What Are Ultra-Processed Foods, And Why Exactly Are They Bad For Us?

Posted on 27 May 2022

In the 1800s, European colonialists in Asia introduced steam powered machines that could more efficiently strip off the outer layers of rice grains, which is how brown rice is turned into white rice. Suddenly, white rice was no longer a food eaten mainly by the wealthy – most people could now enjoy it. Soon after, the incidence of a once rare wasting disease called beriberi increased dramatically throughout Southeast Asia, with devastating consequences. It was decades before the link between these two events was understood: stripping brown rice of its outer layer also stripped it of vitamin B, which led to beriberi, a severe form vitamin B deficiency.

This was an early example of the dangers of food processing, and it’s pretty tame in comparison to what we do to our food today. In factories around the world, foods are blended, filtered, compacted and mixed with preservatives and additives. We are told that processed foods are worse for us than unprocessed foods, but why? Strictly speaking, most of the food we eat is ‘processed’ in some way. The broccoli you buy from the supermarket was still cut, sorted and wrapped, but while this may technically be food processing, the food hasn’t really been altered in any significant way. When it comes to health concerns, we’re mostly talking about ‘ultra-processed’ foods. These are things like industrially made bread and pre-packaged frozen meals – food that is manufactured on a massive scale, usually going through many modifications before it arrives in a store. There’s no hard definition for when something becomes ultra-processed – the more processing a food goes through, the worse it is likely to be for your health. But what exactly about these modifications is bad for us? The answer is not as straightforward as you might think.

Ultra-Processed Foods And Deadly Diseases

So, just how bad for you are ultra-processed foods? Studies suggest that the association between processed foods and severe disease is pretty strong. One study in France followed over 100 000 people who were asked to record their diet over the years. They found that for every 10% increase in ultra-processed foods eaten, the risk of certain chronic diseases went up by 10-15%. These included heart disease, cancer and diabetes – among the world’s biggest killers. This has been confirmed in multiple other studies around the world, and holds true when confounding factors are accounted for, such as people with lower incomes buying more processed food.

These are alarming findings, but there’s also a rather mundane possible explanation that has nothing to do with food processing itself: most junk food is processed food. It’s difficult to make most foodstuffs very unhealthy without processing them in some way, be that pumping them full of sugar or deep frying them in fat. If people ate food with the same nutrient and calorie content, would it make a difference whether that food is processed or not? Fortunately, scientists have already done such an experiment…

More Than Mere Junk Food

In a 2019 study designed to test whether it was the processing of the food itself, or merely the nutrient content of the food that was to blame for the health effects, researchers conducted a study in a tightly controlled environment. 20 adult participants were randomised into two groups that would move into the lab for the duration of the study. Both groups were provided with meals containing the same calorie and nutrient content, and nutrient density, but one group ate an ultra-processed diet, while the other ate an unprocessed diet. The groups maintained these diets for two weeks, and were allowed to eat as much or as little of the provided food as they wanted. Then, the two groups swapped diets. Researchers took blood samples, tracked weight, and studied various health metrics during this time. The result? There was a difference between the two groups, and a large one at that. The ultra processed group ate 500 calories more per day on average than the unprocessed group, and gained an average of 0.9 kilograms (2 pounds). They then lost all of that weight when they swapped over to the unprocessed diet.

Changes in calorie intake and body weight over time for people eating nutrient matched ultra-processed (blue) or unprocessed (red) diets.
Ultra-processed diets cause excess calorie intake and weight gain: An inpatient randomized controlled trial of ad libitum food intake

An extra 500 calories per day is huge: that’s a 25% increase in calorie intake for an adult woman, and 20% for a man. This could be because processed foods aren’t as filling relative to the amount of calories they contain. We know that gaining weight is associated with chronic diseases like those mentioned above, so it seems plausible that weight gain caused by overeating could explain the link between processed foods and disease. However, something doesn’t quite add up. When studies such as the French study mentioned earlier account for people’s weight, ultra-processed foods are still associated with increased risk of disease. This suggests that there’s something else about food processing that makes it intrinsically less healthy.

A Processed Lunch To The Gut

When trying to understand how a component of our food affects health, the gut is often the best place to start for obvious reasons. Not only does the gut determine how our food is absorbed, but the health of the gut has a powerful effects on other organ systems, and can promote or protect against a wide range of diseases. One way in which processed foods could affect the gut is through the additives they contain. Food additives can look frightening when you read their names on the food packaging, but most of them are completely harmless. However, some studies suggest that a group of additives called emulsifiers could be problematic.

Emulsifiers are used in many foods to help different components such as oil and water to mix together. Natural emulsifiers are produced in the gut as part of the bile, which helps to break down fat. But it appears that when people consume emulsifiers from processed foods, they can kill off good gut bacteria and disrupt the lining of the intestines, allowing bad bacteria to gain purchase. This causes inflammation and reduces the gut’s ability to control what should be absorbed into the blood and what should be kept out of the circulation. The evidence for this in humans is still limited, but was enough for the International Organization for the Study of Inflammatory Bowel Disease to recommend that people with IBDs avoid certain common emulsifiers.

