The idea regular trips to the gym can trigger an early menopause is a myth, say scientists.
A study of more than 107,000 women found no link between levels of physical activity and the age at which they became infertile.
Previous research has claimed those who spend a lot of time exercising are more likely to go through the change of life in their 40s – or even sooner.
But the latest analysis is the largest and longest of its kind – based on data from US nurses who were followed for more than two decades.
Epidemiologist Professor Elizabeth Bertone-Johnson, of the University of Massachusetts, said: “Our study provides considerable information in helping us understand the relationship between activity and timing of menopause.
“This is because of its size, its focus on early menopause specifically, and because of its prospective design, which limited the likelihood of bias and allowed us to look at physical activity at different time periods.”
Until now, there have been conflicting results with some studies suggesting very active women may be at lower risk of menopause before the age of 45, while others have found evidence of the opposite effect.
The issue has important implications for health.
An early menopause increases the risk of developing diabetes, cardiovascular disease and osteoporosis because of the loss of oestrogen.
On the other hand the hormone promotes breast tumours, and that may explain why early menopause is tied to a lower risk of breast cancer.
Prof Bertone-Johnson’s team also found a diet high in vegetables, cheese and yoghurt – which are rich in calcium and vitamin D – reduces the risk of an early menopause.
The menopause, which occurs on average at the age of 51, happens when the body stops naturally producing oestrogen and other sex hormones.
About one in ten women in Britain start the menopause before the age of 45, one in 100 before they are 40 and one in 1,000 before they are 30.
While high levels of physical activity – usually five or more hours of exercise per week – have been tied to early menopause they have also has been linked to irregular menstrual cycles, which could lead to later menopause.
Research director Prof Bertone-Johnson said: “Several previous well designed studies have found suggestions that more physical activity is associated with older age at menopause, but even in those studies the size of the effect was very small.
“Our results, in conjunction with other studies, provides substantial evidence that physical activity is not importantly associated with early menopause.”
An earlier Japanese study of over 3,000 participants found those who worked out the most – about eight to 10 hours a week – were 17 per cent more prone to early menopause than sedentary peers.
The new research published in the medical journal Human Reproduction involved more than 30 times as many individuals who were tracked for over twice as long – from 1989 to 2011.
The registered nurses were aged 25 to 42 when they were enrolled in the Nurses’ Health Study II and completed questionnaires about lifestyles and medical conditions every two years.
They were asked about the time they spent in recreational physical activities such as walking, running, cycling, racquet sports, swimming laps, aerobic activities, yoga, weight training and high intensity activities such as lawn mowing.
The researchers also collected information on factors such as race, ethnicity, age, education, height, the age when they had their first periods and whether or not they had been pregnant and how often.
They were also asked about use of oral contraceptives and hormone therapy, their smoking history, weight and body mass index (BMI), diet and use of dietary supplements.
In order to assess the frequency, duration and intensity of the activities, the researchers multiplied the hours per week of each activity by its metabolic equivalent (MET) score to create total MET hours per week.
MET (Metabolic Equivalent of Task) is a measure of energy consumed per hour. A MET score of one is the sort of energy you would expend watching TV.
Anything which gets a score above three counts as moderate activity. Above six and you are in the realms of “vigorous”.
During more than 20 years of follow-up, 2,786 women experienced natural menopause before the age of 45.
There was no significant difference in the risk between, for instance, those reporting less than three MET hours a week of physical activity or 42 or more.
The latter would be equivalent to four or more hours of running or eight or more hours of brisk walking a week.
The amount of physical activity reported in their teenage years was also unrelated to the risk of early menopause.
First author Mingfei Zhao, a graduate student, said: “While our results do not suggest more physical activity is associated with lower risk of early menopause, we would encourage pre menopausal women to be physically active, as exercise is associated with a range of health benefits, such as a lower risk of heart disease, diabetes, breast cancer and other conditions.
“Our results in no way suggest that premenopausal women should not be physically active.”
Researchers are still investigating other factors that might play a role in women experiencing an early menopause.
Added Prof Bertone-Johnson: “Our work has suggested environmental factors are associated with early menopause.
“We found higher intake of calcium and vitamin D from dairy foods to be associated with lower risk.
“Higher intake of vegetable protein was associated with lower risk as well, though animal protein was not.
“Cigarette smoking is associated with higher risk, as is being underweight. We are currently investigating other factors as well.”
During menopause, a woman’s ovaries stop producing eggs and she can no longer get pregnant. It usually begins between the ages of 41 and 55.
An estimated 1.9million women in the UK are going through the menopause at any one time.
Some 80 per cent of these women are thought to experience symptoms, which typically last for about four years, including depression, hot flushes, headaches and night sweats.
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