Scientists know that eating too much red meat can increase people's risk of bowel cancer, but until now they've been unsure exactly why.
A new Scottish study could provide some explanation. It appears that iron sparks the bowel cancer process in people with a faulty APC (Adenomatous polyposis coli) gene.
The researchers studied mice with a defective APC gene and found they were two to three times more likely to develop bowel cancer.
Mice without the defective gene that were fed a high-iron diet remained cancer-free, as did mice who had the defective gene but were fed a low-iron diet.
Lead scientist Professor Owen Sansom, deputy director of the Cancer Research UK Institute in Glasgow, told the UK's Daily Mail that this research helps us understand how bowel cancer develops.
"The APC gene is faulty in around eight out of 10 bowel cancers but until now we haven't known how this causes the disease," he explained.
"It's clear that iron is playing a critical role in controlling the development of bowel cancer in people with a faulty APC gene. And, intriguingly, our study shows that even very high levels of iron in the diet don't cause cancer by themselves, but rely on the APC gene."
The researchers believe the faulty APC gene allows iron to build up in the cells around the gut.
Dietitian Kathy Chapman, an expert on diet, obesity, physical activity and cancer risk, says that scientists have long known excessive amounts of red and processed meats can cause cancer, but they haven't been able to work out exactly why.
"The mechanisms of how red meat might be contributing to the risk of bowel cancer haven't been well understood," she said.
"The link with the iron and certain people having a genetic predisposition is new."
According to Chapman, we don't need to stop eating red meat altogether, but just make sure we don't overdo it.
"We need some iron or else we would all be anaemic and completely lethargic and not able to function very well," she said.
"When we look at studies of people and the link with red meat and bowel cancer, it's those who are the very big consumers of meat. It's those who would have a piece of steak that fills up the whole plate and hardly any vegetables on it."
Chapman suggested limiting meat portions to a quarter of a plate.
"The message is certainly not to avoid red meat, but to eat it in moderation," she said.
"Have your meat take up about a quarter of your plate and the rest of the plate should be vegetables or grain-based foods. If you are somebody who does the opposite, then this is a wake-up call."