While the average height of Japanese adults had grown some 15 cm (nearly 6 inches) over the past century with improved nutrition and public health conditions, it has started to decline for those born in 1980 or later, recent research has shown.

A research team at the National Center for Child Health and Development in Tokyo analyzed data on some 3.15 million adults and concluded the declining trend in average height may be attributed to increases in low-birth-weight infants in the country — those who weighed 2,500 grams (5.5 pounds) or lighter at birth.

The findings by Naho Morisaki and other researchers were published last year in the Journal of Epidemiology and Community Health.

According to the study, the height of an average Japanese adult peaked for those born in 1978 and 1979 at 171.4 cm (5.6 feet) for men and 158.5 cm (5.2 feet) for women. The figures for those born in 1996 stood at 170.82 cm for men and 158.31 cm for women — 0.64 cm and 0.21 cm shorter than peak levels, respectively.

The researchers found “a strong inverse correlation” between the rate of low-birth-weight infants and adult height. While the decline of the average height started among people born in 1980 or later, the number of low-birth-weight babies sharply increased since around that time.

Health ministry statistics showed the ratio of low-birth-weight babies stood at 5.1 percent in the latter half of 1970s, but it jumped nearly twofold to 9.7 percent in 2007.

Past research has indicated that the height of people with low birth weights tends to be shorter when they reach adulthood.

Morisaki, chief of the center’s division of life course epidemiology, and other team members studied trends in average height of Japanese adults born between 1969 and 1996. They also observed trends in birth characteristics using data of around 64.1 million live births between 1969 and 2014.

KEYWORDS children, birth, height