Does processed meat cause cancer?

by Jennifer Bowden / 25 September, 2017

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The finger is pointing at processed meats as a cause of cancer, but small-goods makers aren’t convinced.

Bacon and sausages are favourite brunch-menu items even though studies have identified health risks for these processed foods. A major 2015 review by the International Agency for Research on Cancer (IARC) concluded processed meat could be classified as “carcinogenic to humans” given the evidence that it causes colorectal cancer. However, the Meat Processors Association of New Zealand has questioned the IARC’s findings. So, how good is the IARC’s evidence?

Randomised trials that use placebos are the best study design to prove causation, but that’s not possible for studying the relationship between processed meat and cancer risk. There are two reasons, says Professor Nick Wilson, a University of Otago public health physician. First, having people unaware of their processed-meat intake – that is, feeding them bacon and sausages without their knowing it – is impossible. Second, cancers typically take decades to develop.

“Cohort studies are a key alternative, and although these aren’t perfect, they usually allow for statistical analysis that removes the effect of other causes of cancer, such as smoking or a diet low in whole grains,” says Wilson, the co-author of a recent review of the effect of processed and red meat on New Zealanders’ health.

The IARC based its conclusion on a range of research, all of which contributes to the bigger puzzle, he says. More than 400 epidemiological studies on cancer in humans provided data on processed meat, and 700-plus other studies supplied red-meat data.

“When a scientific agency such as the IARC declares a substance is a cause of cancer, this is a very careful process involving consideration of the strengths and limitations of the various studies.” Wilson has first-hand experience of the Lyon-based agency’s approach. He worked there on a similar review of smoking risks.

Public health physician Nick Wilson.

In addition to assessing the strength of the effect of processed meat on colorectal cancer risk in published studies, the reviewers would have considered how consistent the effect was in different studies; the timeline of exposure to meat and cancer development; whether a clear dose-response relationship existed between processed meat and cancer; and if there was a biologically plausible reason for processed meat to be the cause. The nitrates, haem iron and high salt content of processed meat have all been implicated as potential cancer triggers.

However, the Meat Processors Association, which represents manufacturers of small-goods and processed meats such as bacon, ham and sausages, disputes the findings. “The rating is based on the IARC’s opinion of how certain the relationship is between an agent and cancer,” says spokeswoman Antoinette Bisset. “It’s not intended to show an agent is equally as dangerous as other agents in that classification group. Clearly, processed meat and tobacco do not represent the same risk.

“The IARC does not factor in the level of safe exposure or any positive effects of an agent – hence sunlight is in the same category as processed meat.”

Bisset says the association believes there is no compelling evidence to suggest moderate amounts of processed meat eaten as part of a balanced diet raise the risk of cancer. “No single food has been proven to cause or cure any type of cancer,” she says.

In support, the association points to two published studies. One was a review funded by Meat and Livestock Australia and the other an opinion piece on the limitations of research about meat and health. In contrast, the IARC reviewed more than 400 published studies.

No one is saying the odd sausage or bacon-and-egg pie is lethal. However, there’s plenty of evidence to suggest that certain dietary patterns can reduce our risk of many cancers. That includes eating less processed meat to reduce the chances of getting colorectal cancer.

This article was first published in the August 19, 2017 issue of the New Zealand Listener.

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