A survey in Japan last year found 21.9 percent of female track and field athletes running medium-to-long distances had amenorrhea, or absence of menstruation, according to researchers.
That is nearly 10 times the 2.4 percent rate found among general university students, the Japan Society of Obstetrics and Gynecology and Japan Institute of Sports Sciences said Saturday.
A similar trend was found with regard to stress fractures, or bone cracks resulting from excessive activity, the two institutions said. Some 51 percent of the female athletes had experienced those types of fractures, compared to 4.3 percent of the university students.
Amenorrhea stemming from excessive training or malnutrition is associated with a decrease in female hormonal secretion, increasing the risks of a stress fracture and infertility.
The survey, which was conducted between July and September 2014, covered 1,600 athletes and 500 university students.