1 Cancer Information and Epidemiology Division, National Cancer Center Research Institute, Tokyo, Japan.
2 Epidemiology and Biostatistics Division, National Cancer Center Research Institute East, Kashiwa, Japan.
3 Current affiliation: National Institute of Health and Nutrition, Tokyo, Japan.
Junko Ishihara, Cancer Information and Epidemiology Division, National Cancer Center Research Institute, 511 Tsukiji, Chuo-ku, Tokyo 104-0045, Japan. E-mail: junishih{at}gan2.ncc.go.jp
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Abstract |
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Methods Subjects were the 78 531 participants (4574 years) who completed a self-administered questionnaire in 1995 or 1998 in a 5-year follow-up survey by the Japan Public Health Center-based prospective Study on cancer and cardiovascular disease. The questionnaire included enquiries about supplement use, occupation, height, weight, smoking, alcohol, physical activity, dietary behaviours, working hours, subjective stress, as well as intakes for 138 foods.
Results The supplement users were likely to have formerly smoked or never smoked. Female supplement users were likely to consume alcohol moderately. The prevalence of users was higher in the elderly, the self-employed, those with lower body mass index, greater physical activity, lower frequency of eating prepared food, higher frequency of eating out, and higher stress level in both sexes after mutual adjustment. Mean intakes of energy and nutrients were lower for users than for non-users.
Conclusion The demographics, lifestyles, health characteristics, and dietary intakes may need to be adjusted when evaluating the effect of dietary supplements on disease because they can become potential confounding factors.
Accepted 3 February 2003
Commercially available dietary supplements in Japan have seen tremendous growth over the last decade,1 and their variety and number continue to increase. The prevalence of dietary supplement users differs depending on the study population as well as the definition of supplements and survey methods used. In the US, where dietary supplements are generally very popular, the prevalence varied from 21% to 55% among a number of different studies.2 In our previous report for the baseline survey by the Japan Public Health Center-based prospective Study on cancer and cardiovascular disease (JPHC Study) in 1990 and 1993, the users who took vitamin supplements 1 week ranged from 4.4% to 22.7% by area.3
In epidemiological studies, the use of dietary supplements is an exposure of interest because of its potential effect on disease. When dietary supplement use is associated with both diseases and demographic factors such as sex, age, race, and socioeconomic status,4 and health-related characteristics such as body mass index (BMI), smoking, and alcohol consumption,57 as well as certain psychological factors,8 it also becomes a confounding factor when determining other risk factors. These associations have been investigated extensively in Western countries where supplements have been widely used for decades. However, little is known about the characteristics of dietary supplement users in non-Western countries, including Japan.
In this report, we aimed to determine the characteristics of dietary supplement users among the participants in the JPHC Study cross-sectionally. The objective of our study was to investigate the possible association between supplement use and demographics, lifestyles, health characteristics, and dietary intakes.
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Materials and Methods |
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In the FFQ, general use of any vitamin supplements more than once a week, and use of specific supplements by five categories (Multivitamin, Beta-carotene, Vitamin C, Vitamin E, Others) were probed. For each category, the brand names, frequency, and duration of use were asked. Users of dietary supplements were defined as subjects who used at least one category of dietary supplement 1 week for
1 year. If a subject was a user of at least one category of supplement, he or she was regarded as an overall supplement user. To preclude the incorrect categorization of self-reported dietary supplements, all supplements in the FFQ were re-categorized by the authors using brand names according to the definition of dietary supplements in the Womens Healthy Eating and Living Study.15 This method of defining the supplement users was validated in our previous study.16 Recategorizing self-reported categories was shown to improve sensitivity in identifying dietary supplement users. The results from our validation study also indicated that our questionnaire could even identify non-vitamin supplement users to a certain extent (sensitivity of 75%), although we only enquired about vitamin supplement use.
Self-reported occupations were combined into the following six groups: farming, forestry, and fishing; employee and professional; housewife; self-employed; unemployed; and other occupations. The subjects with 2 occupations across those groups were classified as a combination group. Body mass index for each subject was computed based on self-reported height and weight by dividing weight (kg) by the square of height (m). The questionnaire covered smoking status, frequency of alcohol consumption, physical activity, 14 questions on dietary behaviours (frequency of eating miso soup, breakfast, eating out, consumption of prepared foods, fried foods, deep-fried foods, fat on meat, soup from noodle bowls, adding salt or soy sauce to foods at the table, types of vegetable oils used, frequently used cooking methods, well-doneness of cooked meat, eating charred parts of fish), working hours, and self-reported stress.
Individual intakes of energy and 33 nutrients were calculated from 138 food items in the FFQ. The algorithm of the calculation was reported elsewhere.17 Intake from dietary supplements was not included in that calculation.
