Book excerpt: Are girls entering puberty earlier than they used to? This question, which has been debated recently by doctors and scientists in the pages of Time magazine and the New York Times, proves that there is still a great deal to learn about women's reproductive health. Female Fertility and the Body-Fat Connection is the record of one scientist's groundbreaking and decades-long work on the connections among fertility, body fat, and reproductive health in women. Rose E. Frisch explains here how, in women, a certain amount of body fat is crucial to the reproductive system and sexual maturation. Women who are too lean are infertile and cannot conceive children; young girls who are too thin have a delayed onset of their first period. Female Fertility and the Body-Fat Connection illuminates how and why a "critical fitness" level underlies a woman's reproductive health. In the process Frisch gives readers a comprehensive view of the research done to date on the relationship between body composition and fertility and also describes her own journey as a woman scientist working to advance her critical-fitness hypothesis both to the general public and the scientific community. Frisch answers the questions every woman has about the desirable weight for health and fertility and even includes tables to help women find their own best weight. She also demonstrates how important diet and exercise are for the long-term reproductive health of women, and shows what factors influence the onset of puberty in girls. Each milestone of the reproductive life span is affected by food intake and energy output, the factors affecting the storage of fat. Female Fertility and the Body-Fat Connection is a cornerstone to understanding the health of girls and women.