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We should all know that just as fruit and veg are good for us, so refined carbs, including sugar, are bad for us - there's basically no exception to this rule. But along with this well-evidenced mantra, there are other new "food rules" - and some of them seem to be more knee-jerk reaction than carefully considered guidance.
Public Health England came under fire in 2017 for its campaign on acrylamide after the true (tiny) size of the risks it was warning against, became apparent. But the advice to reduce intake of red and processed meat combined to under 70 g a day resulted in no similar backlash.
Here we look at the evidence and ask if there's more to this advice than 'meets' (no pun intended) the eye.
Confounded by confounders
To be certain one risk factor is real, you need to know that there isn't another explanation for an apparent link between one food (or drug, or other lifestyle factor) and a disease. The completely discredited hysteria over MMR and a possible link with autism is a case in point. Autism was rising, something had to be to blame and a group of maverick scientists, ably abetted by the media, chose MMR as the scapegoat.
Pity the hedgehog
Let's take hedgehogs. In the last 50 years, there has been a steady rise in the number of UK households owning a TV. That rise has precisely matched the number of hedgehogs in the UK killed on the roads. Is the evil gogglebox to blame? You might be wondering, until you realise that UK television ownership has increased at almost exactly the same rate as car ownership - and cars squash hedgehogs. Cars are a "confounding factor" - a common factor that allows other, completely unrelated phenomena to appear to be linked to each other.
So before we rush to lay the blame at the door of any one cause, we need to be very sure of the logic.
It's a lottery
Assessing risk is complicated enough but putting the information across in a way that makes sense is even more challenging. It's all about absolute and relative risk, and the way the figures can be skewed.
Relative risk of disease is your risk of being diagnosed with a condition compared to someone else. Absolute risk is how many more people in a group of, say, 100, will be affected as a result. A 50% relative rise in risk sounds terrifying - until you realise that by buying three lottery tickets rather than two, you have increased your chance of winning the lottery by 50%. In absolute terms, you've increased your 'risk' of winning from, say, 1 in 3 million to 1 in 2 million - all of a sudden it doesn't seem so impressive.
A meaty issue
Where meat is concerned, the headlines came from an International Agency for Research on Cancer (IARC) review on processed meat in 2015. They caused huge public concern when they announced that eating 50 g of processed meat a day increased the risk of bowel cancer by 18%.
But Sir David Spiegelhalter, professor of the public understanding of risk, at Cambridge University, put the risk into perspective: "Around six in every 100 people would be expected to get bowel cancer in their lifetime. If all 100 people ate a three-rasher bacon sandwich every single day of their lives then, according to this report, we would expect that 18% more would get bowel cancer, which is a rise from six cases to seven cases.
"However, according to the National Diet and Nutrition Survey (NDNS), the average red and processed meat intake in adults is 71 g (86 g in men and 56 g in women). So that's just one extra case of bowel cancer in all those 100 lifetime bacon eaters."
Tarred with the same brush
The IARC separates red meat and processed meat, on the basis that the risk of cancer from processed meat is higher.
What's the process?
In its classification, processed meat is a "class 1" carcinogen, which means the evidence suggests it definitely causes cancer. Dr Gunter Kuhnle, a food nutrition scientist at the University of Reading, explains how: "Most processed meat is cured with a curing salt, and the nitrite in curing salt can cause the formation of nitrosamines in the intestinal tracts."
The IARC defines processed meat as: "Meat that has been transformed through salting, curing, fermentation, smoking, or other processes to enhance flavour or improve preservation." By its definition, burgers and normal sausages don't count as processed meat.
In the red
Red meat is classed as a "class 2a carcinogen" - it "probably causes cancer". This includes: "mammalian muscle meat, including, beef, veal, pork, lamb, mutton, horse, and goat." IARC accepts that the evidence that red meat causes bowel cancer is limited, and based largely on theory.
When assessing what we eat, the NDNS lumps processed red meat ("Manufactured, cured and/or dried meat, including bacon and ham") and total red meat ("Beef, burgers, lamb, offal, other red meat, pork, processed red meat, sausages") together. Government recommendations to cut down on red and processed meat don't discriminate between the two.
As bad as tobacco?
The IARC classed processed meat as a "class 1" carcinogen and tobacco, not surprisingly, sits in the same category. But even here, there are degrees. For instance, if nobody smoked, it's estimated that there would be 64,500 fewer cases of cancer a year in the UK. If we all gave up processed and red meat, there would be 8,800 fewer cases. So if you want to take one step to cut your risk of cancer and you're a meat-eating smoker, there's no contest.
The risk and benefit seesaw
When a doctor prescribes a medication, they're weighing up the risks and the benefits. All medications carry risks, and a side effect like losing all your hair, viewed as a "necessary evil" for cancer chemotherapy, would be completely unacceptable for a treatment of the common cold.
Reducing meat intake could also carry risks for some people. Meat is a major source of iron and zinc in the average UK diet and a blanket recommendation to cut meat intake could affect some more than others.
A review of iron and health by the Scientific Advisory Committee on Nutrition (SACN), highlighted a higher risk of iron and zinc deficiency in some groups than in others: "A recommendation to reduce consumption of red and processed meat in order to decrease colorectal cancer risk could have a negative impact on iron and zinc intakes in the UK by increasing the proportion of the population with intakes below the LRNIs for these nutrients."
Health scares have more of an impact on some groups than others, and red meat is no exception. The rise of "clean eating" may well have triggered a lower meat intake among some young women, but government guidance undoubtedly plays a role. The NDNS first assessed our nation's eating habits in 2008-9. Compared to the first two years of the survey, average red/processed meat intake among 19- to 64-year-old women dropped from 58 g to 47 g a day in the most recent two years. Among men of working age, by contrast, there was no significant drop in the 84 g a day intake.
Who's in the frame?
Red meat, it seems, really is a man thing. According to the NDNS, 48% of 11- to 18-year-old females, and 27% of 19- to 64-year-old women, are short of iron in their diet. With periods starting at an average age of 12, and the extra demands of pregnancy, these are precisely the groups who need iron most.
Of course it's possible to get the iron you need in your diet without eating meat, but many women don't. 30% of women eating 40-70 g of red meat a day don't get enough iron in their diet, compared to 38% of women eating under 40 g. For zinc, the equivalent rise in deficiency was from 5% to 20%.
What do we take away?
While they've been lumped together for the sake of some guidelines, there are major health differences between processed meat and lean red meat such as lean pork, steak or even lean mince.
If you're a three-bacon-butties-and-gammon-steak-for-supper-a-day man, you probably haven't dropped your meat intake in the last few years - but you should certainly think about it if you want to cut your risk of cancer. If you're a smoker, going vegetarian is hardly going to touch your risk of cancer compared to giving up the evil weed.
But if you're a woman, you may need a long hard look at your diet. Statistically, you're more likely to be short of iron and zinc. Statistically, you're much less likely to have been eating over the new recommended maximum. And yet, statistically, you're the one most likely to have cut down on meat. If you want to go veggie for ethical reasons, that's completely different. If you want a couple of meat-free days, go for it. But remember that by cutting your meat intake, you might just be jumping out of the frying pan into the mineral deficiency fire.