IARC broke my heart when they made bacon a carcinogen.

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Looking back, it was a much simpler time.

People argued good-naturedly whether a dress was gold and white or blue and black, a New York rat taught us the importance of chasing our dreams and “Netflix and chill” had finally given introverts the tools they needed to go on a date.

It was 2015 and the future seemed rosy. Few could foresee the coming dystopia.

But the signs were there.

On October 26, 2015 the International Agency for Research on Cancer (IARC) issued a press release classifying bacon as a class one carcinogen.

It wasn’t just bacon either, IARC judged that there was strong evidence linking any meat that has been salted, smoked, fermented or cured to colorectal cancer.

In a single stroke IARC had put some of my favourite foods into the same category as cigarettes, asbestos and radioactivity.

Going through the stages of grief my first reaction was denial. Surely there’d been a mistake. IARC would sort this out soon enough, I thought. Then came anger.

Finally, though, I had to accept it, the evidence was pretty good. Bacon is a carcinogen.

But, there are carcinogens and then there are carcinogens. I gave up smoking, but I’m still going to eat bacon.

The IARC classifications

A class 1 carcinogen is one that IARC believes is carcinogenic to humans. The other categories are 2A “probably carcinogenic”, 2B “possibly carcinogenic”, 3 “Not classifiable” and 4 “probably not carcinogenic”.

There is not a lot of joy in IARC classifications, the best you can do is “probably not carcinogenic”.

How Bacon Causes Cancer

Humans have been preserving meat for centuries using a substance called saltpetre.

Also the main constituent of gunpowder, saltpetre is actually a salt formed by combining a nitrate ion (NO₃^–) with either potassium (KNO₃) or sodium (NaNO₃).

Nitrate itself is relatively inert and doesn’t do much. But during the curing process nitrate is slowly converted to nitrite (NO₂^–) by bacteria.

Nitrite is a much busier molecule. It is nitrite that inhibits bad bacterial growth, prevents spoilage and contributes to the characteristic pink colour of cured meats.

These days commercial curing speeds things up by skipping the conversion step and directly adding sodium nitrite (NaNO₂). Though saltpetre is still occasionally used, especially for traditional products.

How Curing Chemistry Converts Nitrites into Carcinogens

So far so good. Nitrite preserves meat and protects us from spoilage and harmful bacteria.

The problem is that nitrite also causes cancer once it gets into the acidic environs of our stomach. The reason? Chemistry. Though I’ll try to keep it simple.

The essence of what happens is that, in the acidic environment of the gut, nitrite molecules react with each other to form a highly reactive molecule called a nitrosonium ion (NO⁺).

This nitrosonium ion then reacts with protein fragments to produce a few different types of molecules. The type depending on which chemical group on the protein fragment is involved.

The two important ones for us are S-nitrosothiols, which form with protein thiols, and N-nitrosamines, which forms with protein amine groups.

The favoured pathway is to produce S-nitrosothiols. The nitrosonium ion has a thermodynamic and kinetic preference for reacting with sulphur ions.

This is good thing. S-nitrosothiols, along with nitric oxide (NO), have important and beneficial roles in cardiovascular and metabolic health.

Problems arise when we have a lot of nitrites in our gut, like if we’ve just eaten an abundance of bacon.

With a lot of nitrites around the production of N-nitrosamines, along with similar molecules collectively called N-nitroso compounds (NOCs), can increase.

This is not such a good thing. N-nitrosamines and the other NOCs are well known carcinogens.

How Do NOCs Cause Cancer?

NOCs cause cancer because they do things to our DNA. In this case they add a chemical group called an alkyl to our DNA.

Over the course of a day the human body will produce about 330 billion new cells. Every time this happens all the DNA in the nucleus of the cell needs to be duplicated so each of the daughter cells receives a full complement of DNA.

To do this the cell has a complex and very effective DNA replication machinery.

Effective, that is, until you start adding alkyl groups to your DNA.

The unexpected chemical group interferes with the DNA replication machinery and you can get errors in the DNA code that is passed down to the daughter cells.

If you get an error in the wrong place, if it disrupts genes important for controlling cell growth for example, you can end up with a mutation that causes the cell to become cancerous.

Vegetables Contain Nitrates, Why Don’t They Cause Cancer?

So, processed meats can cause cancer because they have nitrites in them that cause mutations in our DNA. Seems reasonable and there is a lot of experimental evidence backing up the idea.

But, there’s a problem. Fresh fruit and vegetables have a ton of nitrates and a lot of those nitrates are converted to nitrites by bacteria in your saliva.

So why are diets rich in vegetables and fruit associated with a low cancer risk when they are packed with the same molecules that cause cancer in processed meats? Why isn’t a cabbage a class one carcinogen?

Plants Have Far Fewer Protein Fragments

During curing and fermentation of meat, not only does a lot of nitrate get converted to nitrite but a lot of protein also gets broken down into fragments. This means that NOC production can begin before a nitrite gets anywhere near our gut.

