Second Nature logo



Why are ultra-processed foods bad for you?

Robbie Puddick
Written by

Robbie Puddick

Medically reviewed by

Fiona Moncrieff

7 min read
Last updated June 2024

Jump to: Ultra-processed foods are designed to make us eat more | The gut-brain hunger axis | Other health effects of ultra-processed foods | Take home message

Randomised controlled trials have shown that ultra-processed foods lead to excess calorie intake and weight gain. This effect appears independent of other factors contributing to overeating, like our mood and sleep patterns.

Observational studies also show that individuals who consume the highest amount of UPFs are 79% more likely to be obese than individuals with the lowest intake.

Ultra-processed foods are classified by the NOVA system developed by researchers in Brazil in 2009. They can be loosely defined as commercially manufactured foods that no longer resemble the ingredients used to make them.

A typical example would be cornflakes.

Corn would’ve been harvested, broken down, chemically and mechanically changed to resemble the golden flakes presented to you in a convenient box.

UPFs typically contain ingredients we wouldn’t find in our kitchen and use many chemical stabilisers, emulsifiers, and preservatives.

Many of these chemicals and artificial ingredients have unknown safety properties – we simply don’t know how they’ll impact our bodies in the long term.

Compare this to whole grains like quinoa or brown rice. They’re harvested and packaged, and then we cook them in their whole form.

Whilst often cheap and convenient, UPFs significantly impact our hunger and fullness signals and promote an increased desire to eat more.

The NOVA classification system

Group 1: Unprocessed or minimally processed foods

Fresh, dry or frozen fruits, vegetables, nuts, seeds, grains, legumes, meats, fish, eggs, and milk.

Minimally processed foods like removed inedible parts, drying, crushing, pasteurising, freezing, fermentation, etc. without adding substances

Group 2: Processed Culinary Ingredients

Oils, fats, butter, sugar, and salt extracted from foods or nature. They’re used in kitchens to prepare and season foods.

Group 3: Processed foods

Foods made by adding salt, sugar, or other substances to Group 1 foods

Examples include canned foods, salted nuts, smoked meats, and cheeses.

Group 4: Ultra-processed foods

Industrial formulations made from substances extracted from foods or synthesised from food constituents. They contain little or no intact Group 1 foods.

Examples include soft drinks, chips, frozen meals, hot dogs, pre-prepared desserts, supermarket bread, and most breakfast cereals.

Check my eligibility

Ultra-processed foods are designed to make us eat more

You may have noticed that no matter how full you feel after a meal, you can often still make room for dessert. This is likely due to a psychological phenomenon called ‘sensory-specific satiety’.

As we eat more of a particular flavour, our taste buds slowly get tired of it, so we stop eating that food. But when presented with a new flavour, we get more reward, so we continue eating.

This concept is in action at an all-you-can-eat buffet, where we’re likely to eat more because there’s a variety of flavours to keep our taste buds interested.

However, our taste system can be tricked when salt, fat, and sugar are carefully combined in expertly measured amounts to be ‘just right’.

At this point, despite our body trying to tell us to stop, we keep coming back for more because we’re continuously experiencing pleasure.

This is the ‘bliss point’ – the exact measures of fat, sugar, and salt that make our taste buds tingle and override the brain’s natural ‘stop’ signals.

This bliss point plays a significant role in why we crave certain ultra-processed foods, such as ice cream, chocolate, and crisps.

Even when we don’t feel hungry, these cravings seem impossible to resist, and we can continue eating these foods despite the flavour staying the same.

The science behind the bliss point

Our body responds to foods that hit the bliss point by triggering reward pathways in our brain and stimulating dopamine signalling.

Dopamine is a chemical messenger in the brain involved with feelings of motivation and pleasure. Dopamine is responsible for telling us to repeat certain actions, like eating crisps while watching TV.

The pleasure created by foods that hit the bliss point acts like a high and keeps us returning for more.

Regularly eating these foods can lead to a perpetual cycle of cravings. Eating more ‘bliss point’ foods leads to more cravings, and the cycle continues.

Research shows UPFs lead to excess calorie intake

A randomised controlled trial with 20 weight-stable individuals demonstrated that UPFs can increase hunger, cravings, and overeating.

The participants were randomly assigned to eat a diet rich in UPFs or a whole-food diet. All meals were provided, and participants were instructed to eat as much as they liked.

