Do 'heavily processed' foods increase the risk of an early death?

"Study links heavily processed foods to risk of earlier death," reports The Guardian.

Researchers reported that middle-aged French people who ate 10% more so-called "ultra-processed" food had a slightly increased chance of dying over a 7-year period compared with those who ate less.

The researchers describe ultra-processed food as "food products that contain multiple ingredients that are manufactured through a multitude of industrial processes".

They give examples as including "mass produced and packaged snacks, sugary drinks, breads, confectioneries, ready-made meals and processed meats".

While some of these foods may be unhealthy, it seems unhelpful to group together nutrient-free sugary drinks and ready-made vegetable soups, for example.

As one dietitian points out: "Bread or biscuits baked at home would not be considered ultra-processed, whereby shop bought versions would, despite identical ingredients."

The study adds some information to evidence about the importance of a healthy diet.

Unfortunately, grouping together all "ultra-processed" food in one category makes it hard to make much sense of this study.

Where did the story come from?

The researchers who carried out the study were from Sorbonne Paris Cité university and Hôpital Avicenne, both in France.

There's no information provided about the funding source for the study.

It was published in the peer-reviewed journal JAMA Internal Medicine.

The study was reported with varying degrees of alarm in the UK media.

The Mail Online warned that a "junk food diet is killing us", and stated that eating processed food like "burgers, sugary cereals and pizzas increases risk of fatal diseases".

But the news story does not point out the limitations of the observational nature of the study, or question why "any product involving an industrial procedure" should raise disease risk.

The Daily Mirror's headline that eating processed food "knocks decades off [lifespan]" was wide of the mark, as the difference in lifespan reported in the study amounted to about 18 months.

The Guardian gave a more balanced analysis, giving the absolute numbers of deaths in the study and quoting experts who questioned its findings.

What kind of research was this?

This was a prospective cohort study.

Cohort studies, like all observational studies, can help spot patterns, but they cannot tell us whether ultra-processed food is a direct cause of early death because many other factors could be involved.

What did the research involve?

Researchers used data from the ongoing NutriNet-Santé Study of 44,551 French adults, which began in 2009.

Volunteers aged 45 or older completed a series of online questionnaires about their health, socioeconomic status, family history, lifestyle and other information.

They filled in at least 3 24-hour dietary records during an average 7 years of follow-up until 2017.

Researchers used the questionnaires to calculate the proportion by weight of total food intake categorised as ultra-processed.

After adjusting their figures to take account of a range of potentially confounding factors, they calculated the link between the proportion of ultra-processed food in the diet and the chances of having died during the follow-up period.

Confounding factors included:

• gender and age
• income and education level
• marital status and residence
• body mass index (BMI)
• physical activity level
• smoking status
• total energy intake
• alcohol intake
• season of food records
• first-degree family history of cancer or cardiovascular disease
• number of food records completed
• degree of adherence to French nutritional recommendations (which are much the same as UK guidelines)

What were the basic results?

During the 7 years of follow-up, there were 602 deaths (1.4% of the people who started the study).

The researchers say 219 were caused by cancer and 34 by cardiovascular disease, but did not report causes of death for the other 349, so we do not know whether they could have been related to diet.

Ultra-processed foods comprised 14.4% of total food consumed by weight on average, which translated to 29.1% of calories.

People who ate more ultra-processed food were likely to be younger, on a lower income, have a lower education level, be living alone, have a higher BMI, and do less physical activity.

They were also less likely to adhere closely to the French nutritional recommendations.

The researchers calculated that each additional 10% increase in proportion of ultra-processed food in the diet (by weight) was linked to a 14% increased risk of death (hazard ratio [HR] 1.14, 95% confidence interval [CI] 1.04 to 1.27).

But when they excluded deaths in the first 2 years of the study and people who had cancer or cardiovascular disease at the start of the study, the association was no longer statistically significant – it could have been down to chance.

How did the researchers interpret the results?

The researchers said their results "suggested a positive association between increased ultra-processed food consumption and all-cause mortality risk".

They suggested several theories for why this could be, including the presence of acrylamide (a substance caused by some high-temperature cooking that's been linked to some types of cancer), meat processing, some additives and the presence of hormone-disrupting chemicals in some food packaging.

But these theories are all speculative and not backed by evidence.

Conclusion

It's quite difficult to unpick any useful messages in this study because of its many limitations.

The main limitations are:

• an unclear definition of ultra-processed food, which may not be a particularly helpful term as it bunches together very different foods based on how they were made, rather than what's in them
• the observational nature of the study, which means it cannot show cause and effect
• the self-selecting volunteer population, which is likely to represent people particularly interested in nutrition and health and not the general population
• the fact people could choose which 24-hour period to record their diet, which may mean they were more likely to record a healthy eating day than an unhealthy day

Because so many different types of food are included in the "ultra-processed" category, it's impossible to tell which foods might have contributed to the small increased risk in deaths among the people taking part in the study.

We certainly cannot conclude that all processed food is bad, or that eating processed food is killing us.

But the study is a reminder that relying on pre-prepared food or eating too many snacks, sweets and ready meals can make it easy to consume too much salt, sugar and saturated fat, and not enough fibre, green vegetables and fruit.

Find out more about a healthy diet