Obesity: Is it ultra-processed food, gut bacteria, or genes?

As always when it comes to analysing the relationship between food and health, there were no simple answers.

The report notes that an estimated 51% to 68% of the calories consumed by the UK population come from ultra-processed foods, depending on age and deprivation.

That seems alarming but, as Tom Sanders, a professor emeritus of nutrition and dietetics at King’s College London, pointed out, it is "unlikely that this proportion has changed much in the past fifty years...especially as the consumption of bread was higher in the 1960s".

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Part of the problem, he said, is that it is such a "broad catch all" term ranging from processed meat (burgers, bacon, ham, chicken nuggets, sausages), margarine and read meals to cheese, breakfast cereals, bread, flavoured yoghurts, and baby milk. 

Yet, he added, products such as margarine and spreads "now have a healthier nutritional profile compared to butter and contain fewer calories, less saturated fat and more vitamin E and D".

Gunter Kuhnle, a professor of nutrition and food science at the University of Reading, said that recent discussions around the health impact of ultra-processed foods had been "dominated by hyperbole and unnecessary generalisations".

He noted that in most studies it was "impossible to distinguish [nutritionally] between home-made, artisanal or mass-produced breads, although only the latter is considered 'ultra-processed'".

SACN itself acknowledged that there is "no universally agree definition of processed foods".

That complication aside, most systematic reviews of the evidence to date have found an association between an increased consumption of what are considered "ultra-processed foods" and a higher risk of obesity, diabetes, depression, high blood pressure, cardiovascular problems, cancer deaths, and diseases of the gastrointestinal tract.

Theories for why this might be the case are wide-ranging: contaminants from packaging; the effects of additives such as sweeteners; its sheer "energy density" - that is, calories; a nutrient profile high in saturated fat, salt and free sugars with little fibre; or the effects of high temperatures during production. Maybe all of the above?

Yet none of this is clear-cut either.

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As SACN noted, the "available evidence is almost exclusively observational in nature" and the studies which do exist are riddled by "inconsistent adjustment" for other risk factors such as calorie intake, body mass index, smoking and socioeconomic status, which makes it difficult to pin down a direct cause-and-effect between ultraprocessed foods and ill health or weight problems.

This is not to say that a diet low on fibre but packed with excess salt, sugar, saturated fats, and calories won't make you fat and sick in the long-term; the puzzle for public health is whether there is something uniquely pathogenic about ultra-processed foods.

On that, the jury is out.

SACN called for more research - particularly "good quality randomised control trials" - and the development of a more reliable classification system.

Another theory is that the rise of highly-processed, or "Americanised", diets - particularly from the 1990s onwards - has disrupted our gut health so profoundly as to make us more susceptible to obesity, allergies, chronic disease and inflammatory conditions.

In a book published today - 'Dark Matter: The New Science of the Microbiome' - colorectal surgeon James Kinross argues that diets low in fibre but high in animal fat and animal protein, salt, and refined sugars cause harmful bacteria to bloom in the digestive tract which "communicate with the immune system and promote inflammation".

He suggests that this could explain the rising tide of bowel cancer among young people.

In Scotland rates of the disease - though still rare - have more than doubled over the past 20 years in the 25-29 age group, from 0.9 to 2.5 cases per 100,000.

That compares to 405 cases per 100,000 among 85-89-year-olds, but in the over-70s prevalence has been steadily falling since the 1990s.

Two thirds of adults in Scotland are overweight or obese (Image: Scottish Health Survey (Nov 2022))

It echoes a previous study where rural South African males, who traditionally consume up to 50g of fibre a day, and African-American males living in the United States who have the highest bowel cancer rates of any ethnic group, swapped diets.

Within two weeks, the South Africans' gut inflammation soared while the Americans' inflammation came down.

Lots of fibre (30g a day is the "magic number") plus fermented foods such as kimchi, kefir, kombucha, and sauerkraut appear to promote a diverse and healthy gut microbiome.

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One thing none of us can change, however, are our genes.

A new £495 Intelligent Weight Loss test promises to unravel your DNA and reveal whether you are blighted by "fat genes".

There are more than 40 genes and 50 specific mutations known to influence hunger, appetite and metabolism, mainly due to their effect on two key hormones: ghrelin (which makes us hungry) and leptin (which tells us we're full).

FTO is one of the best understood of the "fat genes".

For an unlucky 16% of the population who have inherited two copies of a particular FTO mutation (catchily known as rs9939609), their lifetime risk of obesity increases by 50%.

The reason is ghrelin. People with the high-risk FTO mutation have higher levels of ghrelin, which means they feel hungrier - even after eating.

Brain scans also show that their neurons light up more enthusiastically in the area of the brain associated with eating, cravings and reward when shown images of high-calorie food.

Other genes reduce sensitivity to leptin.

By age 18, one in 330 people who carry a specific mutation in their MC4R gene are on average nearly three stone heavier than someone without it because they have an increased appetite and lower satiety response.

Professor Giles Yeo, who studies the interplay between genetics and eating behaviour, says that between "40 and 70% of the variation of our bodyweight is down to genes".

But if genes load the dice, its our environment and lifestyles which have enabled obesity to explode in the past 50 years, and - for now - there's little sign of that changing.