Bacon and meat lovers were dealt a devastating blow this fall when the World Health Organization (WHO) released a report evaluating just how carcinogenic eating red and processed meat is.

Mariana Stern, associate professor in preventive medicine at the Keck School of Medicine of USC, was the only Californian appearing on a panel of 22 experts in Lyon, France. She has conducted research on meat intake, cancer and diet for almost a decade.

The panel concluded that for every 50 grams of processed meat eaten daily, the risk of colorectal cancer is increased by 18 percent.

The WHO report says processed meat such as bacon, hot dogs, ham and sausages causes colorectal cancer. Why is processed meat more carcinogenic than red meats?

Processed meats have been altered for preservation and sometimes to enhance flavor by salting, smoking, curing, fermentation or other processes. Some of the salts used have nitrates and nitrites that could convert into cancer-causing compounds called carcinogens. Smoking meat can lead to accumulation of other carcinogenic compounds.

In addition, both unprocessed and processed red meat have heme iron, which can have direct harmful effects in the body and also contributes to the formation of cancer-causing agents in our bowel that are derived from nitrates and nitrites.

In our evaluation, the evidence of association between processed meats and cancer risk was more consistent than the evidence for unprocessed red meat. Thus, processed meats were classified as “carcinogenic to humans” (Group 1), whereas unprocessed, fresh red meat was classified as “probably carcinogenic to humans” (Group 2A).

Globally, tobacco smoking accounts for about 1 million cancer deaths per year. Air pollution accounts for more than 200,000 deaths annually. Diets high in processed meats could be blamed for only 34,000 cancer deaths per year. Why is processed meat put in the same high-cancer risk category as tobacco smoking and formaldehyde?

The WHO classifies Group 1 agents as those for which there is reasonable certainty they cause cancer. The magnitude of the effect of the agent is not a consideration.

Indeed, tobacco smoking is a more powerful cancer-causing agent than processed meats. However, this does not take away from the fact that processed meats can cause cancer.

The panel concluded the association of colorectal cancer with diets high in processed meats was unlikely to be explained by other factors or by random chance, particularly because the phenomenon was observed in large epidemiological studies conducted around the world.

Colorectal cancer likely develops due to a combination of multiple cancer-causing agents. Processed meats are one of these. Therefore, the contribution of processed meats and its fellow carcinogens is probably smaller than smoking, which by itself can contribute to the development of several cancers.

The panel concluded that consumption of red meat probably causes cancer in humans. Doesn’t meat provide necessary nutrition such as protein, iron, vitamin B12 and other nutrients?

It was not the task of our panel to do a comprehensive evaluation of the risks and benefits of eating red meat. It was our task to evaluate the potential role of consumption of red and processed meats as cancer-causing agents.

Moving forward, more evaluations will have to take place to weigh the risks and benefits of eating meat, given its nutritional value and the newly found cancer-causing risks.

How much red and processed meats could people consume on a daily basis without increasing their chances of getting cancer?

Analyses done by the World Cancer Research Fund based on the same epidemiological data we evaluated suggest limiting the intake to 500 grams — about 18 ounces — of red meat per week. That’s about 71 grams or 2.5 ounce per day. For reference, a hamburger is between 65 to 85 grams. They emphasized the majority of this meat should be unprocessed.

Should people become vegans and vegetarians?

The decision to eliminate meat altogether is a personal choice. Those who enjoy eating meat may want to keep the recommended guidelines in mind.

Rather than thinking about this in absolute terms, perhaps it is better to focus on incorporating more fruits, vegetables, grains, legumes and vegetable oils in the diet — which would naturally minimize intake of red meats and processed meats. The evidence for the many beneficial effects from plant-based diets is mounting, not only for preventing cancer but also for prevention of several other diseases and for longevity.

Unfortunately, heaps of processed meats have become staples in many households because of their affordability and convenience, which can make a transition to a more plant-based diet challenging for many. Improving access to healthy and affordable plant-based dietary items will be essential in reducing intake of processed meats.

Scientific studies say coffee is good for us one week and bad for us the next. Should bacon lovers wait until these red and processed meat findings are reversed?

It can be confusing! Often, these “scares” come from individual studies that get publicity because of people’s interest. There is a lot of variability across different populations, so it is no surprise that epidemiological studies looking at dietary factors can often lead to inconsistent results.

Because of this, organizations like the WHO comprehensively evaluate the literature periodically to truly understand what is going on with a particular dietary factor and to find consensus. These comprehensive evaluations are the ones the public should pay attention to, as they consider evidence that accumulates over time across the world.

In the case of our evaluation of red meats, we considered more than 800 studies that were published over many decades. The evidence of the role of red meat and cancer has been mounting for a long time. This comprehensive evaluation confirmed something that had been suspected for many years. The American Cancer Society has been recommending lowering red meat intake and minimizing processed meats for many years now. It is highly unlikely these findings will be reversed.

How do meat cooking methods alter carcinogenic risk?

Cooking meats at high temperature — pan-frying, frying, grilling — facilitates the formation of powerful cancer-causing agents called heterocyclic amines (HAAs) that can cause damage in our body and contribute to cancer formation. Meats grilled over flames can accumulate another group of carcinogens called polycyclic aromatic hydrocarbons, which are also capable of causing damage in our body.

The panel determined HAAs in cooked red meats are involved in cancer causation. Pan-frying has been found to be the cooking method that accumulates the highest level of HAAs.

The formation of these cancer-causing agents is known to be a function of temperature and cooking time. Therefore, it is better to cook meat longer at a lower temperature than to cook it quickly at very high temperatures. Flipping the meat often and using soy-based marinades has also been found to reduce significantly the formation of these carcinogens. In contrast, use of barbecue sauces when cooking meat seems to increase HAA formation.

Should we microwave all of our red meats?

Interestingly, studies have found microwaving meat for a few minutes before cooking it using other methods significantly reduces the formation of HAAs.

— Zen Vuong