“Gym workouts ‘can hit pregnancy hopes’,” warned the Daily Express. It said research has apparently found that “superwoman workouts” triple the likelihood of fertility problems.
This study found that women who performed high-frequency, high-intensity exercise had a lower rate of fertility. However, it does not prove that exercise actually caused these fertility problems, as this type of study can only show associations, not cause and effect. There are also other limitations, including an assumption that the participants’ physical activity levels remained the same over a 10 year period and a failure to take into account the fertility of the women’s partners. Several other factors, such as diet, may also explain the assocation.
This finding should be viewed in the context of other studies in the same field, which have shown that maintaining an optimum weight is good for fertility. Moderate exercising (rather than excessive, exhausting workouts) is also likely to be the most appropriate type of activity for healthy women.
Where did the story come from?
The study was carried out by Dr Sigridur Gudmundsdottir and colleagues from the Norwegian University of Science and Technology and the Emory University in Atlanta. The research was funded by the Norwegian University of Science and Technology, Trondelag County Council and the Norwegian Institute of Public Health. The study was published in the peer-reviewed medical journal Human Reproduction.
What kind of research was this?
This was a cohort study which investigated the association between physical activity, fertility and parity (number of children) in a group of several thousand healthy Norwegian women. These women were recruited to the study between 1984 and 1986 and their final follow-up assessments took place between 1995 and 1997. The researchers are careful throughout their study not to suggest that exercise causes infertility, and are mindful of the numerous other factors (confounders) that may affect this relationship.
What did the research involve?
All male and female residents of the Nord-Trøndelag county of Norway were invited to take part in this research. An initial assessment involved a health questionnaire and a physical examination, which was given to participants between 1984 and1986. They were later asked to participate in further follow-up assessments that took place between 1995 and 1997.
A total of 24,837 women participated in both assessments. This study only looked at the link between physical activity and fertility in a subset of 3,887 participants. These were all healthy, pre-menopausal women who were under the age of 45 at the second assessment. By excluding women who had conditions that were known to affect fertility (including poor health, use of oestrogen tablets, hysterectomy, oophorectomy and fertility problems), the researchers attempted to make their findings relevant to healthy young women. It’s important to note that the number of women with undiagnosed problems would not have been known to the researchers.
Physical activity was assessed at entry into the study (baseline). The participants completed a validated questionnaire, which defined the levels of exercise they did during work and leisure time. This was determined by asking them to report the intensity, duration and frequency of the exercise. Exercise frequency was categorised as ‘never’, ‘less than once a week’, ‘2-3 times a week’ and ‘almost every day’. Exercise intensity was categorised as ‘take it easy’, ‘lose breath’ and ‘to exhaustion’.
Fertility was assessed at the follow-up assessment, where women reported the number of children they had, their age at childbirth, whether they had difficultly conceiving within one year of trying (and at what age), contraceptive use and status of menstruation and pregnancy.
Of the women who had attempted to conceive, those who succeeded within one year were considered ‘fertile’, while those who did not were categorised as ‘infertile’. Infertile women were divided into the ‘involuntarily childless’ (women who had problems conceiving within a year and had no child) or the ‘subfertile’ (if it took longer than one year to conceive). Women who had no problems conceiving and had no children were labelled as ‘voluntarily childless’.
Other factors such as age, education, marital status, body mass index (BMI), smoking and alcohol consumption were considered in the analyses. The fertility status at follow-up was then compared across groups who had different exercise levels at baseline.
What were the basic results?
The average age of the women at baseline was 27.2 years. Average BMI was 22.7 kg/m2 (with a wide range from 14.5 to 44.1). At the follow-up assessment, 90% of the women were classified as fertile, 5% as subfertile, 0.7% as involuntarily childless and 4% as voluntarily childless. In total, 62.4% of the infertile women had visited a doctor for fertility problems.
Increased frequency and intensity of physical activity was associated with increased infertility, even after the researchers adjusted their analysis for likely confounders. Women who were active on most days of the week were 3.2 times more likely to be infertile than inactive women. Women who exercised 'to exhaustion' were 2.3 times more likely to be infertile than women who said they 'take it easy'. The link between physical activity and fertility was not significant for frequencies or intensities of exercise below this level. The effect of exercise on fertility was more pronounced in women under the age of 30.
How did the researchers interpret the results?
The researchers conclude that fertility is negatively affected by physical activity of an extreme intensity and frequency. They say that their results conflict with those of other studies, but that their study has found a link between heavy exercise and infertility. They say that the potential role of regular physical activity in the prevention and treatment of infertility needs further investigation.
This cohort study cannot prove that heavy exercise causes infertility, a suggestion the researchers themselves are careful to avoid making. While this particular study has found an association between heavy exercise and fertility problems, this may be due to another factor, which could mean that the women who exercise most are systematically different from those who exercise less. For example, it is possible that, irrespective of their current weight, those women who exercise the most may be on low-calorie diets, and this intentional dieting may also affect their fertility.
There are other factors which affect the interpretation of these results:
- Of the 3,887 women, 1,000 of them had no available record of the intensity of their physical activity, so the results linking intensity of exercise to fertility should be interpreted with more caution than the other results.
- The researchers do not appear to have adjusted for the multiple statistical testing that they performed. Using multiple statistical testing means that it is more likely that their positive results are due to chance alone.
- About 30% of participants in the baseline survey did not participate in the follow-up. If these women were systematically different from participants in terms of their physical activity or fertility, this could have given the study different results.
- Exercise habits were only measured at baseline and are unlikely to have remained consistent in the 10 years until follow-up, particularly if during that time women had children. Women also self-reported their exercise intensity, which may have led to bias.
- It is possible that women incorrectly remembered their fertility history as they were asked to recall a period of up to 10 years. The researchers say that this is unlikely.
- Importantly, the fertility of the women’s partners was not considered.
The researchers put forward a number of possible theories to explain their findings, including that fertility problems may be caused when women of a normal weight do a lot of exercise but do not consume enough energy (have a negative energy imbalance). This and their other hypotheses remain to be tested.