Soy foods are Korean traditional foods and have been set as daily foods in the forms of not only fermentation products such as soybean sauces and pastes, but also soybean sprouts and processed foods such as soy milk and tofu, but the consumption has been gradually decreased compared to the past because of the influence of westernized dietary lives (Lee et al., 1999, 2000, 2004). Recently, as a variety of functions of isoflavone contained in the soy have been uncovered, soy foods have been spotlighted in the aspects of not only chronic degenerative diseases but also the functions of soy proteins and physiologically active substances (Adlercreutz, 1997; Ho, 1999; Martin et al., 1978; Messina et al., 1991; Maskarinec et al., 1998). An epidemiological study suggested that high intakes of soy foods in Asian people might be one of the reasons Asians had less chronic degenerative diseases and menopausal disorders than western people (Ho, 1999).

Isoflavone is a kind of phytoestrogens, also called a plant estrogen, that play roles similar to those of estrogen in mammals including humans. It has been reported that isoflavone could prevent various chronic diseases such as prostate cancer, breast cancer, cardiovascular diseases, and osteoporosis (Adlercreutz & Mazur, 1997; Sung, 1994). Isoflavones found in the soybean are mainly genistein and daidzein, which have been known to have various roles in disease prevention effects of the soybean, and there have been some trials for finding new isoflavone in soybean fermented food (Esaki et al., 1998; Klaus & Brarz et al., 1998). It has been reported that a Korean dietary life using lots of soybean products could reduce the incidence of prostate cancer or breast cancer, suggesting the cancer preventive possibilities of soybeans (Adlercreutz & Mazur, 1997; Harrison & Cho, 1999; Lee & Harrison, 1998; Lee et al., 1999; Maskarinec et al., 1998; Sung, 1994; Sung et al., 2000).

To date, the database on isoflavone contents in Korean foods has been extremely lacking and thus an objective index for judging the intake level is necessary. Recently, there has been some research on the evaluation of isoflavone intakes in middle-aged women and adults in some regions, but research on soy foods and isoflavone intake levels on a national basis have not been reported yet, except for Korean adolescents in 2004 (Kim & Kwon, 2001; Lee et al., 2000; Lee et al., 2002; Lee et al., 2004; Park et al., 2002; Sung et al., 2000). The isoflavone intake levels in Koreans based on the 1995 National Nutrition Survey data was estimated as 14.9 mg (Kim & Kwon, 2001) and the average daily intakes of isoflavone of adults in the Seoul region, using 6-day dietary records, were 14.4 mg in males and 21.0 mg in females (Lee et al., 2002), The average isoflavone intake levels in middle-aged females, using the food frequency method, was 24.4 mg per day (Lee et al., 2000). Also, the isoflavone intake level of adults in the Daejeon region, using the semiquantitative food frequency method, was 39.03 mg (Park et al., 2002) and the result in menopausal women in the rural area, using the 24-hour recall method, was 27.27 mg (Sung et al., 2000). While in the study of isoflavone intakes in Hawaiian residents (Maskarinec, 1998), the intakes were in the order of 38.2 mg in Chinese origin, 31.3 mg in Japanese origin, 22.2 mg in native Hawaiians, 6.9 mg in Caucasians, and 5.0 mg in Philippino origin. Harrison & Cho (1999) studied the isoflavone intakes using the food frequency questionnaire in the Los Angeles region and reported that the intake was 29.33 mg in Korean- American women, 14.46 mg in Japanese-American women, and 0.419 mg in Caucasian American women.

This study was performed to evaluate the isoflavone intake levels in Korean adults and to analyze variables related to isoflavone intakes, and thus to provide useful information in preparing measures for the promotion of isoflavone intakes.

Isoflavone intake levels of Koreans were estimated using FFQ and the average daily intakes of isoflavone in adults are shown as 23.1 mg. This was higher than 19.3 mg in Hong Kong (Mascarinec, 1998) but much lower than 150~200 mg in the Japanese traditional meal group (Cassidy, 1995). The isoflavone intake level at 50 percentile was 16.9 mg (0~190 mg), 10% of adults took almost 50 mg of isoflavone a day and 10% took about 5 mg a day. The results of the study showed that the distribution of isoflavone intakes in these subjects had positive skews due to a few extremely high values. Thus, it is considered that using the median rather than the mean is reasonable to estimate the isoflavone intake levels in Koreans.

