We all know that eating junk food is bad for you. That burger is full of fat. Put down that candy, it'll only rot your teeth! And while you're at it, put down that muffin, it'll make you...depressed?

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Or at least, that's what you'd think from all the headlines. "Want to cheer up? Stop eating junk food", "Too much junk food can lead to depression", "Sweets and fast food linked to depression". And while this study did find that those who consume more fast food are more likely to be diagnosed with depression...correlation is not causation. Not only that, reading this study shows that there are many other possible correlations that need to be ruled out before we can say that it's the fast food that's really getting you down.

Sanchez-Villegas et al. "Fast-food and commercial baked goods consumption and the risk of depression" Public Health Nutrition, 2012.

The role of diet in the development of psychological disorders is starting to receive some attention. After all, it plays a role in so much else, and wouldn't it be nice if we could just remove one thing from our diets and assured that everything would be ok?

The authors of this study hypothesized that fast food may be more than just bad for your body, it could also be bad for your mental health. They looked at a cohort of 8,964 participants, none of whom had a diagnosis of depression. They took the amount of fast food and commercial baked goods they ate at baseline, as well as other factors like the amount of healthy food, BMI, potential correlates of healthy lifestyle (leisure time vs active), employment status, hours worked, etc, etc. They then followed up with them for up to 6 years.

The authors found, after controlling for baseline BMI, leisure time, employment status, and potential healthy lifestyle factors and healthy food consumption, that people who ate more fast food were more likely to receive a diagnosis of depression in the 6 years followup than those who ate little to no fast food. With fast food there are a dose-response curve, with more fast food eaten correlated with higher rates of diagnosis. With commercial baked goods, they also found an association between consumption and depression diagnosis, though there was no dose effect. It appears as though the data are in: eating bad food means you're at higher risk for depression.

Or is it? While I think the correlation is completely real, I also wonder what other things might be present that influence this correlation. For example, all they corrected for was employment status (employed vs not), not socioeconomic status, which is correlated heavily with both fast food consumption and mental illness diagnoses. They excluded diagnoses of comorbid disorders, but, and this is key, only at baseline. They only took the data on their BMI, comorbid disorders, etc, at the original point. And they then followed them out for two to six YEARS, without taking further data. Considering that comorbid disease is strongly correlated with depression, this would be good factor to control for, as people might develop comorbid disease and then depression. Not only that, they assessed intake of fast food and calories, again, only at baseline. Some of these diagnoses of depression were SIX YEARS following the start of the study, and who knows how much their diets may have changed.

In addition, the people more likely to be diagnosed with depression were also more likely to be smokers...and more likely to work 45/hours per week or more. This indicates a certain amount of stress in their lifestyles that might influence both the fast food consumption and the depression. High stress levels have been previously shown to increase the risk for depression, and they also correlate increased fast food consumption. They didn't control for these factors and I wonder how much a high stress lifestyle influenced the results.

Finally, they didn't actually call it fast food. The study was all self-report, and what they asked about was whether people ate sausages, burgers, and pizza. For baked goods they asked about muffins, doughnuts, croissants, and other commercial baked goods. The question is...how much of this was fast food? Did these people make some of these at home? Not only that, the authors hypothesize that it's the trans fats and carbohydrates in these foods that correlate with the depressive symptoms, but there are many other sources of trans fats and carbohydrates, and there are plenty of carbohydrates in a healthy diet as well. There are also many other types of fast food (though apparently these are the ones consumed most often in Spain, where the study took place).

Don't get me wrong, I believe this correlation. But correlation is not causation. I think it's going to be a lot harder to prove that fast food causes depression, than to prove that things like higher BMI, higher stress levels, lower socioeconomic status, and other factors are correlated with fast food consumption AND with depression. But even though correlation is not causation, and fast food may not cause depression, there are probably enough reasons to put down that burger if you can.

Sánchez-Villegas, A., Toledo, E., de Irala, J., Ruiz-Canela, M., Pla-Vidal, J., & Martínez-González, M. (2011). Fast-food and commercial baked goods consumption and the risk of depression Public Health Nutrition, 15 (03), 424-432 DOI: 10.1017/S1368980011001856