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Am J Public Health. 2004 September; 94(9): 1592–1595.
PMCID: PMC1448499

Thinness Among Young Japanese Women

Abstract

Objectives. We described changes in body mass index (BMI) and the prevalence of thinness among young Japanese women (aged 15–29 years) from 1976 to 2000 by reanalyzing the nationwide data in the National Nutrition Survey, Japan.

Methods. We used height and weight data sets for 30 903 nonpregnant, nonlactating women during the 25-year period. We calculated the mean values of BMI and the prevalence of thinness for 3 age groups (15–19, 20–24, and 25–29 years of age).

Results. Changes in BMI per 10 years were −0.17 kg/m2, −0.22 kg/m2, and −0.34 kg/m2 for each age group, respectively. Extreme thinness (BMI < 17 kg/m2) increased from 2.4% in 1976–1980 to 4.2% in 1996–2000.

Conclusions. Further studies regarding topics such as increased smoking prevalence are needed to identify the underlying causes of increasing thinness.

Obesity has become a major health problem worldwide in recent years. Health professionals have made efforts to inform the public about the adverse health outcomes of obesity and have emphasized the importance of being “slim.” However, such health education programs may produce unfavorable outcomes in some populations, such as young women, because their desire for thinness is greater than that of other age groups.1,2 Fear of being fat may induce unnecessary attempts to reduce body weight,3 producing thinness that in some cases is associated with nutritional deficiencies, irregular menstruation, and eating disorders.4–6 Although the adverse health outcomes of thinness or underweight in industrialized countries are not as clear as those associated with obesity, several studies suggest increased mortality in underweight compared with normal weight subjects, as described in cohort studies involving mainly middle-aged or elderly populations.7–11 Previous studies suggest that underweight in women of childbearing age is a risk factor for adverse pregnancy outcomes, such as intrauterine growth-restricted or low-birthweight infants.12–14

Yoshiike et al.15 reported a declining trend in the average body mass index (BMI) (in kilograms per square meter) of adult women (aged 20–39 years) over 1976 to 1995 on the basis of their analysis of nationwide data from the National Nutrition Survey, Japan (NNS-J). However, in this study, there was insufficient discussion of (1) whether this trend could be related to the population size of the participants’ residence area; (2) whether this trend was starting at younger ages; and (3) whether these changes were caused by an increase in the proportion of extremely thin (BMI < 17 kg/m2) subjects. Therefore, we performed in-depth analyses focusing on women aged 15 to 29 years to describe the changes in anthropometry of Japanese women.

METHODS

We evaluated 30 903 nonpregnant, nonlactating women (aged 15–29 years) for whom information on height and weight was available and who were participants in the NNS-J between 1976 and 2000. The NNS-J is an annual nationwide survey that covers approximately 5000 households in randomly selected census units defined by the Ministry of Health, Labour, and Welfare16 and provides the largest available nationally representative sample with which to monitor dietary intakes, lifestyle factors, and selected biological indicators of Japanese people including anthropometric measurements.

The subjects were divided into 3 age groups (15–19, 20–24, and 25–29 years of age) and were also divided according to the location of their residence (metropolitan, cities, and small towns), which were defined according to population size. Twelve major cities so designated by government ordinance and 23 Tokyo Metropolitan Wards were defined as metropolitan, municipalities with 50 000 to 1 000 000 residents were defined as cities, and those with fewer than 50 000 residents were defined as small towns. The census units were not always the same every year; therefore, the chance of selecting the same subjects twice was small over the 25-year period.

To evaluate the prevalence of thinness, we defined a BMI of 18.5 kg/m2 as the cutoff for thinness according to the criteria set by the Japan Society for the Study of Obesity.17 Furthermore, we analyzed the prevalence of extreme thinness using a BMI cutoff of 17 kg/m2.

To evaluate the trend in mean height, weight, and BMI, a general linear model was used to obtain the increment of each parameter per 10 years, as previously described by Yoshiike et al.15 Logistic regression analysis was used to investigate trends in the prevalence of thinness across survey year periods within the same age and residential area group. All statistical analyses were performed with the SPSS statistical package program (SPSS Inc, Chicago, Ill).

RESULTS

The mean height, weight, and BMI values for each survey year period, grouped according to subject age range and residential area, are shown in Table 1 [triangle]. Increases in mean height were significant in all age and residential area groups. Significant increases in weight were observed in only 3 groups: those aged 15 to 19 years in small towns and those aged 25 to 29 years in both cities and small towns. A trend of decreasing mean BMI was significant for all groups of young women, except for subjects in the 15- to 19-year-old group who lived in small towns. The greatest decrease was observed in subjects aged 25 to 29 years in metropolitan areas, whose change in BMI was −0.42 kg/m2 per 10 years. The decrease in BMI of subjects aged 15 to 19 years and 25 to 29 years was greater in metropolitan areas and cities compared with small towns. No such difference by residential area was observed in the subjects aged 20 to 24 years.

