Causation vs. correlation in the relationship between ultra-processed food and mental health

The New Zealand Herald reported earlier this week:

According to an annual global report, if you’re after mental wellbeing and a flourishing life, you should pay attention to those who live in the Dominican Republic, Sri Lanka, Tanzania and Panama.

The report, by Sapiens Labs, is available here. However, it was this bit of the article that caught my eye:

According to Sapien Labs, adults’ risk of mental health challenges is four times lower if you have close family relationships - but wealthier countries were least likely to say they were close with many adult family members, at just 23 per cent...

Similarly, there is a strong body of research on the impact of processed food and a growing number of studies around technology use.

“We found that over half of those who eat ultra-processed food daily are distressed or struggling with their mental wellbeing, compared to just 18 per cent of those who rarely or never consume ultra-processed food,” the report stated. This is almost a three-fold increase.

I just talked about the difference between causation and correlation with my ECONS101 class a couple of weeks ago. Everything that Sapiens Labs has found is correlation. Sure, you can tell a plausible story about how ultra-processed foods lower mood and lead to worse mental wellbeing. However, there is also a plausible story going in the other direction (reverse causation) - people with worse mental health might comfort eat, thereby consuming more ultra-processed foods. Just because we observe a correlation between higher ultra-processed food consumption and lower mental wellbeing (a negative correlation), it doesn't mean that ultra-processed food consumption causes decreases in mental wellbeing.

Even worse than that, the report itself (but not the New Zealand Herald article) tries to suggest a link between higher consumption of single-use plastics and lower mental wellbeing. I'm not even sure that you can tell a plausible story linking those two variables in that direction - how would plastic straws, forks, and grocery bags make our mental health worse? This could well be spurious correlation. However, I'd be surprised if there is even a correlation there at all. Many countries (including New Zealand) have recently banned single-use plastics. Have we seen an immediate improvement in mental health in those countries? I thought not.

Just because two variables are moving together (either in the same direction, or opposite directions) that doesn't mean that changes in one variable are causing changes in the other one. No matter how much you might want them to, or how much you are looking for a simple explanation. Correlation is not the same as causation.