Changes in body mass index by birth cohort in Japanese adults: results from the National Nutrition Survey of Japan 1956-2005

Abstract

Background: The National Nutrition Survey, Japan (NNS-J) provides annual anthropometric information for a whole nation over 50 years. Based on this survey, the mean body mass index (BMI) of Japanese men and elderly women has increased in recent decades, but that of young women has decreased. We examined the effect of birth cohort on this phenomenon.

Methods: We analysed data from the NNS-J for subjects aged 20-69 years. BMI during 1956-2005 and the prevalence of overweight and obesity (BMI >/= 25 kg/m(2)) during 1976-2005 were estimated.

Results: The BMI increased with age in every birth cohort, with similar increments, and did not peak until 60-69 years of age. However, with cross-sectional age, the BMI usually peaked before 60-69 years of age. The differences among cohorts already existed at 20-29 years of age, and slightly increased in men between 20-29 and 30-39 years of age. The BMI in all male age groups increased from the 1891-1900 through 1971-80 cohorts. However, in women, the figure increased until the 1931-40 cohorts, but later decreased. Changes in prevalence were generally consistent with changes in BMI. The recent increase (decrease in young women) in the mean BMI is attributable to birth cohort, indicating that thinner (fatter) and less recent birth cohorts have been replaced by fatter (thinner) ones.

Conclusions: A cohort effect was quantitatively demonstrated based on a repeated annual survey. In Japan, the differences in BMI among cohorts were already established by young adulthood.

Figure 1

Changes in height with age by birth cohort (1891–1900 to 1971–80) and by survey year (1956–65 to 1996–2005) for each gender from the National Nutrition Survey, Japan, 1956–2005

Figure 2

Changes in weight with age by birth cohort (1891–1900 to 1971–80) and by survey year (1956–65 to 1996–2005) for each gender from the National Nutrition Survey, Japan, 1956–2005

Figure 3

Changes in BMI with age by birth cohort (1891–1900 to 1971–80) and by survey year (1956–65 to 1996–2005) for each gender from the National Nutrition Survey, Japan, 1956–2005

Figure 4

Changes in prevalence of overweight and obesity (BMI ≥ 25 kg/m²) with age by birth cohort (1911–20 to 1971–80) for each gender from the National Nutrition Survey, Japan, 1976–2005

Figure 5

Relation between body mass index (BMI) and prevalence of overweight and obesity (BMI ≥ 25 kg/m²) across gender, age (20–29 to 60–69 years) and survey year (1981–2005) from the National Nutrition Survey, Japan, 1981–2005