Tons of studies have found that there is a relationship between diet quality and junk food. If you go online to the world of medical journals, via Medscape, you’ll find studies that show that junk food is associated with depressive symptoms. But one of the first things a person learns in a graduate statistics course is that ‘associations are not causation.’ So, for example, if junk food and depression are related, which came first? Or, could there be a third cause (sometimes called a ‘confounder’) that might be causing both?
I looked into this last month and found some studies that examined this important relationship. Turns out, many of the studies that find a relationship between depression and diet are ‘cross sectional.’ That means, that the data is collected at one time point. There’s no way to figure out which came first—the depression or the junk food. And even more importantly, many of the studies are small—under 1000 patients. What does that mean for our analysis? It means that important confounders—outside factors that might be causing both depression and poor diet—can’t be measured accurately. What is needed is a big study, that is longitudinal (takes place over time) and can show the direction of causality—and has enough participants so that the accurate measurement of confounders is possible.
A study like this came out in 2017. It publishes results from the ALSPAC study in the UK, a huge cohort study that has been following thousands of British children and their families for years. In this study, researchers looked at about 11000 parents over time, measuring both their diet and their depressive symptoms.
What they found is important. The quality of diet was associated with depressive symptoms. People who ate a ‘healthy’ diet with lots of vegetables and whole grains were less depressed. People who ate a lot of junk food and processed foods were more depressed.
But, the problem was this: wealthier, more educated people ate much healthier diets. Poorer, less educated people ate a lot more junk food. As the researchers note: In brief, increasing scores on the ‘health-conscious’ components were associated with higher educational attainment, being white, owner-occupied housing and older age. “ Processed, high sugar and high fat food patterns showed associations that were the reverse of these.
What this tells us is that depression and poverty are also related. Being poorer makes people more depressed. Being poorer also leads people to consume a poor diet. When ALSPAC researchers entered socio-economic factors entered into the equation, the relationship between diet and depression went away. Gone!
Why do lower income people eat worse quality diets? Maybe because high sugar and high fat food is comforting or delicious—probably for evolutionary reasons, it relieves our stress in the short term. Maybe it’s because of food deserts—neighborhoods where there isn’t enough good food at affordable prices available for low income people to consume. Maybe because of a time crunch—working hard at long hours means less time to shop and cook. For whatever reason, low income people get stuck with poorer diets as well as more depressive symptoms. This has implications for food justice policy. But, there’s no good science showing us that reducing junk food in the diet will do anything for their depression.
What’s the take home here?
A diet heavy in junk food is not good for you. And if you are one of the many people who are finding that eating has become a compulsion, you should consider seeking help. But that doesn’t mean that your diet is responsible for your depression. People struggling with depression often feel highly self critical. The last thing they need is one more thing to feel they fall short on.
If you are depressed, don’t blame the donut you ate for breakfast. Maybe you need to give yourself a break. Grabbing junk food is understandable when you feel low, and it may relieve your low mood for a little while. Try finding other ‘quick fixes,’—more on that later.
If you can tolerate the jargon, here’s a link to the ALPAC article. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5848748/pdf/S1368980017002324a.pdf
