[Effects of age, period, and cohort on the trends in obesity rate and energy intake ratio from fat in Japanese adults].

Objectives: In the present study, we aimed to clarify the specific effects of age, period, and cohort on trends in obesity rate and energy intake ratio from fat in Japanese adults using a Bayesian age-period-cohort (APC) analysis and to evaluate the relationship between changes in obesity rate and energy intake ratio from fat.

Methods: We obtained data regarding obesity rate and calorie intake of fat, animal fat, carbohydrate, protein, animal protein, and total energy intake of Japanese adults from the National Nutrition Survey. The data were tabulated for five 10-year age groups (from 20-29 years to 60-69 years) and 17 annual demographic profiles (from 1995 to 2011), with regard to the energy intake ratio. These standard cohort tables were analyzed using a Bayesian APC model.

Results: With regard to obesity rate, the effect of age was the greatest and increased rapidly in the age group of 30-39 years for both genders. Moreover, the period effect consistently increased in men, but had very little variation in women. The cohort effect indicated a reverse of the decreasing trend in the cohorts born after 1962-1971 in men and indicated a reverse of the increasing trend in the cohorts born after 1965-1974 in women. With regard to the energy intake ratio from fat, the trends for the three effects differed from those for obesity rate for both genders. The age effect generally decreased with increasing age. Furthermore, for both genders, the period effect gradually decreased after 1998 and markedly decreased in 2001, remained constant or slightly increased until 2008, and increased thereafter. However, the cohort effect was the greatest among the three, and although a decreasing trend was observed in the cohorts born after 1976-1985 in women, the energy intake from fat increased in the younger cohorts in both genders. The overall effect on energy intake ratio from animal fat had a slope similar to that of the energy intake ratio from fat.

Conclusion: Each effect affected obesity and energy intake ratio from fat in a different manner, suggesting that factors other than energy intake ratio from fat, such as energy expenditure, contributed to the changes in obesity rate. However, obesity risk markedly increased in the age group of 30-39 years, and younger generations had a higher energy intake ratio from fat. These results suggest that dietary guidelines, particularly the optimal intake of animal products, is needed for younger generations to prevent the development of obesity in adulthood.