TABLE 1
Changes in Height, Weight, and Body Mass Index Among Young Japanese Women: 1976 to 2000

The prevalence of thinness (BMI < 18.5 kg/m2) in each group over the 25-year period is presented in Table 2 [triangle]. A significant increase in the percentage of thin women was observed in all groups. The percentage of thin women in the metropolitan areas was greater in all age groups in the initial survey period (1976–1980), and this difference persisted until 2000. The difference between the metropolitan areas and the other 2 areas was obvious in the youngest age group (15–19 years of age); however, the prevalence of thinness among subjects aged 20 to 24 years and 25 to29 years was similar in metropolitan areas and cities in the most recent survey period (1996–2000). Approximately one fourth of young women surveyed in 1996–2000 were thin in the age groups 20 to 24 and 25 to 29 years.

TABLE 2
Prevalence Rates (%) of Thinness (BMI< 18.5kg/m2) Among Young Japanese Women: 1976 to 2000

Table 3 [triangle] shows the changes in the prevalence of extreme thinness (BMI < 17 kg/m2). There were significant increases in all age groups. Among the subjects aged 15 to 19 years, there were no significant increases when women were grouped according to their residential area. The prevalence of extreme thinness significantly increased among subjects aged 20 to 24 years living in all 3 residential area groups. In the group aged 25 to 29 years, the increases in extreme thinness were significant in women living in cities and small towns but not in those living in metropolitan areas.

TABLE 3
Prevalence Rates (%) of Extreme Thinness (BMI< 17kg/m2) Among Young Japanese Women: 1976 to 2000

DISCUSSION

Our results clearly demonstrate a decrease in the average BMI of young women nationwide (Table 1 [triangle]). The decrease was prominent among women aged 20 to 30 years old. The prevalence of thin women (BMI < 18.5 kg/m2) and extremely thin women (BMI < 17 kg/m2) significantly increased over the 25-year period. There were no differences in BMI changes and prevalence of thinness between the 3 residential areas, with the exception of the youngest age group (Table 1 [triangle]). The decline in BMI or increase in prevalence of thinness was observed in all age groups. The increased prevalence in extremely thin women was not the sole reason for the decline in overall BMI values (Tables 2 [triangle] and 3 [triangle]).

There may be several reasons for the dramatic increase in thinness or the decline in BMI values among Japanese women of childbearing age. First, the women who were surveyed had a misconception regarding self–body image as evidenced by the data shown by the 1998 NNS-J,16 in which over 40% of young women answered that they were overweight despite the fact that the mean BMI was between 20 and 21 kg/m2.16 The mean daily energy intake in this population was approximately 1800 kcal/d, with a standard deviation of approximately 600, indicating that over 15% of this population consumed fewer than 1200 kcal/d.16 Second, the continuous increase in the smoking rate of young women may be the reason for the changes in BMI. Although smoking status is assessed only in subjects aged 20 years or older in the NNS-J, the smoking rate in women aged 20–29 years increased from 11.9% in 1990 to 20.9% in 2000, nearly doubling in those 10 years.18 Third, it has been suggested that the overall increase in eating disorders among young Japanese women in recent years may be the cause of the decline in BMI values. However, this is unlikely to be the cause. Estimates of the incidence of eating disorders in young Japanese women are 17.1 to 30.7 per 100 000 for anorexia and 5.8 per 100 000 for bulimia,19,20 which is strikingly low compared with other industrialized countries, such as the United States (269.9 per 100 000), the United Kingdom (115.4 per 100 000), and Switzerland (70 per 100 000).21–23 However, we cannot exclude the possibility that extremely thin women in our study were not affected by eating disorders, because our survey participants were noninstitutionalized individuals.

At 21.6% in the most recent survey period (1996–2000), the high incidence of thinness in young Japanese women presents a striking contrast to that in other industrialized countries. In the United States, for example, the percentage of thin women (BMI < 18.5 kg/m2) of those aged 18 to 24 years was 7.2% in the National Health Interview Survey, 1997–1998.24 Using the same cutoff of BMI value for thinness, the prevalence in young Australian women (aged 18–23 years) was 12%.25

Health problems associated with thinness or underweight have rarely been investigated in industrialized countries. However, recent reports from Australia suggest that underweight women (BMI < 18.5 kg/m2) are more likely to report irregular menstruation or “low iron”23 and are less likely to use preventive health services such as breast examinations.26 Further studies are needed to identify the presence of health risk behaviors in these thin young Japanese women.

Acknowledgments

This study was funded by the Ministry of Health, Labour, and Welfare, Health and Labour Research Grant, Research on Children and Families.

Human Participant Protection
No protocol approval was needed for this study.

Notes

Contributors
H. Takimoto prepared the article and performed the statistical analysis of the data sets. N. Yoshiike had the original idea for the study and prepared the data sets for analysis. F. Kaneda assisted the statistical analysis. K. Yoshita provided technical advice and drafted the article. All authors helped to conceptualize ideas, interpret findings, and review drafts of the article.

Peer Reviewed

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