“Eating a diet high in processed foods increases the risk of depression,” BBC News reported.
This report is based on data from a long-running study of middle-aged civil servants. An analysis found that eating processed foods was associated with depression five years later, even after other social and health factors were taken into account.
However, the study design has some limitations, and although this type of study (called a cohort study) can make a strong case for causation, it cannot prove that one thing causes another. In addition, it is possible that depression affects a person’s diet rather than the other way around.
A link between diet and depression seems plausible, but further research that gives more conclusive evidence is needed.
The study was carried out by Dr Tasnime Akbaraly and colleagues from University College London. The study was based on data from the Whitehall II study, which was funded by grants from the Medical Research Council, British Heart Foundation, UK Health and Safety Executive, Department of Health and several national funding organisations in the US. The study was published in the peer-reviewed British Journal of Psychiatry .
BBC News gives a balanced report of the study and points out that this sort of study cannot prove cause and effect, but can only show associations.
This was a cohort study, which used data from a larger, long-running cohort study called the Whitehall II study. Whitehall II is a well-established and well-regarded study that was set up to investigate how social class, lifestyle and psychosocial factors contribute to the risk of disease. Many subsequent studies have used its data to produce or dismiss several theories regarding risk factors for disease.
This particular research investigated whether there is an association between diet and depression.
As a cohort study, it can make a strong case for causation, but it cannot prove cause and effect, in this case that poor diet causes depression. In addition, it cannot rule out reverse causation, in other words that depression may have affected the participants’ diets.
Other factors, measured or unmeasured, may also confound the association between an exposure and outcome. The researchers attempted to account for some of these factors by collecting certain sociodemographic factors and health behaviours and adjusting for them in their analysis. This was a strength of the study.
Between 1985 and 1988, the Whitehall II study enrolled 10,308 London-based civil servants aged between 35 and 55. When they signed up, the participants were given a physical examination and a broad questionnaire about their diet and lifestyle. At five-year intervals after this, they were invited for clinical examinations and between these visits were sent postal questionnaires.
This particular study involved 3,486 white European participants who had data collected on dietary patterns and related factors from 1997 to 1999, and on depression from 2002 to 2004.
Food intake was measured using a food frequency questionnaire adapted from another study that asked how much of 127 items the participants ate during the past year. It is not clear whether this food frequency questionnaire had been validated in the UK population, although the researchers report that the questionnaire was ‘anglicised’ (presumably meaning it was made relevant to UK foods). Each participant was given a score according to their responses. This score was used to measure how well they fit two dietary patterns: ‘whole foods’ (a high intake of vegetables, fruits and fish) or ‘processed foods’ (including fried food, chocolate, pies, processed meat and refined grains). Within each group, scores for each pattern were divided into thirds to indicate how well the person fit the pattern.
A statistical method called logistic regression was used to examine the association between dietary pattern and depression. This is an appropriate analytical method for these types of data. Factors that could have affected this link, including sociodemographic factors (such as age, gender and education) and health behaviours (such as smoking and exercise) were taken into account in the analyses. The researchers also carried out analyses that excluded people who had depression at the time of the dietary assessment (defined as having a score above a cut-off point on a depression scale, or receiving antidepressants).
People with the highest intake of whole foods were less likely to have depression. This was the case even after all the factors that may have influenced this link were taken into account (odds ratio [OR] 0.74, 95% confidence interval [CI] 0.56 to 0.99). People who ate the most processed foods were more likely to have depression (OR 1.58, 95% CI 1.11 to 2.23).
This link between processed foods and depression remained statistically significant after those who already had depression when they completed the dietary questionnaire were excluded from the analysis. This was not the case for the whole foods group, where the association with less depression was no longer statistically significant.
The researchers conclude that, in middle-aged people, processed foods are a risk factor for depression five years later, while whole foods can protect against it.
This study suggests that a healthier diet protects against depression, but it cannot prove this due to several limitations:
The researchers conclude that processed foods are a ‘risk factor’ for depression rather than specifically labelling them a ‘cause’. This is a balanced conclusion, considering that unmeasured factors may contribute to this association. A healthy diet has a range of proven benefits and the suggestion from this study that there is a link with improved mental health seems plausible. Randomised controlled trials would provide more conclusive evidence for this.