Friday July 9 2010
The women self-reported their chocolate consumption
'A regular chocolate treat 'could halve a woman's risk of giving birth prematurely,'" reported the Daily Mail.
The story is based on research that looked at whether regular chocolate consumption during pregnancy is associated with reduced risks of pre-eclampsia and high blood pressure. It found that a higher chocolate intake in the first or third trimester was associated with a lower risk of pre-eclampsia and in the first three months of pregnancy with a lower risk of high blood pressure.
This study does not provide firm evidence that chocolate consumption can reduce the risk of high blood pressure in pregnancy or pre-eclampsia. However, it does warrant further research into the possible benefits of chocolate. One important limitation is that it relied on women remembering and reporting how much chocolate they ate during pregnancy, which introduces the risk of error.
Chocolate contains caffeine, which should only be consumed in moderate amounts during pregnancy. It is also high in calories and fats. The current advice about chocolate for both pregnant women and everyone else, is to consume it as an occasional treat rather than on a regular basis. Women thought to be at risk of pre-eclampsia during pregnancy should always follow their doctors’ advice.
Where did the story come from?
The study was carried out by researchers from the University of Iowa College of Public Health and Yale University in the US. It was funded by the US National Institutes of Health. The study was published in the peer-reviewed medical journal Annals of Epidemiology.
The Daily Mail’s coverage was fair, although its headline that regular chocolate could halve the risk of premature birth was inaccurate. Premature birth can occur for many reasons, not just as a result of pre-eclampsia. At the same time, pre-eclampsia does not always lead to premature birth, although women who are at high risk may need to be delivered early.
The Mail did mention that the results may have been skewed by women being asked to remember what they had eaten during pregnancy. The newspaper also correctly pointed out that the study failed to distinguish between dark and light chocolate.
What kind of research was this?
This was part of a larger, prospective cohort study about health in pregnancy. This particular study aimed to investigate whether regular chocolate consumption during pregnancy is associated with a reduced risk of pre-eclampsia and hypertension, and whether the risks varied according to the amount of chocolate consumed. The researchers also wanted to find out if the timing or pattern of chocolate consumption during the first and third trimesters had an effect.
The researchers point out that the risk factors for pre-eclampsia are similar to the risk factors for cardiovascular disease. They say that recent studies indicate that regularly eating chocolate (in particular dark chocolate) reduces the risk of cardiovascular disease. It is thought that it does this in several ways, including lowering blood pressure, insulin resistance, blood fats and indicators of inflammation.
Many of these features also apply to pre-eclampsia, providing a ‘strong rationale’ to test for a possible protective effect of chocolate intake. To date, there have been two studies in this area, which reported conflicting results.
What did the research involve?
For their initial interview, the researchers recruited 3,591 women who were less than 16 weeks pregnant. A total of 2,967 women completed the interview, which was conducted in-person by trained personnel, usually at the women’s homes. The women were asked about their medical and reproductive history, height and weight, smoking habits, exercise habits, and alcohol and caffeine intake. They were also asked detailed questions about their chocolate consumption during pregnancy, including both drinks and foods, and asked to recall their average weekly intake of chocolate since becoming pregnant.
The women were interviewed again with the same questions directly after giving birth and asked to recall the last three months of pregnancy. The final analysis was restricted to the 2,508 women who had singleton deliveries and who had hospital delivery records available.
The researchers used the answers from both interviews to calculate consumption patterns separately for the first and third trimesters. The answers were categorised as: less than one serving of chocolate a week, one to three servings a week, and four or more servings a week. They also calculated chocolate consumption for both trimesters combined.
The researchers used blood pressure and urinary protein readings from prenatal and hospital delivery charts to categorise the women as having either high blood pressure, pre-eclampsia or normal blood pressure during pregnancy. Accepted diagnostic definitions were used to do this and the results were validated in a second sample.
The researchers used standard statistical techniques to analyse any potential association between chocolate consumption and the risk of high blood pressure and pre-eclampsia. They adjusted their figures for various potential confounders, including established risk factors for pre-eclampsia such as body mass index (BMI) and maternal age.
What were the basic results?
The researchers found that chocolate intake in the first and third trimesters of pregnancy was more frequent among women with normal blood pressure than among women who developed high blood pressure or pre-eclampsia. Of those who developed pre-eclampsia, 37.5% did not consume chocolate regularly, compared to 19.3% of women who had normal blood pressure and 24.2% of those with high blood pressure.
After adjustment, women who reported regular chocolate consumption (equal to or more than one to three servings a week) had about a 50% reduced risk of pre-eclampsia during the first trimester (odd ratio [OR] 0.55,95% confidence interval [CI] 0.32 to 0.95) and the third trimester (OR 0.56, 95% CI 0.32 to 0.97). Only intake of chocolate during the first trimester was associated with a reduced risk of high blood pressure (OR 0.65, 95% CI 0.45 to 0.87).
Since the researchers found no difference in the size of risk between chocolate foods and drinks, they combined both sources in their analysis.
How did the researchers interpret the results?
The researchers say their findings provide additional evidence of the benefits of chocolate and that further studies are needed to confirm and explain the protective effects of chocolate intake on the risk of pre-eclampsia.
They say that the current understanding of pre-eclampsia as a ‘2-stage disease process’ makes it biologically plausible that trimesters one and two would be ‘critical windows’ for possibly lowering the risk.
The findings from this well-conducted study warrant further research, but do not provide firm evidence that chocolate can protect against pre-eclampsia. One problem is the possibility of ‘reverse causality’, with women who developed high blood pressure in pregnancy possibly being less likely to consume chocolate after diagnosis. Although the researchers say that they took account of this possibility by excluding women with high blood pressure before 20 weeks gestation, it is not certain that this applies to the later analyses. They also claim that the protective effects of chocolate were apparent in the first trimester.
A strength of the study is its size, with a large cohort of women being asked detailed questions about chocolate consumption both in early pregnancy and just after delivery. Classification of pre-eclampsia and high blood pressure were also based on accepted definitions and the researchers controlled for risk factors that might influence the outcomes they were studying.
As the authors note, the study has several limitations:
- The women self-reported their chocolate consumption and had to recall their consumption over a relatively long period of time, which raises the chance that errors were introduced.
- It did not differentiate between dark and other types of chocolate.
- No direct measures of any biomarkers were taken (such as theobromine) to validate associations between self-reported chocolate consumption and the risk of pre-eclampsia and high blood pressure.
- It did not assess what else the women were eating during pregnancy, other than caffeine, which could have skewed the results, although the researchers point out that diet is not currently thought of as a risk factor for pre-eclampsia.
- The findings could be biased by underreporting of chocolate intake by overweight women although the researchers say re-ran their analyses to take account of this and got the same results.
- Although many confounders were taken into account, the results could still have been affected by some of these or other unmeasured confounders, such as other foods or drinks associated with chocolate eating that were not recorded.
Women thought to be at risk of pre-eclampsia during pregnancy should always follow their doctors’ advice.