MOST people would assume that eating too much "ultra processed" food is bad for their health.

Too much salt can lead to high blood pressure, while diets overloaded in saturated fat and sugar are likely to cause obesity which in turn increases the risk of everything from heart attacks to type 2 diabetes.

One study published this week in the BMJ - led by researchers based in Boston - has also drawn links between the consumption of ultra processed food and colorectal cancer.

This might seem obvious, but defining what exactly is meant by "ultra processed" is controversial to begin with, let alone proving a direct causal relationship between certain foodstuffs and certain cancers.

Colorectal cancer is currently the second most common cause of cancer death worldwide, although incidence rates among older adults are actually falling in many wealthier countries.

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In Scotland, where routine bowel cancer screening was introduced on the NHS from 2007 and is currently available to everyone aged 50 to 74, incidence rates of the disease (per 100,000 and age-standardised to account for the ageing population) have been falling steadily since the late 1990s in everyone aged 55 and over, including by around a fifth in the over-90s - the steepest decline seen in any age group.

You are still around 30 times more likely to be diagnosed with colorectal cancer if you are over 90 compared to an adult aged 35 to 39, but one of the significant trends of the past 20 years has been the growing numbers of young adults found to have the disease.

There is almost a precise, and widening, divide between the over- and under-50s, down through the generations:among 30-34-year-olds, rates of colorectal cancer - though still low at around six cases per 100,000 - are twice what they were in the late 1990s.

In 25 to 29-year-olds, case rates have climbed by 150 per cent over the same period, and even 15 to 19-year-olds have seen an eight-fold increase from 0.2 to 1.7 diagnoses per 100,000.

One crude observation is that the generational split roughly coincides with the arrival of the UK's first ever McDonald's outlet in 1974 - a watershed moment in the nation's culinary habits which marked a new era of fast food chains, supersized portions, and increasing calorie intakes as food in general became cheaper, and snacking on crisps and confectionary more common. Food also became more "processed".

This is a correlation, however; not evidence of cause.

As Gunter Kuhnle, a professor of nutrition and food science at Reading University noted in response to the US study, it would be "misleading" to conclude that ultra-processed foods cause these diseases.

"A more correct summary would be that people who consume food classed as ultra-processed are at higher risk of disease," he added.

However, he also added that the definition used for ultra-processed food "fairly vague and ambiguous", ranging for example from the number of ingredients to "most supermarket breads...even though many are not fundamentally different from other breads".

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Dr Duane Mellor, a dietician and lecturer at Aston University in Birmingham also noted that it is still "unclear" from a public health point of view whether there is any benefit in warning against the consumption of so-called ultra-processed foods than it is to encourage people to simply cut down on foods with added sugar, salt and fat and increase their intakes of fruit, vegetables, and fibre-rich wholegrains and pulses.

He added: "The risk with the ‘ultra processed foods’ definition is that it would suggest a shop bought sponge cake would be ultra-processed, but one made at home is not, even though the average consumer is aware that both contain sugar and fat and are not ideal as a regular part of most people’s diet."

The study - which defined processed food as industrially produced ready-to-eat/heat items such as biscuits or instant noodles as well as carbonated drinks, milkshakes or fruit juices - found that US men with the highest levels of consumption (based on regular, self-reported dietary questionnaires over a period of 24-28 years) were 29% more likely to have developed colorectal cancer compared to those with the lowest intakes.

Oddly, no correlation was found overall for women. The researchers suggest that oestrogen may have a protective effect in women, or that it may be down to differing choices of ultra-processed foods between the sexes (for example, more calcium-rich dairy-based desserts in females).

However, there did appear to be an association with colorectal cancer in women consuming the highest levels of takeaway fast foods and ready meals specifically.

So what to do? The researchers themselves simply say that "further studies are needed" to unravel the interplay between ultra-processed food and the onset of bowel cancer.

Brazil-based academics Carlos Monteiro and Geoffrey Cannon, commenting on the findings, draw an analogy with tobacco, saying that the "rational solution" would be official guidelines and laws "designed to reduce production and consumption of ultra-processed foods and to restrict or preferably prohibit their promotion".

Such measures have been resisted to date, and will arguably become even harder to pursue in a cost of living crisis: ultra-processed food (fish fingers, for example) tend to be cheaper because they have a longer shelf-life.

Prof Gunter Kuhnle said: “Banning ultra-processed foods would increase the cost of foods and many people rely on ultra-processed foods."

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Then again, as Monteiro and Cannon point out, "nobody sensible wants foods that cause illness".

Yet this only brings us back to the same intractable problem we started with: how exactly to define ultra-processed food, and how to prove it causes harm.

"The overall positive solution," they write, "includes making supplies of fresh and minimally processed foods available, attractive, and affordable."

That, at least, is harder to argue against.