"Eating potatoes before pregnancy increases risk of diabetes," The Daily Telegraph reports. Researchers found a small, but significant, increase in gestational diabetes risk in mothers who reported eating a potato-rich diet before their pregnancy.

Gestational diabetes is caused by raised blood glucose levels during pregnancy. It doesn't usually cause any symptoms, but can cause complications if left untreated.

The condition does not usually pose a problem for women in England, as diabetes can be routinely screened for. If it is diagnosed, it can normally be controlled by diet and exercise.

This latest study involved researchers in the US looking at records for 21,693 pregnancies. They found that women who said they regularly ate potatoes were more likely to have had gestational diabetes.

The researchers estimated that women who regularly ate five or more portions of potatoes a week had a 50% increase in gestational diabetes risk compared to women who ate none. While this may sound high, the overall rate of gestational diabetes in the study was reported to be 5.5%.

Researchers looked at potatoes because they have high glycaemic index (GI), so release a lot of glucose into the blood shortly after being eaten. Some experts think this might increase the chances of diabetes.

Although the study found an association between potatoes and diabetes, it cannot prove cause and effect.

There's no need to stop eating potatoes as a result of this study. On the other hand, a little variety in the types of food you eat, with plenty of vegetables and pulses, makes it easier to get a healthy and balanced diet that includes all the nutrients you need.

Where did the story come from?

The study was carried out by researchers from the National Institutes for Health, Brigham and Women's Harvard Medical School and Harvard TH Chan School of Public Health, and was funded by the National Institutes of Health and the American Diabetes Association. The study was published in the peer-reviewed British Medical Journal (BMJ) on an open-access basis, meaning it is free to read online (PDF, 304kb).

The Daily Mirror and the Daily Telegraph both over-stated the certainty of the results, with the Mirror referring to "type 2 diabetes" instead of "gestational diabetes". While there are similarities between the two conditions, their causes and likely outlooks are different.

However, the Mail Online and the BBC News gave good, balanced reports.

What kind of research was this?

This is a prospective cohort study, which looked to see whether there was a link between regularly eating potatoes and the chances of having gestational diabetes. Prospective cohort studies provide useful information about links between different factors, but cannot prove that one factor causes something – in this case, that eating potatoes causes gestational diabetes.

What did the research involve?

The researchers looked at records from a large group of women in the US. They looked at how often they ate potatoes (measured in diet questionnaires every four years) and whether they’d had diabetes in pregnancy. After adjusting for other confounding factors, they looked for links between diabetes in pregnancy and eating potatoes.

They used data from an ongoing study of 116,430 nurses in the US, picking a 10-year time period from 1991 to 2001. Researchers only looked at pregnancies during that time in women who had not had gestational diabetes before, and had not been diagnosed with cancer, diabetes or heart disease at the start of the study.

As well as looking at how often they ate potatoes, the researchers also took account of how healthy their diet was overall, how much they exercised, their weight, age, ethnic group and any family history of diabetes.

They ran several different analyses of the data, to see which factors affected the chances of women getting gestational diabetes. They used the findings to calculate the chances of getting gestational diabetes if they ate potatoes once a week, two to four times a week, or five or more times a week. They also looked to see what the effect might be if women swapped two portions of potatoes a week for other healthy food, such as wholegrains, vegetables or pulses.

What were the basic results?

Women who said they regularly ate two to four portions of potatoes a week were 27% more likely to have had gestational diabetes (relative risk [RR] 1.27, 95% confidence interval [CI] 1.04 to 1.55) and women who’d eaten five portions a week or more were 50% more likely to have had gestational diabetes (RR 1.50, 95% CI 1.15 to 1.96). One portion a week may also have had an effect, but the results for this group are not statistically significant, meaning the finding may be down to chance.

The overall risk of diabetes in pregnancy was quite low. There were 21,693 pregnancies and 854 cases of gestational diabetes over the 10 years of the study. The researchers said the rate of gestational diabetes in the study was 5.5%. An increase in risk of 50% from eating five or more portions a week would mean a risk of around 8%.

The researchers calculated that swapping two portions a week of potatoes for wholegrains, vegetables or pulses would reduce the relative risk by 9%, to 12%, depending on the type of food substituted.

How did the researchers interpret the results?

The researchers make it clear that these results do not show that potatoes cause diabetes in pregnancy. However, they say the suggestion that they might is "biologically plausible" because potatoes are starchy foods and quickly digested.

They said that "findings from the current study raise concerns" about dietary guidelines in the UK and the US, which advise people to eat plenty of potatoes.

Conclusion

Although we cannot say from this study whether eating potatoes could cause gestational diabetes, it does seem worth taking seriously.

This study has a number of strengths. It's big enough to give statistically significant results and the researchers were able to adjust their results to check for many factors which could have had an effect on women's chances of getting gestational diabetes.

They carried out sensitivity analyses to check that no one factor was skewing the results. Also, as the researchers say, there is a possible plausible scientific reason for thinking that potatoes might increase the risk of diabetes.

However, there are drawbacks to the study. The results are based on the women's own estimates of how often they ate potatoes, and also whether they had diabetes in pregnancy. It's possible they may have forgotten or misreported this. We also don't know how bad the women’s diabetes was, so we can't tell whether eating more potatoes affects the severity of diabetes in pregnancy.

Also, most of the women in the study were white Americans, so we can't be sure the results would apply to everyone. This is particularly important, as it is known that the risks of gestational diabetes are higher in some ethnic groups, such as black women or women of South Asian origin.

Finally, even the best observational study cannot have adjusted for all possible factors affecting the outcome. That's why we can't say that potatoes are the cause of the increased risk of gestational diabetes.

Further research is needed to find out more about the potential links between potatoes and gestational diabetes. But what should women do if they want to get pregnant and are worried about their risk?

Advice from Public Health England remains unchanged – people should continue to eat starchy foods, including potatoes and wholegrains, to get plenty of fibre. If you are worried about how often you eat potatoes, swapping one or two portions a week for wholegrain rice, sweet potatoes, pasta or bread means you will still be following official advice, while eating a more varied diet.

There's no need to stop eating potatoes as a result of this study. On the other hand, a little variety in the types of food you eat, with plenty of vegetables and pulses, makes it easier to get a healthy and balanced diet that includes all the nutrients you need.

Analysis by Bazian

Edited by NHS Website