Karin Michels, the lead researcher and an associate professor at Harvard Medical School, told The Washington Post that the research team wanted to explore the role of diet and lifestyle in sexual development and puberty.
"We've been puzzled now for awhile as to why girls go into puberty earlier and earlier over the last decade or two," she said. The increase in childhood obesity may be a reason girls are getting their first periods (or menarches, if you want to get technical) earlier; but, Michels added, it doesn't fully explain the trend.
The researchers found the link between sugary drinks and earlier periods, even when controlling for factors such as physical activity levels, birth weight and height.
Michels said the self-reporting aspect of the study probably means the results underestimate the strength of the association. (The girls who participated in the survey came from the Growing Up Today study, which began tracking more than 16,000 boys and girls starting in 1996.)
For now, researchers can only speculate about why this link may exist. One plausible theory, Michels said, has to do with the relationship between sugary drinks and insulin levels and hormones. Table sugar, the kind that gets added to drinks, has sucrose and can lead to spikes in insulin, which in turn can lead to higher concentrations of sex hormones connected to earlier periods. The authors noted previous research showing the link between insulin levels and the timing of first periods, and some research has shown that girls with Type 1 diabetes who are deficient in insulin also have later periods.
Naturally sweetened drinks contain fructose, which doesn't interact with the body's insulin in the same way, Michels noted.
Additionally, earlier menstruation and later menopause has been associated with a slightly elevated risk of breast cancer, but Michels emphasized that they didn't establish a link between sugary drinks and the cancer.
More broadly, the Harvard researchers hope their findings can help scientists figure out why girls are having earlier periods.
"We are the only species where the sexual maturation is actually much earlier than the social expectation to produce, or even before the rest of the body being ready," Michels said. "Basically, these girls could reproduce at age 11 but the rest of the body is not ready, and we wouldn't want that. They're not mentally ready either; from a social standpoint they are immature."