Being fit and healthy will boost your fertility, right? Absolutely! But excessive exercise can actually have the opposite effect.

The reproductive system of both men and women is tightly tied up with energy balance. Both extremes of the energy spectrum can impact negatively on fertility: at the one end is chronic energy excess, due to lack of activity against calorie intake; at the other end is chronic energy deficiency, due either to inadequate calories or excessive activity against calorie intake. Both extremes can lead to reproductive disorders, including polycystic ovarian syndrome in women and erectile dysfunction and spermatogenesis in men. It’s been noted that many female athletes suffer interrupted menstrual cycles, whether their sport emphasises strength or leanness, although usually for slightly different reasons.

A study in Norway found that increased frequency, duration and intensity of physical activity were associated with increased subfertility among women, with those who were active on most days being 3.2 times more likely to have fertility problems than their inactive peers. Significantly though, the researchers concluded that the increased risk of fertility was only among the small group of women reporting the highest exercise levels.

This finding was supported by a study of 2,232 women undergoing IVF treatment in the US. Moderate exercise did not impact on success rates, but women who exercised four hours or more per week for a year or more before beginning treatment were 40% less likely to have a live birth than those who did no exercise, while those who did cardiovascular exercise where 30% less likely to become mothers.

A study of 3,628 Danish women, aged 18-40, found that moderate physical activity led to a small increase in fertility, but increasingly vigorous exercise was associated with a delayed time to pregnancy. Some of the studies have suggested that if your infertility is caused by excessive exercise, you may be able to counter this by altering your diet so your food intake matches the demands you are placing on your body. But your body is a delicate machine, and it is worthwhile respecting its natural balance, especially when you are trying to conceive.

The consistent message from all the research is that moderate exercise as part of a healthy lifestyle can improve your fertility, but excessive exercise can have a negative impact.