“Eating junk food has a negative effect on mental health, making those who consume it regularly feel depressed,” said The Daily Telegraph.

The news is based on a Spanish study that looked at how 9,000 people’s consumption of fast food and baked goods, such as pies and pastries, related to their risk of depression. In a week when tax on pasties and pies has been the source of great distress for some, researchers found that people who consumed the most fast food and baked good were 37% more likely to become depressed over a six-year period than people with the lowest consumption.

This study had some strengths. For example, it established people’s diets before they were followed to see if they developed depression, which means their diets preceded their depression. However, it cannot conclusively show that fast food directly causes depression. For example, it is just as plausible that diet and depression are both the result of a common factor. Therefore, it's too early to rebrand the burger and fries as an "unhappy meal".

Where did the story come from?

This Spanish study was carried out by researchers from the University of Las Palmas in Gran Canaria and the University of Navarra. It was funded by the Spanish Government’s Carlos III Institute of Health.

The study was published in the peer-reviewed journal Public Health Nutrition.

The study's methods were covered appropriately by the media. However, the 51% increase in risk of depression that was quoted by the Telegraph and Daily Mail did not appear in the research paper. The paper reported an increased risk of 37%.

What kind of research was this?

This prospective cohort study assessed the relationship between eating fast food or processed pastries and developing clinical depression. The research project, called Seguimiento Universidad de Navarra (SUN), is a long-running cohort study that involves university graduates in Spain. The study continuously recruits new participants, and collects data on a variety of factors using mailed questionnaires.

Prospective cohort studies assess participants and then look at the development of various factors over time. They have the advantage of initially measuring the exposure of interest (in this case, consumption of fast foods or processed pastries) in a group of people who do not already have the outcome of interest (in this case, clinical depression). This allows the researchers to be certain that the exposure came before the outcome, which is important for determining a cause-and-effect relationship.

Cohort studies can collect data on a number of other factors that may also account for the relationship between the exposure and outcome. These factors are known as confounders. Adjusting their results to account for the influence of confounders allows researchers to be fairly certain that these confounding factors do not influence the results. However, they cannot take into account factors that weren’t measured during the study. Therefore, it is possible that, during a cohort study, unknown factors may account for the relationship seen, rather than the exposure of interest.

What did the research involve?

The researchers used data from the SUN study to identify participants for their research. They included people who did not have a clinical diagnosis of depression and who were not taking antidepressant medication (to ensure that the participants were free of depression at the beginning of the study). All participants were also free of cardiovascular disease, diabetes and hypertension.

The participants completed the food frequency questionnaire at the beginning of the study. They assessed two exposure variables: fast food consumption (which included hamburgers, sausages and pizza) and consumption of commercial baked goods (which included muffins, doughnuts, croissants and other baked goods). The researchers then divided the cohort into five groups (quintiles), based on the amount of each food group that they usually consumed.

The participants were then followed up for a median of 6.2 years. The researchers used a mailed questionnaire to determine whether the person had been diagnosed with clinical depression or had been prescribed antidepressant medication during this time.

The researchers collected data on other variables they thought might influence the relationship between eating habits and depression. These included age, sex, body mass index, smoking status, physical activity level, total energy intake and healthy food consumption. They then adjusted for the influence of these variables during the statistical analysis.

What were the basic results?

In total, 8,964 participants were included in the study. Participants with the highest consumption (quintile 5) of fast food and baked goods were more likely to be single, younger, less active and have worse dietary habits than participants with the lowest consumption (quintile 1).

After a median follow-up of 6.2 years, 493 cases of clinical depression were reported.

When assessing the relationship between fast food consumption and the development of depression, the researchers found:

There were 97 cases of depression in the group with the lowest consumption (quintile 1) compared with 118 cases in the group with the highest consumption (quintile 5). When the sizes of the quintiles were taken into account, this equated to people with the highest levels of consumption having a 37% greater risk of developing depression than those with the lowest levels of consumption (hazard ratio [HR] 1.37, 95% confidence interval [CI] 1.01 to 1.85).

Intermediate levels of consumption (quintiles 2, 3 or 4) were not associated with significantly increased risk of developing depression compared to the lowest consumption level.

When assessing the relationship between commercial pastry consumption and the development of depression, the researchers found:

People with the highest level of consumption (quintile 5) had a 37% increased risk of developing depression compared to the lowest consumption group (quintile 1) (HR 1.37, 95% CI 1.01 to 1.85).

How did the researchers interpret the results?

The researchers concluded that their results demonstrate “a positive dose-response relationship between the consumption of fast food and the risk of depression”. In other words, as consumption of fast food increases, so does the risk of depression. They also said that “consumption of commercial baked goods was also positively associated to depressive disorders.”

Conclusion

This study has found an association between consuming high levels of fast food and baked goods and the risk of developing depression. Even though this was a prospective study, it cannot conclusively show that eating lots of hamburgers, sausages and pizza causes depression. The tendency to consume fast food and develop depression may both have stemmed from some common factor, rather than fast food directly causing depression. For example, participants with the highest fast food consumption were generally all single, younger and less active, which may have influenced both their diet and their risk of depression.

Several important factors should be noted:

This study used a questionnaire to determine whether a person had clinical depression. This method may be less reliable than either a clinical interview or a diagnosis confirmed by medical records. Some people with depression may not have reported that they had been given a diagnosis. Alternatively, other people may have considered themselves to have depression without having a clinical diagnosis from a doctor. Equally, some people who would have met diagnostic criteria for depression had they seen a doctor may not have realised that they had the condition.

Though the researchers adjusted their results for lifestyle and socioeconomic factors that may have influenced diet and depression risk (potentially confounding the relationship between the two), depression may be triggered by many factors. It is difficult to ensure that all possible confounders were taken into account.

If there is a direct association between these dietary items and risk of depression, the underlying mechanism by which eating these foods could lead to depression is not known.

The cohort excluded people with multiple underlying illnesses and conditions, such as cardiovascular disease and high blood pressure. While this allowed the researcher to ensure these conditions did not influence their results, it makes it difficult to generalise the results to the wider population. Also, these types of illnesses may influence both diet and risk of depression, so it is arguable that including people with them could have been a valid option.

The cohort was divided into groups based on their relative consumption of fast foods and commercial baked goods, and not on an absolute level of consumption. Therefore, the results of this study would only apply to a population that had a similar pattern of consumption.

Overall, this study suggests that there may be an association between eating a lot of fast food or baked goods and developing depression. It is, however, difficult to apply the findings to other groups of people, and it is unclear if the relationship would remain under different conditions.

Analysis by Bazian

Edited by NHS Website