Archives   The Onset of Puberty Posted April 1, 1998 In the 1600s young women usually started menstruation at the age of 18. Currently in the United States, the average girl begins her first menstrual cycle at the age of 12. In the early 1970s, Rose Frisch, a Harvard University biologist, offered a hypothesis to explain this shift in age. She suggested that girls must have a minimal fat reserve in their bodies before they could begin ovulation and the menstrual cycle. Studies of African women who generally consume a low protein, low calorie diets have confirmed a delayed onset of the first menstrual cycle when fat in the diet is restricted. In industrialized nations, young women eat diets with abundant fats. In these countries, there has been a continuous trend towards the earlier onset of menstrual cycles. Researchers have found that the hormone leptin triggers the onset of menstrual cycles. Fat cells secrete this hormone as a signal to the brain. The amount of leptin secreted, which is directly related to the amount of fat in the body, lets the brain know how much fat is available. A four-year study of girls aged 8 to 13, who had not yet started their menstrual cycles, was recently completed. Throughout this study, the scientists measured the girls' height, weight, body fat and leptin levels. This study has confirmed the connection between fat, leptin, and reproductive readiness. The girls with lower leptin levels reached puberty last or not at all during the study time period. This study suggested that a minimum leptin threshold was required for the start of the menstrual cycle, an event controlled by the brain. A similar role for leptin may occur in boys. The connection, however, may be more difficult to find, since the onset of puberty is much more difficult to pinpoint in males. References Seppa, N. "Leptin linked to onset of monthly cycles," Science News, Vol.152, October 18, 1997, p.247.