Other potential culprits include nitrates and nitrites, which can react with other molecules in the body to produce carcinogens. One study suggested that certain cancers are more common when people consume more of these additives, though we have to keep in mind that observational studies such as this are vulnerable to confounding factors, even when attempts are made to account for them.

And The Ultra-Processed Porridge Was Just Right

So, additives could help explain why ultra-processed foods seem to be bad for us, but there’s another line of investigation: the microscopic structure of the food itself. Food processing often changes the microscopic structure of the food (food matrix), giving it a noticeably smoother texture. Scientists have speculated that this could change how the food is digested. One study recruited people who had an ileostomy, in which the small intestine (where nutrients are extracted from the food) is diverted through the abdomen so that food never reaches the large intestine. This allowed scientists to study food mid-digestion, which isn’t normally possible. Scientists then randomised participants to eat porridge that was either processed (smooth) or unprocessed (chunky), and looked at how it was digested. The two groups then swapped over.

These graphs show how participants’ blood sugar (left) and insulin levels (right) changed over time after eating ‘smooth’ processed porridge (black dots) or ‘coarse’ unprocessed porridge (white dots). The bar chart to the top right of each graph represents the total glucose/insulin amount measured over the duration of the experiment.
Manipulation of starch bioaccessibility in wheat endosperm to regulate starch digestion, postprandial glycemia, insulinemia, and gut hormone responses: a randomized controlled trial in healthy ileostomy participants

They found that the smooth, highly processed porridge was digested much more rapidly and, importantly, led to a higher and more rapid spike in blood sugar. Such blood sugar spikes can eventually cause cells to become resistant to the blood sugar lowering hormone insulin. Insulin resistance is best known as the cause of type II diabetes, but even if you don’t get diabetes, insulin resistance still increases your risk of developing a wide range of diseases. So, it seems that processing of some foods might alter their microscopic structure and change the way they are digested, leading to a more rapid release of nutrients, and this might explain some of these foods’ harmful effects.

Packaging Perils


Not all of the harm done by processed foods is necessarily done by the food itself. One hypothesis is that plastics making their way into processed foods are partly to blame. Plastics can contain molecules called endocrine disruptors, which look very similar to our hormones and can attach to hormone receptors to cause problems. Of particular concern are endocrine disruptors that can interfere with insulin signalling, and therefore disrupt the body’s ability to control blood sugar.

We know that these chemicals are indeed more common in processed foods. Studies looking at people’s urine, for example, have shown that eating more processed food is linked with higher concentrations of endocrine disruptors in their urine (20% to 40% more for some molecules) in the next few days. Some of these chemicals will come from plastic packaging, but many of them may also be introduced in the factory from sources like conveyor belts, storage containers and even gloves. In essence, the more processing steps the food goes through before it arrives on someone’s plate, the more plastic it’s likely to come into contact with. Unfortunately, you can’t really avoid these chemicals just by avoiding processed foods – in fact, you may not even be able to avoid them at all without running your own farm. Plastic gets used all throughout the food industry. In fact, one study from 2013 even suggested that certain less processed foods, such as cream and butter, might actually contain more plastic chemicals than ultra-processed foods.

Keeping Things In Perspective

Before you grab your pitchfork, set fire to your nearest food factory and disappear into the forest, it’s important to take a step back for a moment and put things into perspective. While food processing has the potential to cause a great deal of harm, the world wouldn’t be better off without it any more than we would be better off without industrialisation. Our ability to manipulate food and change its properties is a very good thing. For one, it allows food to be stored for longer, and helps make a diverse diet affordable to a larger number of people. Just as food processing can strip food of its nutrients, it can also be used to enrich them. Remember how turning brown rice into white rice depleted its vitamin B? We can now add that vitamin B back in, and many countries do. This too is food processing. Whether the evidence presented here is enough for you to stop eating ultra-processed foods altogether is up to you.


Processed Food: A Load of Baloney?

Consumption of ultra-processed foods and cancer risk: results from NutriNet-Santé prospective cohort:

Ultra-processed diets cause excess calorie intake and weight gain: An inpatient randomized controlled trial of ad libitum food intake:

Randomized Controlled-Feeding Study of Dietary Emulsifier Carboxymethylcellulose Reveals Detrimental Impacts on the Gut Microbiota and Metabolome:

Emulsifiers and Intestinal Health: An Introduction:

Nitrites and nitrates from food additives and natural sources and cancer risk: results from the NutriNet-Santé cohort:

Manipulation of starch bioaccessibility in wheat endosperm to regulate starch digestion, postprandial glycemia, insulinemia, and gut hormone responses: a randomized controlled trial in healthy ileostomy participants:

Unexpected results in a randomized dietary trial to reduce phthalate and bisphenol A exposures:

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