Statistical analysis
SAS version 8.02 (SAS Institute Inc., Cary, NC) was used to conduct all the statistical analyses. We excluded subjects who were confirmed to be ineligible during the follow-up because they were not Japanese nationals, had already moved away at the baseline, or were not in the intended age group for this study. Subjects were also excluded for the following reasons: BMI of <10 or >100; failure to supply data in the questionnaire for any of the variables used such as occupation, height, weight, smoking, alcohol consumption, physical activity, dietary behaviours, working hours, and stress; men with a total energy intake of <900 kcal or 4000 kcal; women with a total energy intake of <800 kcal, or
3600 kcal. Of 103 769 subjects who completed the questionnaire, a total of 78 531 subjects (37 298 men and 41 233 women) were finally included in the analysis. The proportion of demographic, lifestyle, health characteristics, and supplement use among those who were excluded was similar to those included in the analysis.
We calculated the prevalence of dietary supplement users for each of the demographic factors, lifestyles, and health characteristics by sex. The odds ratios (OR) with 95% CI were computed using a logistic regression model for the association between dietary supplement use and those factors. P-values for linear trends from the lowest to the highest levels of each variable were calculated using logistic regression.
Individual intakes of energy and nutrients were transformed using the natural log scale to normalize skewed distribution. Energy intake was adjusted for area, age group, occupation, smoking, alcohol consumption, physical activity, dietary behaviour, working hours, and stress level using LSMEANS of PROC GLM in SAS. Nutrient intakes were adjusted for the same variables and for energy intake using the same procedure. The geometric means of energy and nutrient intakes by sex were calculated for users and non-users of dietary supplements by the back-transformation of least-square means. The percentage difference was also calculated by dividing the difference in mean intakes between user and non-user by the mean intake of users.
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Results |
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In the initial analysis of 14 dietary behaviours and the use of dietary supplements, no association was observed with the frequency of eating breakfast, consumption of fried foods, deep-fried foods, and fat on meat, of eating soup from noodle bowls, adding salt or soy sauce to foods at the table, types of vegetable oils used, frequently used cooking methods, well-doneness of cooked meat, and eating charred parts of fish (data not shown). Therefore, we included in the logistic model the only three variables as dietary habits associated with supplement use (frequency of eating miso soup, consumption of prepared foods such as freeze-dried noodles and retort-pouched foods, and eating out). For miso soup, although the prevalence of dietary supplement users was highest in the subjects who consumed the least, the difference was not significant after the adjustment for other variables. For prepared foods, the prevalence of users was highest in subjects who never used them. The prevalence of users was highest in the groups with the highest frequency of eating out. There was also a significant linear increase of users in the groups of subjects who eat out more frequently.
Working hours were not associated with dietary supplement use after adjustment for other variables. For stress level, the groups reporting high stress were most likely to be users.
The geometric means of energy and nutrient intakes from diet for users and non-users of any dietary supplement and their percentage difference by sex are shown in Table 2. The mean intake was significantly lower for users as was intake of energy and most nutrients except for sodium and niacin for both sexes, carbohydrate for men, and polyunsaturated fatty acids (PUFA) and selenium for women.
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Discussion |
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A high prevalence of dietary supplement users was observed in metropolitan regions (Suita and Katsushika) and in areas strongly influenced by Western lifestyles (Ishikawa) with ready access to dietary supplements. Associations between supplement use and other demographic factors (sex, age, and occupation) were consistent with results in other studies.2,4,1823 In terms of occupation, the lower prevalence in the farming, forestry, and fishing group might reflect their conservative health habits. It might be also a case of ones occupation serving as a surrogate for ones socioeconomic status (SES). A number of studies have indicated a strong association between supplement use and higher income level,2,4,5,7,20,23 social class,19 and education.4,5,20,23,24 Educational background, another factor for SES available only for Cohort I, was also associated with the supplement use.
As for lifestyle and health characteristics, supplement use was associated with healthy lifestyle, which was similar to the earlier-reported tendency for smoking,4,22,24 BMI, and physical activity.2,4,7,24,25 Dietary supplement use did not associate with alcohol consumption in men, and was higher in women who drink moderately. Prior studies had reported alcohol consumption in dietary supplement users as either having no association24 or as showing more users among moderate drinkers.4,22,26 Supplement users have been characterized as having a positive attitude towards their health. In our study, however, users also showed negative lifestyle factors such as frequent eating out and stressful life. It was assumed that these associations, including moderate drinking among female supplement users, were influenced by urban lifestyle. Such people might be aware of their unhealthy behaviour, and therefore intentionally seek to compensate for it with dietary supplements.
