"White bread and pasta 'may increase the risk of depression'," reports the Mail Online today.
It doesn’t take much to realise that feeling down from time to time is probably not caused by the last cheese sarnie or bowl of spag bol you ate. But in this case, the news outlet is reporting on a well-conducted study of post-menopausal women’s diets and their depressive symptoms over time.
While the research did find a significant link between symptoms of depression and high dietary glycaemic index (GI) and glycaemic load, it can’t prove an inevitable cause and effect. The study also found that depression symptoms were particularly high in women who were less physically active, had a higher BMI, consumed more fatty foods, and less fruit and vegetables.
The relationship between diet and lifestyle, and other physical and mental health symptoms and conditions is complex, and it is not easy to single out direct effects.
Basic advice on a healthy diet includes a significant amount of starchy food, so do not be put off your morning toast by this news. Find out about the five steps to mental wellbeing if you want to know which activities, such as learning and exercise, may improve how you feel.
Where did the story come from?
The study was carried out by researchers from Columbia University, Stony Brook University, University of California-Davis, New York University Langone Medical Center, Duke University Medical Center and the University of Minnesota, all in the US. It was funded by the US National Heart, Lung, and Blood Institute.
The study was published in the peer-reviewed medical journal The American Journal of Clinical Nutrition.
Overall, the UK media reported the story accurately, but the study's limitations were not fully explained.
The Mail Online reported a quote from one of the researchers, Dr James Gangwisch, of Columbia University: "This suggests that dietary interventions could serve as treatments and preventive measures for depression." He added that, "Further study is needed to examine the potential of this novel option for treatment and prevention, and to see if similar results are found in the broader population."
The lack of clarity over whether a high-GI diet directly causes depression, or whether there could be some reverse association, or the involvement of other factors, makes it difficult to say whether such interventions could show promise.
What kind of research was this?
This was an longitudinal cohort study looking at the association between dietary GI and glycaemic load, and the prevalence and incidence of depression in post-menopausal women.
Researchers say that previous studies have shown positive association between consumption of sweetened beverages, processed foods (such as sweetened desserts and processed meats), and processed pastries (muffins, doughnuts, croissants and other commercial baked goods) and the risk of developing depression.
This was a longitudinal cohort study, so data was collected from the same people repeatedly over time. These studies can have variable length of follow-up to look at both short- or long-term impacts of an exposure (such as diet). One of the main drawbacks of this type of study design is that they do not fully explain whether the exposure (e.g. diet) causes the effects seen. Randomised controlled trials (RCTs) are a better way to understand the causal link, but RCTs on dietary links with health conditions trials can be unfeasible and unethical.
What did the research involve?
This study included 69,954 socioeconomically and racially/ethnically diverse post-menopausal women aged 50-79 years from 40 medical centres across the US between September 1994 and December 1998, as part of the Women’s Health Initiative.
Women with symptoms of depression at the time of recruitment – as assessed by the eight-item questionnaire used in the study – were excluded. Data was collected on characteristics such as education level, presence of health conditions and smoking status.
The women completed a 145-item food frequency questionnaire at the start of the study. This questionnaire was designed to work out the women’s intake of carbohydrate and dietary fibre and specific foods (whole grains, vegetables, nuts, seeds and legumes). This was then used to calculate GI and glycaemic load. The researchers analysed women’s diets in five groups or "quintiles", based on the levels of GI in their diets.
Depression symptoms after three years of follow-up were measured using the same Burnam eight-item scale for depressive disorders that was given at the study’s start.
Researchers used statistical methods to examine the relationship between GI and glycaemic load and depression symptoms at follow-up.
What were the basic results?
At the start of the study, women with higher GI quintiles tended to:
- be younger
- have higher BMI
- do less physical activity
- eat more fatty foods
- eat less fruit, vegetables, legumes, nuts, seeds and dietary fibre
They were also more likely to be black, have lower education, lower income, high blood pressure, and have previously had a heart attack. They were less likely to be on hormone replacement therapy, but more likely to smoke and have had stressful life events, and they were less likely to have good social support.
After three years, women who consumed more dietary added sugars were significantly more likely to experience symptoms of depression (odds ratio (OR) for the highest GI compared with lowest intake, 1.23, 95% confidence interval (CI) 1.07 to 1.41). Those who consumed a higher GI were also significantly more likely to experience depression symptoms (OR for the highest compared with lowest intake, 1.22, 95% CI 1.09 to 1.37).
Eating more dietary fibre and fruits and vegetables was associated with decreased odds for depression symptoms.
How did the researchers interpret the results?
Researchers concluded that, "The results from this study suggest that high-GI diets could be a risk factor for depression in post-menopausal women."
They added that, "Randomised trials should be undertaken to examine the question of whether diets rich in low-GI foods, such as legumes, cereals high in viscous sticky fibres, and temperate-climate fruit, could serve as treatments and primary preventive measures for depression in post-menopausal women."
This observational study has found that over three years of follow-up, post-menopausal women who consumed a high-GI diet and larger amounts of dietary sugar were more likely to have depression symptoms three years later.
They study has several strengths, including a large sample size, a socioeconomically and racially/ethnically mixed population, and a relatively long follow-up period of up to three years.
However, this observational study cannot prove that a high-GI diet directly causes depression. Other limitations include its observational nature, meaning that it couldn’t weed out all the factors that might have influenced the link. It is not easy to single out direct causative effects, or completely exclude the influence of all other factors without conducting an RCT.
It’s also worth pointing out that the women reported their own diets, which allows for potentially inaccurate reporting. Also, the study used a brief questionnaire to assess depression symptoms, but didn't examine diagnoses of depression. It is also possible that this short scale could not have fully assessed all mental health symptoms that a person may have had at the time of study enrolment.
Finally, the results are only relevant to post-menopausal women (as they were the only people it looked at) and cannot be generalised to men nor to pre-menopausal women.
Overall, this study explores the possible links between eating habits and the risk of depression symptoms, but it can’t provide any firm answers.
We all recognise the link between what we eat and how we feel (for example, through so-called comfort eating). NHS Choices has a range of advice on getting a balanced diet, including information on starchy foods, as well as a series of audio guides to boost your mood.
Analysis by Bazian
Edited by NHS Website
Links to the science
American Journal of Clinical Nutrition. Published June 24 2015