JOHN YANG: The food we eat affects us in many ways, eating a lot of highly processed foods has been shown to increase the risk of cancer, stroke and heart disease.

But what about mental health?

Ali Rogin takes a deeper look at new research into that question.

ALI ROGIN: A recent study from Massachusetts General Hospital and Harvard Medical School finds a link between ultra-processed foods and depression, it found a nearly 50 percent increase in the risk of depression for those who consumed nine portions or more of ultra-processed foods a day.

Researchers also observed that foods and drinks containing artificial sweeteners could increase the incidence of depression.

The study observe the eating habits and mental health status of more than 31,000 middle aged mostly white women for nearly 15 years.

Olivia Okereke worked on the study and is an associate professor of Psychiatry at Harvard Medical School.

Olivia, thank you so much for joining us.

Where did the idea to study this connection come from?

OLIVIA OKEREKE, Harvard Medical School: Well, depression we know is one of the leading causes of disability worldwide.

There are hundreds of millions of people affected by depression.

And so we need to identify ways to prevent it to modify the risk.

And diet is one of those things that people can readily modify.

So, we wanted to look at an important dietary factor in this case, the frequency of consumption of processed foods.

If we looked at all of these ultra-processed foods combined, that a higher number of servings of these nine or more compared to four or fewer per day, were related to this increased risk of depression if there were certain types of processed foods that had a particularly high risk.

So the artificially sweetened beverages in particular, we saw had an increased risk of depression.

ALI ROGIN: Let's just break down really quick, what are the reasons why processed food is so bad for us?

OLIVIA OKEREKE: So we know that processed foods involve adding artificial flavorings, preservatives, whitening, they sometimes involve modification to the food itself, stripping it of some of its nutrient components.

The other thing is that the chemicals involved in ultra-processing of foods also may be unhelpful.

So we think that they may be triggering some unhealthy biological processes that are the same things that could be predisposing to depression.

ALI ROGIN: And of course, there are a number of different risks when it comes to depression.

There's family history, how are you able to rule out that participants in this study, we're not experiencing one of those other risk factors?

OLIVIA OKEREKE: Well, there's a couple of things that we do.

So the first is that we make sure that at the start of following up the participants, that they're not affected by depression already.

So we're really looking into what we call incident depression, that means the person is developing depression during the follow up.

And then the second thing that we do is we look at a whole range of other factors that may correlate with diet.

So things like smoking, physical activity, there's a whole range of these factors.

And they're all very carefully measured in the same women.

ALI ROGIN: Now, the cohort we're talking about, it's an existing observational study.

It's called the Nurses Health Study.

And you made clear in the study the limitations of this cohort, because it's primarily, as we said, middle aged white women, what are the drawbacks of such a limited cohort?

OLIVIA OKEREKE: Well, one of the strengths is that it's very large, these women have been followed for nearly 50 years.

And so we know a lot about a whole range of factors.

And that's critical for doing careful analyses.

Of course, there aren't any men, and it may not be as diverse as we see in other cohorts.

And so that is a limitation.

And that means if there's a stronger association, for example, of these ultra-processed foods and depression risk among young people, or among other groups in the population, we won't really be able to see anything about that.

ALI ROGIN: And that really leads to my next question, which is how clear is the link between ultra-processed foods and mental health right now and what sort of additional study needs to be conducted?

OLIVIA OKEREKE: So, one of the strengths of this particular study is that there really weren't prior studies of this type.

So we've alerted people to this potential issue.

And so some of the next steps are the following.

The first is to look at a broader sample of people.

So for example, other studies that have data on diet and on depression, but maybe more diverse in terms of age ranges, gender, racial or ethnic backgrounds.

Another type of follow up that can be done is to try to understand the mechanisms.

For example, ultra-processed foods may increase inflammation in the body, they may cause metabolic changes in the body that are the same things that we suspect are related to risk of depression.

But additional studies to probe those mechanisms can also be done.

ALI ROGIN: Olivia Okereke with Harvard Medical School, thank you so much for your time.

OLIVIA OKEREKE: Thank you.