• Ultra-processed foods are a common component of many people’s diets.
• Although the definition varies, they are generally industrially produced foods that contain ingredients not found in the typical home kitchen.
• Recently, research has raised concerns about the health effects of a diet high in ultra-processed foods.
• Now, a large Australian study has found that high consumption of ultra-processed foods is linked to depressive symptoms.

Almost everything we eat is processed — merely by preparing and cooking a food, you are processing it. However, ultra-processed food is a different matter.

Ultra-processed food is food that has been industrially produced, containing ingredients that you would not find in your home kitchen. It is designed to be “highly profitable (low-cost ingredients, long shelf-life, emphatic branding), convenient (ready-to-consume), and hyper-palatable.”

The NOVA classification categorizes foods by the extent and purpose of industrial processing into four categories, from the least to the most processed.

Group one includes unprocessed or minimally processed foods, such as fresh, frozen, or dried fruits and vegetables, whole grains, nuts, whole cuts of meat and fish, eggs, milk, and natural yogurt. These foods are altered by processes “designed to preserve natural foods, to make them suitable for storage, or to make them safe or edible or more pleasant to consume”.

Ultra-processed foods make up group four, and are described by NOVA as “industrial formulations typically with five or more and usually many ingredients, […] [which] often include those also used in processed foods, such as sugar, oils, fats, salt, antioxidants, stabilizers, and preservatives.”

“Ingredients only found in ultra-processed products include substances not commonly used in culinary preparations, and additives whose purpose is to imitate sensory qualities of group 1 foods or of culinary preparations of these foods, or to disguise undesirable sensory qualities of the final product,” the definition continues.

Ultra-processed foods and mental health

That may not sound too appetizing, but consumption of ultra-processed foods has increased rapidly over the past few decades — a recent study found that ultra-processed foods accounted for almost 60% of the diet of most adults in the United States.

Now, a large-scale study in Australia has found that people who eat a high proportion of ultra-processed foods have a significantly greater risk of depression than those who eat the least. The study is published in the Journal of Affective Disorders.

Dr. Eamon Laird, a visiting research fellow at Trinity College, Dublin, Ireland, who was not involved in the study, told Medical News Today: 

“This is a very interesting study examining a very complicated issue. The authors found that in a cohort of mostly older adults (age [over] 50 years) higher consumption of ultra-processed food at baseline was associated with elevated psychological distress, as a marker for depression, at 15 years follow-up.“

Ultra-processed food linked to depression

This latest Australian study looked at 15 years of data from more than 23,000 people in the Melbourne Collaborative Cohort Study, a survey looking at the effect of diet and lifestyle on chronic disease risk.

At the outset, none of the participants were taking medication for depression or anxiety. 

The researchers divided the cohort into quartiles based on what proportion of their energy intake came from ultra-processed foods.

For those in the highest quartile, 37.1% by mass of their diet came from ultra-processed foods, providing almost half of all their energy intake. Those in the lowest quartile ate on average 15.9% by mass (30.8% by energy). 

After adjusting for sociodemographic characteristics, lifestyle, and health-related behaviors, the researchers found that those in the highest quartile for ultra-processed food consumption were 23% more likely to show “elevated psychological distress,” a marker for depression, at follow-up. 

How does it affect the brain?

Dr. Melissa Lane, from the Institute for Mental and Physical Health and Clinical Translation (IMPACT), Food & Mood Centre, School of Medicine, at Deakin University corresponding author of the study, notes:

“Even after accounting for factors like smoking and lower education, income, and physical activity, which are linked to poor health outcomes, the findings show greater consumption of ultra-processed food is associated with a higher risk of depression.”

Ultra-processed foods are generally high in carbohydrates, saturated fat, and energy, and low in protein and fiber.

All these features are likely to increase inflammation, which has been linked to depression and other mental health issues.

They are also often low in micronutrients, such as vitamins B12, vitamin D, vitamin E, niacin, pyridoxine, copper, iron, phosphorus, magnesium, selenium, and zinc.

Both these factors can impact mental health, as Dr. Laird explained.

“The authors are correct in stating that some ultra-processed foods are high in fat and low in micronutrients. For example, we know that vitamin D, B vitamins, and amino acids are essential for optimal mental health in helping prevent depression. This is through different mechanisms such as reduced inflammation, less oxidative damage, improvements in gut microbiota, etc,” he told us.

Recent research has focused on the gut-brain axis, with many studies showing the effect of the gut microbiota on mental health.

One study even went so far as to call the Western diet, high in ultra-processed foods “an evolutionarily unique selection ground for microbes that can promote diverse forms of inflammatory disease.”

Ultra-processed foods and general health

Many of the foods that people eat daily are ultra-processed, even some that are marketed as healthy. They include:

• carbonated soft drinks and sweetened juices
• sweet or savory packaged snacks
• chocolate, candies, and ice cream
• mass-produced packaged breads, buns, cookies, pastries, and cakes
• margarine and other spreads
• breakfast cereals (and cereal bars)
• pre-prepared pies and pasta and pizza dishes
• poultry and fish “nuggets” and “sticks,” sausages, burgers, hot dogs, and other reconstituted meat products
• powdered and packaged instant soups, noodles, and desserts.

Research has raised concerns about the effect of a diet high in these foods, with ultra-processed food consumption linked to many health conditions.

As well as increasing the risk of obesity, itself a risk factor for many health conditions, ultra-processed food has been linked to a raised risk of all-cause mortality and cardiovascular disease, colorectal cancer, and death from all cancers, but particularly breast and ovarian cancer.

How diet affects mental and brain health

Increasingly, research is showing the effect of diet on not only physical, but also mental health. Studies have shown that a healthy diet, such as the Mediterranean diet, is associated with a reduced risk of depression. 

Conversely, a less healthy diet, or typical Western diet high in ultra-processed foods, is linked to cognitive decline and mental health issues, such as depression. 

A recent study found that people who ate the most ultra-processed foods had a 28% faster rate of cognitive decline than those who ate the least.

Others have linked the consumption of fried foods and a typical Western diet high in processed foods to greater levels of anxiety and depression.

Eating to protect your mental health

If a typical Western diet, high in ultra-processed foods, can increase the likelihood of depression, can a healthful diet, high in wholegrain, fruit, vegetables, and fresh foods decrease the likelihood of mental health issues?

The answer appears to be “yes.” 

A healthy diet, such as the Mediterranean diet, encourages a healthy gut microbiome, and this in turn has a positive effect on both depression and anxiety. 

However, Dr. Laird advised that not all ultra-processed foods are created equal, so even a healthful diet could include some ultra-processed foods: “Future work needs to classify specific ultra-processed food groups (e.g breakfast cereals vs. fast foods/fizzy drinks) in order to identify the exact ultra-processed food patterns associated with mental health, depression and other health conditions.”

“It’s a complicated issue, as some ultra-processed foods are actually beneficial for the population and thus we need more research to try and pick out the beneficial ultra-processed foods from the negative health consequence ones! It’s not as clear cut as most people think,” he added.

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