Weight and fertility facts

While it's a wellknown fact that a woman's periods will stop if she becomes seriously underweight through dieting or anorexia, a new book claims there is a very fine line between this and becoming permanently infertile. A healthy woman has a body fat ratio of up to 40 per cent (the corresponding figure being 30 per cent for men) and, when pregnant, will need to consume an additional 50,000 calories over nine months.

According to the theory, if calorie intake falls below a certain level, the brain starts to reduce the amount of leptin flowing through the body. Once this hormone drops beneath a critical threshold, the woman continues to menstruate but will no longer produce eggs. When even more weight is lost, the periods stop too. If this continues for too long, the ability to conceive could be permanently affected.

The exact levels are governed by a woman's height and age and, according to Harvard fertility expert, Professor Rose Frisch the critical weight loss could be as little as three pounds. Her research found women with a body mass index (BMI) of 18 or 19 will menstruate but are infertile. Menstruation stops below this BMI. This is of particular relevance to those on ultra-low-fat diets.