Some research has hinted that breastfeeding or milk drinking might affect when kids hit puberty, but a new study casts doubt on that.
Studies have suggested that girls who are breastfed tend to start their menstrual periods later, while others have tied cow's milk intake to earlier menstruation.
But not all studies have found such connections.
Earlier this month, a research review found that childhood obesity may be a major factor in accelerating puberty in girls. Read the study here.
These questions are important, researchers say, because earlier puberty has been tied to certain health risks. Women who started menstruating at a younger age, for instance, seem to have a relatively greater risk of breast cancer -- possibly because of longer exposure to the hormone estrogen.
"Early puberty is associated with a higher risk of obesity, cardiovascular disease and diabetes," explained C. Mary Schooling, a researcher at the University of Hong Kong who worked on the study.
Again, those studies don't prove cause-and-effect. Still, Schooling said in an email, "all in all, it would be best to avoid exposures that induce early puberty."
But her team's findings, reported in the journal Pediatrics, do not suggest cow's milk is one of those exposures. Nor do they support the theory that breastfeeding might delay puberty.
The results are based on 7,523 Hong Kong children who'd been followed since birth, in 1997. When the kids were 11 or 12, parents were asked to remember and report on their children's breastfeeding and cow's milk intake at the ages of six months and three and five years.
Overall, the researchers found no relationship between breastfeeding and the timing of puberty - measured by physical signs like breast and pubic hair development. The same was true when they looked at milk intake.
One problem with studies of early nutrition and puberty timing in Western countries has been that both breastfeeding and milk drinking are related to economic factors.
In higher-income families, moms tend to breastfeed longer and kids' milk intake tends to be lower, Schooling's team notes in its report. And kids from those families tend to start puberty later.
In Hong Kong, though, things are different, the researchers say. Timing of puberty appears to be unrelated to socioeconomics. And, if anything, moms with more education tend to stop breastfeeding earlier.
When Schooling and her colleagues factored in family income, parents' education and other variables, there was still no link between breastfeeding or milk intake and puberty timing.