The research linking meat-rich diets to a higher risk of bowel cancer often raises more questions than answers. Is there something about meat itself that causes cancer to take hold in the bowel – or is it more to do with what we’re not eating?
In other words, when meat fills a lot of the plate does it edge out other foods that might protect against cancer?
Australian research has shed new light on this thanks to a study of what happens in the gut when people eat large serves of meat with resistant starch, a type of fibre found in foods such as lentils, beans and whole grains.
They found that while big helpings of meat triggered the rise of cancer-promoting molecules in the bowel, the resistant starch helped to counteract this.
"This finding supports the consumption of resistant starch as a way of reducing the risks associated with a high red meat diet,” says Karen Humphreys, a research associate at the Flinders Centre for Innovation in Cancer at Adelaide's Flinders University.
Other research has already found a benefit from eating resistant starch which provides food for friendly microbes living in the gut. These bugs then do us a favour – they ferment the fibre, turning it into a substance called butyrate, which helps keep the bowel wall healthy.
In this study 23 healthy adults were split into two groups and ate either a red meat diet or a red meat plus resistant starch diet for four weeks. After a four-week break each group switched to the other diet for another four weeks.
After eating 300 grams of lean red meat every day for four weeks, the people in the first group had a 30 per cent increase in the levels of cancer-promoting molecules called miR-17-92 in their rectal tissue, as well as an increase in cell growth. But when they changed to eating 40g of butyrated resistant starch daily with their meat for four weeks, the levels of these molecules dropped to where they were before the study began.
"When levels of these molecules increase within cells they promote increased cell growth, and uncontrolled cell growth can eventually lead to cancer development," Dr Humphreys says. “Patients with colourectal cancer often have very high levels of these molecules in the cancerous tissue.
"In our study, high red meat intake increased miR-17-92 levels in otherwise healthy cells in the human gut.”
But resistant starch was unable to lower levels of another cancer-promoting molecule called miR-21, Dr Humphreys adds, and this needs more research.
"It's also likely that red meat is having other effects on cells such as damage to DNA, and we are doing more studies to see if these effects can be changed by resistant starch."
What is it about meat that could cause levels of these molecules to rise? One possible reason is an increase in levels of haem iron (the type of iron found in animal foods but not plant foods).
"This has been linked to gene changes and increased growth of the cells lining the human gut, the place where bowel cancer starts," Dr Humphreys says.
"Red meat has also been linked to changes in gut bacteria, and to increased inflammation which could also contribute to changes in the cells lining the gut."
Still, the daily amount of 300g of red meat is high. Although 300g is the raw weight, which translates into around 225 grams when cooked, it is much higher than the serves recommended by the Cancer Council Victoria:✓ 65g of cooked red meat no more than three to four times a week.
But while 300g is more than what many of us eat each day, in some restaurants a large steak can be this size or more, Dr Humphreys says.
"Total meat consumption in the US, Europe and the developed world has increased from the 1960s," she says. "In the light of this research, red-meat eaters should make a conscious effort to get more resistant starch into their diets."
Good sources of resistant starch include bananas that are still slightly green, cooked and cooled potatoes in dishes such as potato salad (cooling increases levels of the starch), whole grains, beans, chickpeas and lentils.
