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. And even if there is a link, it's not clear that breastfeeding or cow's milk directly affect kids' maturation.
The question is 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.
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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.
HARD TO COMPARE STUDIES
Cow's milk is not part of the traditional Chinese diet. But few kids in this study drank no milk: at the age of three, 68 percent were drinking it every day, as were 45 percent at age five.
"Of course our findings have most relevance for Chinese cultures," Schooling said.
But, she added, "we think they should also be generally relevant to all countries and cultures, although ideally they should be confirmed in other settings."
Andrea S. Wiley, director of the human biology program at Indiana University in Bloomington, said it was "great" to see a study that followed a single cohort of children from birth - and from an Asian population, rather than Western.
But she said that when it comes to cow's milk intake, most research has looked at its relationship with the timing of girls' first menstrual periods - which is different from the start of puberty.
So it's hard to compare this study with previous ones, according to Wiley, who was not involved in the work.
In her own research using government survey data on U.S. kids, Wiley found that nine- to 12-year-old girls who drank the most milk had a slightly higher risk of early menstruation - before age 12 - than girls with more moderate milk intake.
She told Reuters Health in an email that it's unclear why you would expect milk intake at age five or younger - which this study measured - to show a connection to puberty. The average age of puberty in this study was just before age 10 for girls, and just shy of 12 for boys.
"I think it's safe to say that we don't know what the relationship is (if any) between milk consumption at various ages in childhood and sexual maturation," Wiley said.
As for breastfeeding, few moms in this study kept it up long-term: six percent of kids were breastfed exclusively for at least three months. It's possible that limited the study's ability to detect an effect of breastfeeding on puberty, if there is one, the researchers note.
The World Health Organization recommends exclusive breastfeeding for six months, followed by at least partial breastfeeding for up to two years or more.
Other studies in the same Hong Kong group have been able to link exclusive breastfeeding to a lower risk of serious infant infections.
"Breastfeeding is undoubtedly beneficial for mothers and babies," Schooling said. But, she added, "it would not appear that the benefits extend to the timing of puberty."
When it comes to puberty, Schooling noted, a generally healthy diet and healthy weight may be more important. Other research has linked excess childhood pounds to earlier puberty.