"Women who eat too much junk food are twice as likely to be infertile," is the deeply misleading headline from the Mail Online.
The study it's reporting on did not look at women who could not get pregnant. In fact, it was a study of nearly 6,000 pregnant women. It questioned what they ate in the month before they became pregnant and how long it took them to get pregnant after they started trying.
The majority of women in the study got pregnant within a couple of months of beginning to try, and the difference in time to conception between those consuming no fast food and those consuming the highest amount was actually only 2 to 4 weeks.
Fast food consumers were at slightly higher risk of having fertility problems, but this was based on a very small subset of women who took longer than 12 months to conceive. In any case, it's not possible to remove the influence of the many other personal, health and lifestyle factors that may contribute to fertility problems.
It's well known that fast food can be high in saturated and trans fats, sugars and salt, and therefore should be eaten in moderation. However, this study doesn't provide convincing evidence that the odd burger and fries will slash your chances of conceiving.
You don't need to go on a special diet if you are trying for a baby. Just make sure you eat a balanced diet, with at least 5 portions of fruit or vegetables a day.
Where did the study come from?
The study was conducted by researchers from the University of Adelaide and Monash University in Australia, and other institutions in Australia, New Zealand and the UK.
The study had multiple sources of funding, including from the NHS, the Biotechnology and Biological Sciences Research Council, University of Manchester Proof of Concept Funding, Guy's and St Thomas' Charity, and Tommy's charity. It was published in the peer-reviewed journal Human Reproduction and is free to read online.
The Mail Online's headline seems to have missed the point of the study, but other UK media sources provided more accurate reports, although none of them discussed the study's limitations, such as the small sample size. They did however note that the difference eating fast food made was small.
What kind of research was this?
This was a large cohort study of pregnant women, conducted across Australia, Ireland, the UK and New Zealand. It looked at whether healthier food, such as fruit and vegetables, or typically unhealthier food, like fast food, can affect the time it takes a woman to become pregnant.
As the researchers said, various female and male factors have been linked with reduced fertility, including smoking, excess alcohol and obesity, but the effect of specific dietary patterns hasn't been studied much.
The main limitation with this method is that it can't attribute pregnancy outcomes to specific foods because a wide range of personal, health and lifestyle factors are likely to influence a woman's ability to conceive.
What did the research involve?
The Screening for Pregnancy Endpoints (SCOPE) study included 5,258 women in the early stages of their first pregnancy, most of whom (94%) conceived without any fertility treatment. Data was collected by questionnaire between weeks 14 and 16 of pregnancy.
Women were asked to recall their diet in the month immediately before conception. Frequency of eating fast food, fish, fruit and green vegetables was specifically assessed.
Time taken to get pregnant was assessed by answering "Duration of sex without contraception before conception with father of baby". Infertility was defined as taking more than 12 months to become pregnant. No further information on causes or actual duration of fertility problems was given.
Although definitions of infertility can vary, UK guidelines advise that women who have not conceived after a year of regular unprotected sex should be referred for assessment.
In assessing the link between diet and time to get pregnant, the researchers took account of other potential confounding factors, including ethnicity, body mass index, smoking, socioeconomic status and past reproductive history, including any miscarriages.
What were the basic results?
The average time for all women in the cohort to get pregnant was 2 months. It took just 1 month for 39% of the women, while 8% (468) took longer than 12 months.
When looking at time to get pregnant, there was a general trend that consuming less fruit was associated with taking longer to get pregnant. But the actual difference was tiny: it only took about 0.2 to 0.6 months longer for those who ate less fruit.
Similarly, consuming less fast food was associated with getting pregnant faster – but again, the difference in time between those eating the most junk food and those eating the least was tiny, between 0.4 and 0.9 months.
Looking at infertility (defined as not being to conceive after 12 months of trying), there was a general trend for increasing intake of fruit and reduced chance of infertility. But none of the results by portion of fruit intake reached statistical significance, so this wasn't a strong finding.
Women who consumed no fast food had a 41% reduced risk of infertility compared with those consuming fast food 4 or more times a week (relative risk [RR] 0.59, 95% confidence interval [CI] 0.37 to 0.94). Women consuming fast food 1 to 3 times a week had no difference in risk compared to those consuming it 4 or more times a week.
There were no significant links with leafy green vegetables or fish.
How did the researchers interpret the results?
The researchers said their findings "underscore the importance of considering preconception diet for [the likelihood of pregnancy] and preconception guidance. Further research is needed assessing a broader range of foods and food groups in the preconception period".
It's well established that a diet high in fruit and vegetables, and low in saturated and trans fats, sugars and salt, is beneficial for overall health. It's also well known that fast food often contains high amounts of the latter so should be consumed in moderation.
However, this study does not provide convincing evidence that fast food consumption is linked to a higher chance of having problems conceiving.
The study did benefit from a large sample of women across countries representative of the UK, but there were a number of limitations to consider.
Observational studies like this can't prove cause and effect – many personal, health and lifestyle factors may influence a woman's chances of getting pregnant. The study cannot account for them all and therefore cannot pin the cause on specific foods.
The women also may not have accurately recalled what they ate, especially when estimating portions per day or week. And in any case, the study only looked at the month before pregnancy, so we don't know whether these were long-term eating patterns.
Furthermore, the majority of women in the cohort got pregnant within a couple of months and, when comparing the effects of different amounts of fast food, this translated to a tiny difference in the time it took to conceive.
The time it took to conceive was also only an estimate and may have been inaccurate. Similarly, "fast food" is a very broad category and could have meant different things to different women.
Finally, only a very small number of the women had fertility problems, and further dividing them according to their diet resulted in such small numbers that the likelihood of findings arising by chance was higher. We also don't know anything about possible causes of these women's fertility problems.
Overall, the study supports general healthy eating advice – factors such as maintaining a healthy weight, eating a healthy diet, exercising regularly, not smoking and limiting alcohol are likely to increase your chances of reproductive success. However, there's no need to worry that having the odd burger might be the cause of problems conceiving.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
BBC News, 4 May 2018
Mail Online, 4 May 2018
Daily Mirror, 4 May 2018
The Daily Telegraph, 4 May 2018
The Independent, 4 May 2018
The Times (subscription required), 4 May 2018
Links to the science
Human Reproduction. Published online May 4 2018