat and carbohydrate intakes were calculated and compared by age and residential areas in each gender.

After the distribution of BMI was analyzed by original WHO standards for western countries and recent suggestions for Asians by WHO Expert Panel (BMI≥30 as obesity for westerners, 30>BMI≥25 as overweight for westerners or obesity for Asians, 25>BMI≥23 as overweight for Asians, 23>BMI≥18.5 as normal weight for Asians, BMI<18.5 as underweight for westerners and Asians), the prevalence of obesity using the Asian classification BMI≥25 was compared by age and region categories in each gender to examine age, gender and residential specific prevalence.

The percentage intake of fat from total energy was classified as >20% of energy from fat and ≤20% of energy from fat. Self-reported alcohol consumption, smoking, exercise, the degree of usual daily activity and weight reduction information were obtained from the questionnaire and the proportions were compared by age and region as well. Alcohol consumption was classified as heavy (heavy drinking once or more than once per month) and non-heavy drinkers (drinking less than once per month or none) from the question "How often do you drink heavily?". Smoking status was classified by the participants' self-report as current versus former or nonsmokers. Exercise was classified as regular (at least 20 min at a time, 3 or more times per week) and non-regular (the rest of them). The degree of daily activity was classified as intense versus nonintense from the question "how intensely can you rate your daily activity?" Information on weight reduction status was classified into the current weight reduction group when the answer was weight reduction from a self-reported question "which weight control do you try?"

The predominant finding of this study was that the prevalence of obesity (BMI ≥25) was significantly different by age and residential areas in Korean men and women. Aging seemed to be a significant contributor in the increase of obesity for both Korean men and women despite the gender difference pattern. This increasing trend by aging in a cross-sectional study was a common phenomenon in other countries as well (9, 21, 22). The 1999-2000 National Health and Nutrition Examination Survey of the U.S.A. showed a continuously increasing prevalence of obesity by aging for men. The 1998 nutrition survey in Japan showed that the prevalence of obesity in men increased to 35-45 yr of age, followed by plateau. On the contrary, women's BMI increased continuously as age increased in both countries.

When the effects of residential areas were considered for obesity trends, these aging effects were different by residential areas in Korea. Although younger people aged 20-49 yr did not show a prevalence difference of obesity by residential areas, people at the ages of 50-64 yr living in urban areas, which are more industrialized, showed the highest prevalence of obesity for men. In rural areas, which usually keep a more traditional lifestyle, men aged 50-64 yr showed the lowest prevalence of obesity. However, in women, these urban-rural effects were applied differently in each age group. In Korean women, a higher prevalence was found in rural areas compared to urban areas in the younger age group (20-49 yr), but not in the older age group.

The urban-rural differences of obesity prevalence in this study demonstrate the various stages of behavioral transition that Korea is currently undergoing, which may be explained by lifestyle and certain social factors. Urban areas had significantly higher rates of people who ate more than 20% energy from fat than rural areas did, especially in older population for both men and women, although age had a greater effect on fat intake than residential areas did. Among lifestyle factors, self-rated usual activity as intense showed the biggest urban-rural differences, which may explain the lower prevalence of obesity in rural older population. The higher exercise rate in urban areas might show that Koreans in urban areas did exercise more in their leisure time than those in rural areas, while non-leisure daily activity level were higher in rural areas. We believe that the industrialized lifestyles including high fat intakes and low usual intense activity led to a high prevalence of obesity, whereas, this higher obesity status combined with higher health concerns got older urban people more involved in exercise.

Higher prevalence of obesity in younger rural Korean women was not explained by lifestyle factors that were analyzed in this study. We think this may reflect one industrialized trend, having a strong desire to be "thinner" due to social pressure, hence, young urban women showed a lower prevalence of obesity than rural women did (23). However, since we only had information about the rate of weight reduction, and not on the degree of weight control or desire in this study, further research is needed to be conducted. Longitudinal trend analysis on BMI and lifestyle factors in Japan by Yoshiike et al. from 1976-2000 also reported similar obesity trends by residential areas to those of Koreans. They showed a very prominent increasing trend in overweight among males in rural populations and a significant decrease in overweight among females in the metropolitan areas in Japan (14).

Although the overall prevalence of obesity (BMI≥25) in the Korean population is quite low compared with western societies, it reached around one-fourth of Korean adults, which is higher than the prevalence in other Asian countries such as China, the Philippines and Japan (2, 14). A higher prevalence in Korea than even in Japan, a developed country, was a surprising result, which should be taken into consideration for obesity control at national levels.

The strength of this study was that it analyzed a national representative sample obtained from the nationwide health examination survey in Korea (1998). External validity is therefore sufficient to interpret the findings for the general population of Korea. Our study provided variable information about how the distribution of obesity would be characterized in Korea, connected with other studies using this data mostly providing a significant relationship between obesity and metabolic disorders (24-26).

A major limitation of this study would be its cross-sectional nature, suggesting that it does not provide sufficient evidence of causality. Other limitations include the recall bias of the single 24-hr recall method although we used the information for only estimating population means. The reliance on self-reporting answers with undefined self-perceived degree on questions such as "heavy" for alcohol drink and "intense" for usual activity could be also the limitation to interpret our results.

In conclusion, our study revealed that different age-specific trends of obesity coexists depending on residential areas in Korea, especially more in older generation. These urban-rural differences on obesity might reflect the difference of other health related behaviors such as lifestyle traditions, in which rural residents necessarily impose a more active and traditional lifestyle.