On October 29, 2015 the World Heath Organization (WHO) published a report claiming that the consumption of processed meat causes colorectal cancer. Published through its subsidiary the International Agency for Research on Cancer (IARC) the report is actually a reaffirmation of a 2002 report on the same subject, “Diet, nutrition and the prevention of chronic diseases”. This report advised people to moderate consumption of preserved meat to reduce the risk of cancer (http://bit.ly/1MmQ8X6 ). 
Science has advanced in the last 13 years and this new report is out of step with the data. More importantly it offers confusion as the strong conclusions were drawn from association studies with no apparent understanding of the cause or mechanism. The recommendations are at the same time strong & confusing, dogmatic & misleading.
Cutting to the chase I think the answer to the associations they are seeing are related to what is NOT in the diet as opposed to a direct action of red or processed meat. Without a mechanism to explain this association with high meat intake, it is just that – an association. A situation where alternative explanations make more sense. 
So let me tease apart some of the issues and problems. The IARC felt the evidence for processed meat was worth a Group 1 classification. This is the same grade as ASBESTOS and TOBACCO SMOKE. Really? They do back track that the magnitude of the risk may not be as great, but yet they feel that the evidence is as compelling. I beg to differ, and if you read through the report and the Q&A’s associated with it you will be left confused. The IARC classified red meat as Group 2A, the latter was classified lower because the evidence was less compelling. 
With asbestos and tobacco smoke we understand the causes behind their links to cancer. So risk and causality are directly connected. We get it. There is no such connection between processed and red meat with cancer. For that reason the report was received with skepticism. 
The possible mechanisms that were offered are the equivalent of scientific rumor; random thoughts presented with insufficient evidence. What we need to consider is it possible that there are other factors involved that were not assessed? Put another way is the relationship indirect as opposed to the inference of a direct relationship in the report. 
PROPOSED MECHANISM 1: The cooking method was discussed. High heat (like BBQ), results in the formation of polycyclic aromatic hydrocarbons and heterocyclic aromatic amines. In the appropriate dose we know that these can be carcinogenic, but will the odd BBQ replicate these lab studies? There was no clear trend for cooking method as the cause of colon cancer. As a result the association was inconclusive and the recommendation vague. 
PROPOSED MECHANISM 2: One of the old vestiges of misinformation that was resurrected again, is that preserved meat may have N-nitroso compounds (nitrosamines) some of which are known carcinogens. This legacy thinking is an example of poor biochemistry because it is an attempt to link the nitrite (NO-) and nitrate (NO-) that is used to preserve the meat back to nitrosamines. But it makes little sense. Remember the solution to the Watergate puzzle – follow the money! – Well in this case it is follow the electrons. Converting a negatively charged NO2- to a positively charged nitrogen oxide is very demanding and complicated. Let’s move on from old conjectural science. 
Stepping back from the molecular analysis and discuss food, which is the center of the theme. If the generation of nitrosamines that causes the cancer, then all dietary sources that are rich in nitrate should show the same action. Some of the richest sources of dietary nitrate, and far more than what is present say in bacon, are the green leafy vegetables and beetroot. Simply put there is vastly more nitrate in a piece of lettuce than a piece of bacon, but the WHO is not condemning the consumption of leafy greens? What we have is a significant disconnect. 
Without a clear underlying cause, then associations can be misleading, obscured and recommendations off-base. For example, there are so many forms of preserving meat from salting, fermenting, curing, and smoking. Are they all problematic? How do they do it? No answers were provided. 
A critical question to ask is “How is this recommendation valuable?” If it does not make sense or is adequately explained, then the public will reject it. They feel that they have been burned by recommendations that are reversed later on. They are skeptical and I completely understand their reaction. 
So if there really is an association between these diets and population based cancer incidence is there an alternative explanation? If the glove does not fit for nitrate/nitrite then we must acquit! But someone did the crime? What is the story? 
Let me offer some “food for thought”. Firstly, before we completely throw out the nitrogen oxides element to the story let’s look at in other ways, focusing not on the dietary intake but rather the immune response. It is not commonly appreciated by the general public, scientists understand that the immune/inflammatory response uses various nitrogen oxide chemical bombs as defenses. 
In my National Cancer Institute grant on antioxidants, nitrogen oxide and stomach cancer it was clear that infection with the bacteria H. pylori was associated with the chemical fingerprints of nitrosating and nitrating, reactive oxides of nitrogen. These are same fingerprints that the WHO was eluding to for preserved meats. But hang on, the source was not the diet, but the immune cells in the mucosa that were trying to kill the bacterial infection. We know the enzymatic source is a form of nitric oxide synthase (iNOS) that produces massive amounts of reactive nitrogen species and is the likely cause of gastric cancer with this infection. Similarly the increased incidence of liver cancer with hepatitis C infection follows similar chemical fingerprints. 
Well even if the diet did not cause the response can diet influence this response? Yes and in a positive, not negative, way. Supplementing the diet with vitamin C reduced this chemical fingerprint while promoting regression of the precancerous process. Further those that are risk for gastric cancer have low levels of dietary antioxidants. 
So here is a thought “Could the segment population associated with red meat ingestion in large amounts also exhibit a simultaneous reduction in dietary antioxidants?” and is that dietary imbalance behind the observations reported by the WHO? Is this profile a simple reflection of food choices, so it has a behavioral component not a direct causal effect of red meat? 
Let’s look at the problem from the other end. Instead of determining the dietary components that cause colon cancer, let’s evaluate what are the dietary components that can reduce the incidence of colon cancer? Well one of the most profound interventions is increased fiber in the diet. The fiber changes both the nature of the gut flora and its metabolism by this flora produces short-chained organic acids that help maintain the software that runs our genes (epigenetic actions). Cancer is when a cell loses its self-control and replicates without end. We now know that this “software defect” is of major importance in the development of cancer, and that our microbes can dictate this, not just locally but the fiber connection holds true for systemic forms of cancer like breast cancer. We know that the metabolism and make up of our gut flora is affected by what we eat, so is the problem child here not processed meats but rather an altered gut microbial flora dictated by the low fiber content of the diets? 
MY PROPOSAL & CONCLUSION: This association between processed meat and possibly red meat, with colon cancer is not due to the N-nitroso compounds or BBQ-generated polycyclic amines. In the simplest form the risk for colon cancer is centered not one what is in the diet – in this case red meat – but what is NOT in the diet – antioxidant fruits and fiber-rich vegetables. 
Linked mechanistically to this is that a diet rich in meat changes our gut flora resulting in limited production of short-chained fatty acids, the one or two carbon donors that regulate epigenetically the software that governs cell replication. In essence the issue is not a meat thing it is a microbial thing. 
And the final irony is that plant sources that are not only rich in fiber but they also contain far more nitrate/nitrite than preserved meats, and that diets that are rich in these plants lower the incidence colon cancer rather than raise it. 
The bottom line is if you do not know the mechanisms and you issue guidelines based on associations, then you are bound to make mistakes. I would take the WHO conclusion and turn it on its head. The problem is not what is in the meat that promotes cancer, but rather the problem is what is not in the diet – FIBRE & ANTIOXIDANTS – to reduce the incidence of cancer. 
So my recommendation is to balance your diet. Meat is fine but don’t forget your fruit and vegetables. Oh…. and my Mother would be so proud……