RECORD numbers of girls not yet old enough for middle school are starting puberty. Almost by definition, puberty arrives at an awkward age, but these are grade-school girls, thrown into a developmental gantlet before they are ready for the change.

As mothers of young girls, we see this phenomenon and fret over the implications. Early puberty can lead to eating disorders, depression, substance abuse, early sexual activity and, later in life, breast cancer. But as doctors, we wince at misleading stories that blame substances that are not likely to bear the primary responsibility — hormones in our meat or soy in our diets, for instance. The real culprits include two problems that are often overlooked: obesity and family stress.

Chemical exposures may also disrupt girls’ puberty, but less is known in this area. To better understand the early puberty phenomenon, we, together with colleagues, are conducting a continuing study of more than 1,200 girls tracked since 2005. Puberty used to begin at age 10 or 11. In our study, we found that by the age of 7, 23 percent of black girls, 15 percent of Hispanic girls and 10 percent of white girls had started to develop breasts. It is clear from the data that overweight girls are more susceptible to early puberty. Body fat secretes estrogen, a hormone that is normally released from the ovaries during puberty and is responsible for breast development. Excess body fat serves as an additional source of estrogen, and the result is earlier breasts.

Sugary drinks contribute to today’s kids’ being more overweight than in previous generations. In fact, according to one study, at least 20 percent of the weight gained by the population from 1977 to 2007 can be attributed to sweetened beverages. The role of sweeteners is intriguing. New research announced last week by the Harvard School of Public Health suggested that girls who drank more sodas were also more likely to reach puberty early, regardless of whether they were overweight.