Longer and more variable menstrual cycles occur more often in lean women, physically active women, and women who do not drink alcohol than in those who do not fit these profiles, says a study that will appear in the November issue of Epidemiology.
Researchers led by Glinda S. Cooper of the National Institute of Environmental Health Sciences poured over dusty records of 750 women enrolled in a study in the 1930s and 1940s that charted their menstrual cycles regularly. Cooper's team contacted 95% of the women, now in their 80s and 90s, and asked them a series of questions about their lifestyles then, including height, weight, caffeine and nicotine use, and whether they exercised.
The findings "raise as a possibility" that variability in menstrual cycles could help explain the reported associations between breast cancer risk and exercise, alcohol consumption, and weight, Cooper says. Longer, more variable cycles--associated with these behavioral factors--may reduce a womans exposure to estrogen over her life, thus decreasing her risk of breast cancer, the study concluded.
Cooper cautioned against making any clinical inferences from the article, however, calling it "a descriptive study of patterns and associations rather than something that is establishing firmly any kind of relationship."
Siobhan Harlow, an assistant professor of epidemiology at the University of Michigans School of Public Health, agreed that the study's chief contribution was that it looked at an older study population than those of prior reports, which have focused mostly on menstruation frequency and behavioral characteristics in teenage women. The study "points to the importance of understanding a woman's reproductive environment over her life-span," said Harlow, who also works on menstrual cycles.