My research has shown that undernutrition and intense physical activity can have a limiting effect on female fertility. Women who have too little body fat, because of injudicious dieting and/or intensive physical activity, also have disruption or impairment of their reproductive ability; they are infertile due to hypothalamic dysfunction which is correlated with weight loss or excessive leanness. The effect is reversible with gain in weight and fatness. Fat is an active metabolic tissue that makes estrogen, and as recently discovered, a hormone, leptin. Human reproduction has a metabolic cost: it takes about 50,000 calories over the daily caloric maintenance cost to grow a human infant to term. Lactation costs about 500-1000 calories a day. I hypothesized that a critical, minimum amount of body fat is necessary for, and directly influences, female reproduction.
My research results are predictive and are now used clinically for evaluation of nutritional infertility and the restoration of fertility. Research on the long-term health of 5,498 U.S. college alumnae showed that moderate athletic regular activity resulted in a lower risk of breast cancer and cancers of the reproductive system and a lower risk of late onset diabetes. Environmental factors of nutrition, physical activity and disease can affect each reproductive milestone from menarche to menopause, hence as I have documented the natural fertility of populations.
In accord with the connection fatness and fertility is the 1994 discovery of J. Friedman et. al. that body fat produces a hormone, leptin, which has receptors in the hypothalamus, the ovary and the testis. Leptin controls appetite and energy metabolism, and the hypothalamic control of reproduction. My book "Female Fertility and the Body Fat Connection" (2002 & 2004 paperback, University of Chicago Press) details this research.
