[Osteoporosis and nutrition: trends of calcium intake and bone mineral densities].

To predict the future incidence and prevalence of osteoporosis in Japan, bone mineral densities (BMDs) in the same age ranges, but attained by different birth cohorts in 1990 or in 2000, were compared. In men, the mean value of BMD at the lumbar spine in the birth cohort of 1930-1939, when they reached the age stratum 60-69, was significantly higher than that of the older cohort born in 1920-1929 when in the same age stratum (p<0.05). The same tendency was observed in BMD of the femoral neck. In women ,the mean value of BMD of the lumbar spine L2-4 in the birth cohort of 1940-1949, when they reached the age stratum 50-59, was significantly higher than that of the cohort born in 1930-1939 when in the same age stratum (p<0.05). Again, the same tendency was observed in BMD at the femoral neck. The comparatively higher levels of BMD observed in women in the 1940-1949 birth cohort when they reached their fifties, may reflect nutritional improvements in Japan. The nationwide nutritional survey reports mean values of calcium intake as 253 mg/day in 1946 (1st survey), 338 mg/day in 1955, 465 mg/day in 1965 and 552 mg/day in 1975 , a dramatic increase. These nutritional improvements might be expected to increase BMD in all age groups;but nevertheless, for females born in 1940-1949 calcium intake per life year would be the highest among the birth cohorts of the present study. The improvement observed in men in their sixties, rather than their fifties as in women, might be attributed to differences in the timing of growth spurts in younger generations. It is encouraging that both men and women in later birth cohorts showed higher BMDs in middle-age, for this may predict a decrease in the incidence and prevalence of osteoporosis in the near future in Japan.