It's a common assumption that processed foods are unhealthy and a cause of overweight and obesity. And it is sometimes true: Processed foods can, for instance, contain four times more calories than unprocessed foods.

Processed foods can be any kind of food that has been altered in production.

That can be food that's had vitamins or coloring added, or food that's been made safe for human consumption, such as pasteurized milk.

Strictly speaking, a freshly-baked loaf of organic whole grain bread is processed, because the individual ingredients have been altered in the baking process. But we still tend to say that "processed foods" are bad for us, when in fact it depends on the extent to which the foods have been processed.

The results of a new study aim to show why processed foods and even ultra-processed foods aren't unhealthy per se — but that they are far more likely to contain more calories and lack nutrients and vitamins.

Published in the British Journal of Nutrition, the study found that ultra-processed foods in the UK contained more food energy than mildly processed foods.

On average, ultra-processed foods contained four times more calories than minimally processed foods — the ultra-processed foods contained on average 378 calories per 100g compared to 94 calories per 100g of minimally processed food.

Not all ultra-processed foods in the study contained high amounts of energy, however. A number of the products had similar nutritional values as mildly processed foods.

"[It's not] clear whether the effects can be attributed to the ultra-processing per se, or whether the effects of processing can be attributed to the nutrient profiles of the foods which tend to be ultra-processed [such as] ready-meals, pre-packaged cakes, crisps, biscuits, cookies, sweets chocolate," said Nerys Astbury, a nutrition scientist at the University of Oxford, in a statement.

Astbury was not directly involved in the study.

Previous research suggests there is no evidence that processing food has an adverse impact on health.

But it may affect that the food is made of and, possibly, its texture. And food texture has been shown to affect the rate at which people eat food. That can result in over-eating, and over-eating can be bad for your health.

The most common processed foods with healthier profiles in the study included packet sandwiches, plant-based milk alternatives and high-fiber breakfast cereals.

But the study was limited, said Thomas Sanders, a professor of nutrition sciences at King's College London, in a statement. It did not look at the nutritional value of processed foods.

"It does not show the contribution made by the different processed food categories to key nutrients, such as protein, calcium, iron, vitamins and fiber," said Sanders.

That means it's hard to know whether the calorific content of different foods can be linked to their nutritional value.

Sanders was not directly involved in the study.

Ultra-processed food is an industrially created food, derived from natural and/or synthesized foods. Most ultra-processed foods contain a lot of calories but very little nutritional value.

Think frozen pizzas, donuts and chips — they are far from their raw ingredients: wheat, tomatoes, dairy and potatoes.

Processed foods often contain high levels of chemical additives and can be addictive as well.

In a study published in 2021, scientists linked the consumption of high levels of ultra-processed foods with deficiencies in nutrients , such as potassium, zinc, and magnesium, and vitamins, such as:

• Vitamin A: important for defense against colds and for good eyesight
• Vitamin C: important for skin and bones
• Vitamin D: important for bones, teeth and muscles
• Vitamin E: important for the immune system
• Vitamin B12: important for red blood cells and the nervous system

Foods that are high in fat, sugar and salt — whether they are ultra-processed or not — have been linked to many illnesses, from obesity to cardiovascular disease, cancer, a decline in cognitive health and increased all-cause mortality. About 40% of the global population is overweight or obese, and experts say that processed foods are largely to blame.

But the causes of obesity are complex — "consuming a diet containing lots of ultra-processed foods can also be a sign of other unhealthy dietary patterns and lifestyle behaviors," Astbury said.