Additionally, all the food was matched for calorie content, fibre, protein, fat, and carbohydrate so the researchers could determine whether the nutrients in the food or the level of processing influenced the results.

The results showed that individuals eating the UPF diet consumed 459 calories per day more than those consuming the whole-food diet. Unsurprisingly, this led to weight gain in the UPF group, while the whole-food group lost weight.

Despite eating more, the UPF group had higher circulating levels of the hunger hormone ghrelin and lower satiety (fullness) hormones PYY and GLP-1.

UPFs are associated with obesity, and clinical trials have demonstrated that they can increase hunger, resulting in higher calorie intake and weight gain.

The gut-brain hunger axis

Hunger is regulated by a complex web of communication pathways between the gut, the central nervous system (CNS), and the hypothalamus in the brain.

These systems interact through several hormones, such as ghrelin, released from the stomach to signal hunger, and those secreted from the gut to signal fullness, like GLP-1, GIP, PYY, and CCK (we’ve used the abbreviated names of these hormones for simplicity).

Ghrelin is our hunger hormone. When ghrelin is high, it increases the feeling of hunger and drives us to eat.

GLP-1, GIP, PYY, and CCK are our fullness (satiety) hormones. When they’re high, hunger is low, we feel full or satiated, and our desire to eat is lower.

When our stomachs are empty, a message is sent to the brain to trigger the release of our hunger hormone, which drives us to eat.

On the other hand, when our stomachs are full, a message is sent to the brain to trigger our fullness hormones, and we respond by stopping eating.

This relationship between the gut and the brain is a continuous loop. The brain constantly receives signals from the gut to know what signals to send to the body, impacting our eating behaviour.

Ultra-processed foods bypass our hunger-regulation network

Ultra-processed foods are also more rapidly digested by the body due to them essentially being ‘pre-digested’ in their manufactured form.

They may be digested further up the digestive tract, avoiding the interaction with the necessary cells of the gut that would trigger the release of the hormones we need to make us feel full after a meal.

Additionally, research has shown that high consumption of UPFs is associated with lower cognitive function and can disrupt the balance of our gut microbiome.

These effects will likely disrupt the delicate relationship between the gut and brain and its ability to manage hunger.

Other health effects of ultra-processed foods

Considering the propensity of ultra-processed foods in our diets, a surprisingly low amount of research has been conducted on their impact on our health.

The human trial described above is the only one published to date investigating the effects of ultra-processed foods in a controlled setting.

The research we have on ultra-processed foods is largely observational, which can investigate associations between ultra-processed food consumption and health outcomes.

While we can’t determine cause and effect from observational research, if all the research points in the same direction, we can be more confident that a causal relationship may be present.

Based on the currently available research, there are very few chronic diseases that ultra-processed foods aren’t associated with.

Research suggests that a higher intake of ultra-processed foods is associated with heart disease, some forms of cancer, type 2 diabetes, anxiety, depression, and cognitive decline.

It’s important to note that a higher intake of ultra-processed foods is also associated with a poorer overall lifestyle. Those who eat more ultra-processed foods also tend to smoke more, drink more, exercise less, and get poorer sleep.

So, it’s difficult to disentangle the other lifestyle factors contributing to poorer health in this research.

Take home message

Despite the lack of research investigating the link between ultra-processed food consumption and our health, we can confidently say that they’re an independent causal factor in the development of obesity.

They bypass our hunger regulation network by disrupting the communication between the gut and brain designed to manage our intake and maintain a stable weight.

Manufacturing ultra-processed foods to make them ‘just right’ hijacks our brain’s reward circuitry to make us return for more.

They may not be as addictive as drugs like cocaine, but they have a much higher addictive potential than whole foods like meat and vegetables.

We can also be confident that consuming high levels of ultra-processed food is unlikely to bring any health benefits and is likely to contribute to the development of a whole host of chronic diseases such as heart disease and cancer.

Meal Plan

Download our free, indulgent 7-day meal plan

It includes expert advice from our team of registered dietitians to make losing weight feel easier. Subscribe to our newsletter to get access today.

I've read and agreed to the Terms of Service & Privacy Policy.

You might also like

Make losing weight feel Second Nature

The first step on your Second Nature journey is to take our health quiz.

Hand holding phone

Write a response

As seen on

The GuardianThe TimesChannel 4The Sunday Telegraph
Evening Standard