The Truth About Processed Foods

I don’t believe in depriving myself of any food or being imprisoned by a diet.
––Joely Fisher

Processed foods.
 
Our worst enemy? Harmless? Somewhere in between?
 
Read on for my science-informed take.

What even are processed foods?

According to the NHS website in the UK, a processed food is “any food or drink that has been changed in some way when it’s made or prepared.”

By this definition, pasteurised milk and a packet of crisps are both considered processed, even though they provide very different nutritional benefits.

To make finer distinctions among processed foods, the most commonly used classification system is NOVA, which categorises foods and beverages based on the extent of their processing and the purpose behind it.

NOVA includes four main food groups:
 

1. Unprocessed or natural foods and minimally processed foods
2. Processed culinary ingredients: oils, fats, salt, and sugar
3. Processed foods
4. Ultra-processed foods (UPFs)

 
You can read a detailed breakdown and example foods for each group in this paper.

These distinctions are important because not all processing is inherently unhealthy. For example, pasteurising milk kills harmful bacteria and pathogens.

On the other hand, many UPFs (Group 4), such as cookies, ice-cream, and frozen pizza, are heavily processed in order to make them more palatable and thus easier to sell rather than better for human health.

Based on this, you might think, “Surely Group 4 is the ‘worst’ then,” but this isn’t entirely correct.

While most Group 4 foods tend to be nutrient-poor, high-fat, and high-sugar items, others are not, such as fruit-flavoured yogurts, sports and energy drinks, and packaged breads.

This is because NOVA isn’t trying to differentiate foods based on their degree of “healthfulness”––which is a tricky term to define to begin with––but rather based on their degree of processing.

Understanding NOVA is useful because much research on processed foods utilises it as a reference. Indeed, both of the studies I’ll discuss in the next section of this article used it.

However, I think that the best practical definition of those processed foods you may want to consume in moderation, is “commonsensical”.

For instance, while fruit-flavoured yogurts are technically UPFs, I think most will agree with me that a 0% fat, high-protein, strawberry-flavoured yogurt is more nutritious than a bottle of vodka (also a Group 4 UPF).

We all know that fruit is good for you and that it’s best not to eat your weight in deep-fried chicken and pastries on a daily basis.

So don’t get caught up in influencer-chatter demonising the processed nature of pasteurised milk, oats, and whatever else they’re “warning you against” in their reels.

What does research suggest?

While health is a multi-faceted concept that extends beyond weight and size, excess fat is proven to be harmful. Therefore, in this article, I’m going to focus on the link between UPFs and excess adiposity.

Most studies show that ultra-processed diets tend to cause more weight gain than less processed diets due to an overconsumption of calories.

But what are the factors driving the overconsumption?

Compared to less processed foods, UPFs have:
 

• A greater energy density, which I’ll define later in the article
• Less protein per gram
• Less fibre per gram

 
Furthermore, many UPFs, like French fries and cookies, are highly palatable, which makes them easier to over-eat than less processed and less tempting foods.

Taken together, these characteristics make ultra-processed diets less satiating and easier to over-indulge on.

But that’s not all.

There are other factors that seem to affect our inclination to over-eat on ultra-processed diets.

The following two studies shed light on these:

• A 2019 paper by Hall and colleagues, which is one of the most well-known studies on ultra-processed diets to date
• A 2025 paper by Hamano and colleagues, which has a very similar study design

 
In both studies:
 

• The researchers provided all meals and snacks to the participants.
• The participants could eat as much or as little as they wanted.
• For half of each study, the participants followed an ultra-processed diet. For the other half, they followed a minimally processed diet.
• In the Hall study, the researchers matched the two diets for energy, macronutrients, sugar, fat, and fibre.
• In the Hamano study, the three daily meals were matched for total energy, macronutrients, and energy density, whereas the snacks were matched for total energy and macronutrients, but not energy density.

