Why processed foods are bad: they massacre our microbes

    4 November 2015

    Last week the people of the world were in shock when many of their favourite meats were labelled as carcinogenic by a World Health Organisation report. How dare they criticise bacon and sausages, the heart of the great British breakfast? The Germans were upset about their Wurst, Italians their beloved salami and the Spanish their jamón. Eating regular processed meats (defined as meat prepared with chemical additives or processes to alter flavour or shelf life) apparently carried an increased risk of colon cancer of 18 per cent. This news wouldn’t have surprised people if we were talking of fast food burgers or long-life frozen pizzas. But surely not our favourite national foods.

    The so-called WHO report (they didn’t actually write it) was very short and lacked any new data. The organisation do little research themselves and prefer to issue regular warnings that we usually ignore on the dangers of fat, sugar, and alcohol. The report that went viral is just a summary of expert opinion on tests on lab animals and epidemiological associations rather than long-term clinical trials.

    Based on past track record these could be true — like for cigarettes and lung cancer — but could also be false and misleading, like for coffee and cancer or fat intake and heart disease.

    Even larger studies in the US and Europe have consistently shown that eating processed foods regularly increases your risk of heart disease by 20-40 per cent, which is relatively much more likely to kill you. However, risk is relative. Each time you choose to eat a bacon sandwich it is estimated you reduce your life by one hour compared to five hours per cigarette packet smoked.

    If we assume processed meats are bad for you (which the evidence suggests they are), is there a unifying factor that explains how they can cause both cancer and heart disease and maybe predispose to obesity? The old scare story of high fat content had some truth in it when we had many foods with artificial hydrogenated (trans) fats in them, but these deadly man-made poisons are disappearing in most sensible countries and, while they were a problem in some dodgy sausages, were never a major content of most salamis, bacons and hams.

    We now know after being told the opposite for years that natural saturated fat as part of a balanced diet (as in the Mediterranean diet) is not harmful for most people. So what is the common factor in processed meats that is upsetting our bodies?

    All substances that get put into manufactured foods need to be approved as safe for consumption in humans. These tests usually involve giving enormous amounts to poor lab rodents to see how their livers and organs react. But we have probably been testing these chemicals in the wrong animals in the wrong place.

    In the last year several labs have started to test them unofficially in tiny animals — ones you need a microscope to see — called microbes. One hundred trillion or so (10 times more than human cells) live happily in our guts feeding off our food and producing vitamins and chemicals that keep our immune systems healthy. It turns out that microbes don’t like these ‘harmless’ substances like emulsifiers, preservatives and artificial sweeteners, making them produce unusual chemicals and killing off many friendly species.

    The dangerous components of the British banger haven’t been dissected yet, but chances are the unnatural additives are the ones that our microbes don’t like, not the fatty bits.

    Back in human guts, losing your diversity of microbial species (there are usually thousands) is associated with an increased risk of many diseases as well as obesity. Our gut microbes are pretty sensitive. I fed my (initially willing) student son Tom a McDonald’s burger and nuggets diet for 10 days and he lost 40 per cent of his species diversity. This could have been due as much to the surge of constant additives as the effects of microbial starvation by denying them fibre. Two weeks later, despite his unusual cravings for salads and veggies, Tom’s poor microbes hadn’t recovered to pre-Big Mac levels.

    We assume that eating processed foods is purely cultural but our twin studies have clearly shown that there is a considerable genetic basis for choosing and consuming these foods and tastes. This could be due to our differing taste buds or could even be due to our microbes sending our brain chemical signals saying ‘more please!’

    So should you be really scared of eating bangers and bacon for breakfast? If, like many people, you have a non-diverse, low-fibre diet, then the answer is yes. But studies in lab mice show that bad effects of junk food on the gut can mainly be prevented if served with large amounts of pulses providing fibre. So a Sunday fry-up is fine as long as you have it with lashings of kale and spinach and a high-fibre diverse diet the rest of the week. Enjoy!

    Tim Spector is professor of genetic epidemiology at King’s College London and author of The Diet Myth: The Real Science Behind What We Eat