Our results indicated that dietary supplement use could confound the association between dietary intake and disease even after every possible related factor was adjusted. Several earlier studies had found that supplement users consume a more nutrient-dense diet, i.e. low in energy and high in micronutrients.7,22 In the present study, intakes of both energy and most nutrients were significantly lower for users than non-users of dietary supplements after various factors were adjusted. The results did not change when adjustment was made only for biological factors (age and BMI). The contradiction in the results was assumed to be caused by the complex characteristics of supplement users. Although the SES is usually associated positively with the quality of the diet, some factors such as eating out, which is influenced greatly by SES, can make the association negative.27 The subgroup analysis in our study indicated that high dietary intake with a higher frequency of eating out made intake of users higher, while low intake with intensive labour such as in farming, forestry, and fishing made intake of users relatively lower.
To our knowledge, the present study is the first investigation into the supplement use and its associations with demographics, lifestyles, and health characteristics in a large population in Japan. Only a few smaller studies have so far reported the characteristics of dietary supplement users in Japan.16,28 The strengths of our study were its population-based large sample, and various and extensive data on potential confounding factors such as demographics, lifestyles, and health characteristics of individuals, as well as dietary supplement use. Those factors can be adjusted later on when the association between supplement use and mortality or disease is investigated. Furthermore, if such association is found, supplement use itself may need to be adjusted when investigating the association between those factors and disease.
One of the limitations of our study was its lack of sensitive SES data, such as income or education level of the participants. Socioeconomic status is strongly associated with supplement use as a result of difference in perception of health and economic status. The positive association with frequency of eating out might be the influence of higher SES level in users, whereas the negative association with prepared food, which tended to be consumed by those who eat at home, might be the influence of low SES.
Generalization of the results could be limited because of the non-respondents to the questionnaire, as well as the representativeness of the study sample. A difference in mortality was observed between the respondents and non-respondents to our baseline questionnaire.29 For the rural areas where the response rate was 7790%, the results were probably good estimates for those populations. For the urban areas, however, where the response rate was as low as 40%, supplement use might have been lower in the non-respondents since both the respondents and supplement users tended to have a healthier lifestyle. Furthermore, the overall prevalence might not represent general population in Japan because no statistical weighting was made for population estimates. However, the characteristics of supplement users were presumably generalizable because the results of subgroup analysis were similar among all areas.
In the present study, we only focused on dichotomous information on dietary supplement use (i.e. user versus non-user) since we aimed to characterize the behaviour of individuals who use dietary supplements. Although we did not examine the amounts consumed or length of dietary supplement use, they may well be of great importance because the association with a disease might depend on them. Further investigation should be done using available data on brand names, frequency, and duration of usage in our study. The development of a database for supplement composition is necessary since it is not currently available.
KEY MESSAGES
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Acknowledgments |
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The JPHC Study Group
The investigators and participating institutions in the JPHC Study Group (Principal investigator: S Tsugane), were as follows: S Tsugane, S Sasaki, Epidemiology and Biostatistics Division, National Cancer Center Research Institute East, Kashiwa; T Sobue, Cancer Information and Epidemiology Division, National Cancer Center Research Institute, Tokyo; J Ogata, S Baba, National Cardiovascular Center, Suita; F Saito, A Koizumi, Y Sano, Iwate Prefectural Ninohe PHC, Ninohe; Y Miyajima, N Suzuki, S Nagasawa, Y Furusugi, Akita Prefectural Yokote PHC, Yokote; H Sanada, Y Hatayama, Kobayashi, H Uchino, Y Shirai, T Kondo, R. Sasaki, Nagano Prefectural Saku PHC, Saku; Y Kishimoto, E Tanaka, M Kinjo, T Fukuyama, Okinawa Prefectural Ishikawa PHC, Ishikawa; K Imoto, H Yazawa, T Seo, A Seiko, F Ito, Katsushika PHC, Tokyo; A Murata, K Minato, K Motegi, T Fujieda, Ibaraki Prefectural Mito PHC, Mito; K Matsui, T Abe, Niigata Prefectural Kashiwazaki PHC, Kashiwazaki; M Doi, Y Ishikawa, A Terao, Kochi Prefectural Chuo-higashi PHC, Tosayamada; H Sueta, H Doi, M Urata, Nagasaki Prefectural Kamigoto PHC, Arikawa; H Sakiyama, N Onga, H Takaesu, Okinawa Prefectural Miyako PHC, Hirara; F Horii, I Asano, H Yamaguchi, K Aoki, S Maruyama, Osaka Prefectural Suita PHC, Suita; S Matsushima, S Natsukawa, Saku General Hospital, Usuda; S Watanabe, M Akabane, Tokyo University of Agriculture, Tokyo; M Konishi, Ehime University, Matusyama; H Iso, Tsukuba University, Tsukuba; H Sugimura, Hamamatsu University, Hamamatsu; Y Tsubono, Tohoku University, Sendai; N Kabuto, National Institute for Environmental Studies, Tsukuba; S Tominaga, Aichi Cancer Center Research Institute, Nagoya; M Iida, S Sato, Osaka Medical Center for Cancer and Cardiovascular Diseases, Osaka; the late M Yamaguchi and Y Matsumura, National Institute of Health and Nutrition, Tokyo.
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