 
By attempting to match the ultra-processed and minimally processed diets for all these different characteristics, the authors of both studies were trying to answer the following questions:

1. Will participants still over-eat on the ultra-processed diet compared to the less processed one, even when both provide the same total energy and macronutrients (in addition to sugar, fat, and fibre in the Hall study)?
2. If they do over-eat on the ultra-processed diet, what are the factors beyond energy and macronutrient composition that might be causing this?

 
In both studies, the participants consumed far more calories on the ultra-processed diet than on the less processed diet.

Specifically, the participants in the Hall study ate 500+ extra calories per day, whereas in the Hamano study they consumed 800+ extra calories per day.

Why did the subjects eat so many more calories on the ultra-processed diets?

In other words, which factors do the two studies have in common, which seem to contribute to overconsumption?

• Eating speed

Likely due to the extra processing, which makes UPFs softer and easier to chew and swallow, the participants ate faster on the ultra-processed diet.

This probably prevented them from noticing that they were full as quickly as they might have on the minimally processed diet, so they ended up eating more.

• Satiation effect

In both studies, the researchers took blood samples to assess the participants’ appetite-related hormones.

Based on the findings, the ultra-processed diets tended to produce lower levels of appetite-suppressing hormones (the fullness hormones) compared to the less processed diets.

• Overconsumption of carbs and fats

On the ultra-processed diet, the participants of both studies over-ate carbs and fat, but there were no significant differences in protein intake.

• Energy density

The concept of energy density refers to the amount of calories per gram within a certain food or beverage. The higher the calories per gram, the higher the energy density.

UPFs have a greater energy density than less processed foods, which means you’ll have to eat a smaller portion of a UPF if you wanted to calorie-match it to a less processed food.

For instance, 100g of minimally processed fresh strawberries––equivalent to about ten small fruits––contains 30 kcals. You can get the same 30 kcals from a measly 5g of chocolate, which is about three quarters of a small square.

Assuming everything else is equal, a bigger portion of food usually results in greater satiety than a smaller portion. Therefore, it’s easier to over-consume calories when eating UPFs because each portion contains a lot more calories than the same-weight portion of a less processed food.

While the authors in the Hall study tried to match the two diets for energy density by adding fibre powder to the participants’ beverages, they also acknowledged that fluids don’t have the same satiating effect as solids.

Therefore, it’s likely that the increase in energy density wasn’t enough to offset the overall lower satiating properties of the ultra-processed diet.

On the other hand, in the Hamano study, energy density was 50% higher on the ultra-processed diet.

What can we make of this?

In summary, ultra-processed diets tend to cause more weight gain than less processed diets for the following reasons:

• They’re lower in protein and fibre;
• They’re higher in energy density;
• They’re less satiating;
• They result in a greater eating speed, so you tend to eat more within the same timeframe;
• Many UPFs included in these diets are highly palatable.

 
Importantly, I’ve been careful to use the term ultra-processed diets instead of ultra-processed foods because the overall dietary pattern seems to be far more important for the maintenance of a healthy weight and body fat level than any single food.

In the Hall and Hamano studies, it was consuming a solely ultra-processed diet that caused the caloric overconsumption, not one food in particular.

For this reason, I don’t think you need to worry about having frozen pizza every now and again if the rest of your diet is nutritious and satiating.

Conclusion

While there is a well-established link between UPFs, caloric overconsumption, and weight gain, what seems to affect weight and body fat levels the most isn’t a single food or food group, but rather the characteristics of your overall dietary pattern.

Therefore, as long as this pattern is nutritious and satiating enough for you, you’re likely to offset the negative outcomes of consuming the odd highly processed ice-cream.

If you want to learn more about all the evidence-based ways in which you can design your diet to check these boxes, I have two free resources for you:

• A podcast interview on the science of appetite regulation with Calvin Scheller
• A blog article or solo podcast on maximising satiety on a diet

 
Thank you for reading. May you make the best gains.
 
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