Experts reveal the 100 effects ultra-processed foods have on your body... and what to do to reverse them

It is no secret that high-fat, high-sugar ultra-processed foods make a person sluggish and depressed. It is also not a surprise that a sedentary lifestyle compounds those effects.

In a recent study, University College Cork researchers discovered that a two-month junk food diet, one that mirrored an ultra-processed food (UPF) diet typical in the US, drastically altered the gut environment of rats, changing 100 out of 175 measured bacterial compounds and depleting specific metabolites that affect brain function.

The brain and gut are closely connected via what scientists call the gut-brain axis, a complex and constant 'conversation' between the digestive system and brain, where gut bacteria produce compounds that directly influence mood, stress levels and cognition.

In the latest study, exercise combatted depression caused by a diet mirroring the one that Americans often adhere to, by rebalancing crucial metabolic hormones and restoring beneficial gut compounds.

Physical activity also reduced anxiety- and depression-like behavior and enhanced learning and memory in the rats who ate the junk food diet.

Until now, it has been unclear whether exercise can reverse the unhealthy diet’s effects on mood, as well as cognitive behaviors, but this latest study suggests that it can.

The researchers concluded that a primary way exercise combats depression is by repairing the gut microbiome, which is damaged by a poor diet.

The restored gut bacteria then release beneficial substances that travel from the gut to the brain, acting as crucial chemical signals to improve mood.

Ultra-processed foods (UPFs) dominate the American diet, accounting for roughly 70 percent of grocery items. The industrially engineered products contain artificial flavors and additives rarely found in home kitchens.

Their hyper-palatable, salty, and sweet design makes them hard to resist, with emerging research suggesting they may be addictive.

Americans get about 55 percent of their daily calories from cheap, tasty and unhealthy manufactured foods. Diets made up mainly of these offenders are strongly linked to an increased risk of heart disease, heart attacks, stroke and cardiovascular-related deaths.

There is a consistent and growing body of evidence linking a diet high in UPFs to an increased risk of overall cancer and specific cancer types such as colorectal.

A 2023 analysis found that every 10 percent increase in the proportion of UPFs in the diet was associated with a four percent higher risk of colorectal cancer.

Diets heavy in UPFs have also been associated with markedly higher rates of depression. In August, Turkish researchers reported that a 10 percent increase in UPF consumption per daily calorie intake increased the depression risk by 11 percent.

To measure depression in rats, the Irish team conducted a swim test in which they measured the amount of time a rat spent immobile, floating passively and making only the minimal movements necessary to keep its head above water, an indication of despair.

The researchers divided the rats into four distinct groups: one ate a healthy diet without access to a running wheel for exercise, a second ate an unhealthy 'cafeteria diet' without exercise, a third ate a healthy diet with voluntary running wheel access, and the fourth ate the unhealthy diet with exercise access.

Rats on the unhealthy diet spent significantly more time floating passively compared to the healthy diet group. However, when the rats with unhealthy diets had access to a running wheel between meals and then were put in the pool, this inactivity was reversed.

They spent less time floating and more time actively swimming, just like the healthy rats. Rats were less likely to give up, which researchers interpret as indicating they were less depressed in the context of the model.

While exercise had a minimal effect on the gut microbiomes of rats eating healthy food, it had a powerful restorative effect on the rats eating junk food, reversing dozens of the harmful changes caused by the poor diet.

The unhealthy diet depleted three key gut compounds: anserine, a brain-protecting antioxidant; deoxyinosine, a building block for stable mood; and indole-3-carboxylate, which aids in the production of serotonin. The loss of these beneficial molecules disrupted crucial communication between the gut and the brain.

Exercise restored these compounds. Since these molecules are known to influence mood and brain function, this appears to be a primary way exercise combats diet-induced depression.

The unhealthy diet spiked insulin and leptin levels, hormones that, when chronically elevated, are linked to depression.

When the rats on the unhealthy diet exercised, researchers observed that insulin did not spike after eating, leptin levels were reduced and the body increased production of beneficial hormones that were reduced by the poor diet, such as GLP-1, which helps regulate blood sugar and promotes feelings of fullness.

Exercise effectively normalized these hormones, improving metabolic health and directly contributing to a better mood.

In addition to performing a forced swim test after eight weeks, they also performed tests for anxiety and spatial memory. Rats were placed in a large pool of opaque water.

A hidden platform was submerged just below the surface, invisible to the rats. The rats had to learn and remember the location of the platform based on visual cues placed around the room.

Over the four training days, all groups of rats learned to find the hidden platform at a similar rate. The unhealthy diet did not severely impair their ability to learn the task.

The most telling result came from analyzing how the rats searched.

The researchers found that sedentary rats on an unhealthy diet tended to use less efficient, more random swimming patterns, such as circling, while exercised rats on the same diet used more direct and purposeful search paths.

However, the study’s results must be interpreted cautiously, as human studies are still needed.

The study’s lead author, Yvonne Nolan, professor in anatomy and neuroscience at University College Cork, said: ‘We studied young adult male rats only. While animal models provide valuable mechanistic insights, we cannot assume identical effects in humans, females, or different age groups.

‘Our rats exercised voluntarily with continuous wheel access, which differs from structured human exercise programs.’

Their research was published in the journal Brain Medicine.