The average daily intakes of isoflavone of adults in the Seoul region using the dietary record method were 14.4 mg in males and 21.0 mg in females, and the average isoflavone intake levels in middle-aged Korean females using the food frequency method were 24.4 mg per day (Lee et al., 2000). Also, the isoflavone intake levels of adults in the Daejeon region using the semiquantitative food frequency method were 39.03 mg (Park et al., 2002). The result in menopausal women in the rural area using 24-hour recall method was 27.27 mg (Sung et al., 2000) and the isoflavone intake levels in Koreans based on the 1995 National Nutrition Survey data was estimated as 14.9 mg (Kim & Kwon, 2001). Also, Ahn & Park (2003) reported that the average isoflavone intake in female college students was 33.46 mg/day. According to the survey on the estimated dietary isoflavone intake among Korean adolescents using the same FFQ by Lee, et al. (2004), the mean daily isoflavone intake of the subjects was 28.1 mg and the median value of isoflavone intake was 19.7 mg.

While in the study of isoflavone intake in Hawaiian residents (Maskarinec, 1998), the intakes were in the order of 38.2 mg in Chinese origin, 31.3 mg in Japanese origin, 22.2 mg in Hawaiian aborigines, 6.9 mg in Caucasians, and 5.0 mg in Philippino origin, and the isoflavone intakes in Asians of the soybean culture was about 20~100 mg.

The major food sources for isoflavone of Koreans were in the order of soybean, soybean paste, soy milk, soybean curd (tofu), and bean sprouts, and adult subjects took 83% from these. The intake was different depending on age, educational background, occupation, economic standard, and family type among socio-demographic characteristics. According to the study of Lee, et al. (2000), the intake frequencies of soybean foods in Korean middle-aged women showed that 81.2% of subjects ate soybean foods everyday in the order of soybean paste stew, soybean curd (tofu), and bean sprouts, and the intake distribution by soybean foods showed higher intakes for soybean paste stew as 3-4 times a week and for soybean curd (tofu) and bean sprouts as twice a week. The isoflavone intakes by frequently eating soybean foods in Korean Middle-aged women showed 10.68 mg from soybean paste stew, 3.34 mg from braised soybean, 2.44 mg from soybean curd residue, 2.42 mg from soybean curd (tofu), 2.42 mg from soy milk, 1.12 mg from Dambuk, 1.02 mg from bean sprouts, 0.29 mg from uncurdled soybean curd, and 0.33 mg from soybean broth (Lee et al., 2000). Also, according to the study in adults in the Daejeon area, it was 13.01 mg from bean sprouts, 10.17 mg from soybean curd (tofu), 5.83 mg from soybean paste stew, 4.57 mg from uncurdled soybean curd, and 2.54 mg from braised soybean (Park et al., 2002), showing slightly different aspects of major isoflavone food sources from those in this study.

The result showed higher isoflavone intake levels in the group over 30 s, and the differences by occupations of subjects showed the highest average isoflavone intakes in subjects working in agriculture/livestock/fishery, followed by housewives. According to the differences by family type, the family with elderly members showed 50% higher isoflavone intakes than the young family type with friends or siblings. Therefore, it was analyzed that the isoflavone intake levels of Koreans were different depending on various ecological variables and thus it is considered that educational materials for the promotion of isoflavone intake considering various ecological factors should be developed.

The results of the study are expected to be used as reference material in establishing the recommended intake level of isoflavone for Koreans, and the development of effective nutrition education programs and continuous education are required to recommend sufficient intakes of soybean foods on the basis of several physiologically active substances of soybeans, particularly preventive and treatment effects related to isoflavone, and to promote the recognition for the excellence of soybean foods as traditional foods. Also, continuous research is needed to develop simple and easy cooking methods for a variety of intakes of soybean foods, along with diversity and convenience.