The 2007 WCRF report on diet and cancer made it clear that bowel cancer is more common amongst people who eat a large amount of red and processed meat.
But why? What component in these foods is the ‘villain’ that can lead to cancer? And what is it about white meats (e.g. chicken and fish) that means they’re OK? A large number of studies have been published on this topic, so let’s have a look at the suspects.
Burnt or charred meat
There’s a commonly held view that burned or charred food, particularly meat, is responsible for cancers of the digestive tract. A fair bit of evidence supports this view, but there are several reasons why we believe this isn’t the major factor.
For a start, the chemicals produced when white and red meat are burnt are virtually identical, but white meat isn’t linked to bowel cancer. And there are large variations in how meat is cooked and eaten across the world, but the link between red meat is pretty much universal.
Also, although the chemicals involved in charring of meat have been shown to cause DNA damage in the laboratory, that’s a long way from proving they’re the culprit in real life.
The link between being overweight and developing cancer is now pretty well-known. But there’s an important difference between eating fat and being fat.
People who are bigger are more likely to get several types of cancer, including bowel cancer. But if you look a people who eat a low fat diet, and take into account their bodyweight, as a team of US researchers did in 2006, you find that people who eat less fat, rather than people who are skinny, tend not to have any reduction in bowel cancer risk.
Even more confusingly, a huge Swedish study from 2005 found that bowel cancer risk was lower amongst women who ate the highest amounts of fatty foods, regardless of their actual weight.
So it’s unlikely that the animal fat contained in red and processed meat can directly affect the lining of the bowels and cause cancer. It seems that fat needs to become part of your body before it affects your cancer risk.
Of course, as Ed pointed out the other week, a low-fat, regularly exercising lifestyle is still good for you in many other ways.
Nitrite and nitrate preservatives
The evidence suggests that processed meats (for example, ham, bacon, sausages, pate and tinned meat such as Spam) are more strongly linked to bowel cancer than red meats. So maybe something in the processing is to blame?
Processed foods often contain nitrogen-based preservatives to stop them going off during transport and storage. Are these chemicals linked to bowel cancer?
The evidence for this is patchy, but its difficult to rule them out completely. A 2006 Swedish study found that woman who consumed the highest levels of nitro-containing foods had higher rates of stomach cancer. But again there’s still no ‘smoking gun’ for bowel cancer.
Haem is the pigment found in haemoglobin, the protein that makes our blood red and carries oxygen round our bodies. It’s also the pigment that makes ‘red meat’ red.
Scientists, including the late Sheila Bingham, found that haem is broken down in our gut to form chemicals called ‘N-nitroso compounds’. These have been found to damage the DNA of the cells that line the digestive system. And DNA damage is the first step on the road to cancer.
To complicate things a bit it seems, that when the lining of our gut senses it’s been damaged, it reacts by telling the existing cells to divide more rapidly to make new cells. This ‘extra’ cell division might also increase the chances of cancer developing, because every time a cell divides, it runs the risk of making a copying error in its DNA.
Professor Bingham ran studies that directly looked at the amount of N-nitroso compounds in people’s faeces, and looked at whether N-nitroso-linked DNA damage was occurring. It was.
There’s still some evidence missing here – no-one’s yet proved that people with high levels of N-nitroso compounds in their stools actually have a higher chance of developing bowel cancer itself. But as evidence goes, it’s probably the best we’ve got.
Intriguingly, there’s some evidence that chlorophyll, a green pigment found in plants, might block the breakdown of haem in the gut. This could be one of the reasons why a diet high in fruit and vegetables can protect against bowel cancer.
So, looking at all the evidence, it seems that haem breakdown is probably responsible for the increased risk of bowel canceramongst people who eat large amounts of meat. There might be an additional effect from nitrogen-based preservatives, but the haem evidence is certainly the most compelling of the theories we have so far.
But as with so much about diet and cancer, we should stress that they’re just that – theories.
Until we’ve really pinned down the culprit or culprits, the best advice remains to keep red and processed meat to a sensible level in our diets.
How much meat?
But what is a ‘sensible level’ of red and processed meat? Again, we’re venturing into rather uncertain territory.
EPIC – the big, Europe-wide study on diet and cancer that Cancer Research UK is helping to fund – has found that bowel cancer rates were 30 per cent higher among people who ate two daily 80g portions of these meats, compared to the rate in people who ate just 20g a day.
And a French team found that every daily 80g portion of processed meat increased bowel cancer risk by two thirds.
So there are some ball-park figures. But it’s important to remember that these are studies that look at overall risks in large numbers of people, not specific risks in individuals.
Given that we all carry different genes, our individual, personal risk of bowel cancer is currently impossible to work out.
So all we can say for sure, in answer to the question ‘what should I do?’ that inevitably follows stories like this, is that a diet low in red and processed meat is definitely a good starting point if you’re worried about bowel cancer.
There’s a lot more work to do – we’re really only just starting to understand how the complicated jigsaw of diet, genetics and cancer fits together.
PS If you want to read a really detailed (but slightly technical) breakdown of a lot of the evidence discussed in this post, have a look at the Bowel cancer – Risks & Causes page of the CancerStats section of News & Resources.