When we eat processed meat, not only will we produce NOCs in our gut but we will also get a healthy dose of pre-formed NOCs that can get to work on our DNA straight away.

Fresh produce, on the other hand, has a lot less protein fragments, the proteins are intact, and they also don’t have bacteria converting nitrate to nitrite. Because of this fresh produce has almost no pre-formed NOCs when we eat them.

Haem Seems To Be Necessary As Well

Fresh produce also doesn’t stimulate the production of NOCs when they get to our gut. This is because fresh produce doesn’t contain a molecule called haem.

Haem is an iron containing molecule that is able to bind oxygen and it is famous for it’s role as a part of haemoglobin, the protein that transports oxygen around our blood. Skeletal and muscles cells are also rich in haem because they use a related protein called myoglobin to store oxygen.

So processed meat has plenty of haem and it reacts with NO⁺ to form a molecule called nitrosylated-haem which is actually the molecule that goes on to react with protein fragments and form NOCs.

When considered this way, it seems that it is haem that is driving NOC formation rather than nitrites. A fact that could be important when we look at red meat in a future post.

Either way, haem is part of the reason that fresh produce doesn’t stimulate NOC production, no fruit or vegetable contains haem.

Plants Have a Lot of Antioxidants

Antioxidants are molecules that are able to neutralise other highly reactive molecules. Plants are generally stuffed with antioxidants.

These antioxidants, things like polyphenols, vitamins C and E and anthocyanins, can inhibit the production of NOCs by neutralising reactive nitrogen species, like N2O3 and NO⁺. This prevents them from reacting with haem and stops NOC production.

If you are wondering if this means we can eat lots of vegetables with our bacon and we’ll be fine? The answer is probably no.

The antioxidants may prevent NOC formation in the gut but they can’t do anything about the pre-formed NOCs that you get when you consume processed meat.

Eating vegetables is the path to good health but they can’t save you from dietary NOCs in processed meats.

Factor	Processed Meat	Fruits & Vegetables
Pre-formed NOCs	High (formed during fermentation/curing)	Near Zero (no intact protein breakdown)
Haem Iron Content	High (catalyzes NOC production)	Zero
Antioxidants	Low	High (Polyphenols, Vit C, Vit E)
Cancer Association	Increased Colorectal Risk	Protective / Lowered Risk

Understanding the Risk

OK, the evidence is pretty good that processed meats are carcinogenic. But what are the risks? Am I doomed to a short life, dying with a cancer-riddled gastrointestinal tract because of my love of charcuterie?

Relative and Absolute Risk

Well here the story gets a tiny bit better.

There are a lot of observational studies on processed meats so the statistics vary from one paper to another. But taking one, essentially at random, it reports a relative risk of 18% for colorectal cancer.

Before we all panic, this doesn’t mean we have an 18% greater chance of colorectal cancer if we eat processed meats. It means our risk of getting colorectal cancer increases by 18% if we eat processed meat.

To convert this to an absolute risk we need to know what the risk of getting colorectal cancer is without eating processed meats.

Working with Australian data, 1 in 23 people (about 4.3%) will develop colorectal cancer by age 85. So, an 18% increase in lifetime relative risk means the absolute risk will go up to 5.07% (4.3% x 1.18). That’s a 0.8% increase in your absolute chance of getting colorectal cancer if you eat processed meats.

This is real back of the envelope stuff because my baseline includes people who ate processed meats anyway. But a 2005 UK study found that the ten-year absolute risk for colorectal cancer went from 1.28% to 1.71% between the lowest and highest consumers of red and processed meats.

Personally, I don’t think these are huge increases in absolute risk.

But this is my comfort level, everyone has a level of risk that they are comfortable with. Others may be at particular risk if they have a history of colorectal cancer in the family.

Processed meats have also been associated with a range of other cancers so, even if you are comfortable with a low absolute risk of colorectal cancer, the cumulative risk of other cancers may get you thinking about how much processed meat you are eating.

Does It Matter How Much You Eat?

If I’ve accepted that processed meats can cause cancer how much processed meat should I be eating? If I’m going to stop gorging myself on cabanossi every day how much gorging can I get away with?

Well the bad news is most advisory bodies recommend not eating processed meat at all.

But the epidemiological evidence suggest that there is a dose-response relationship so the more processed meat you eat the greater the risk. The Australian Cancer estimates that each 50g daily serving (about two bacon rashers) of processed meats increases the risk by 18%.

This is, again, a relative risk equating to around a ~1% increase in the absolute risk if we use the above rough calculations.

Conclusion

We all need to decide how we want to behave and what level of risk we want to accept. I’m not going to tell anyone what they should do but, personally, I’m not going to stop eating processed meats completely.

I’m certainly going to eat less processed meats. I don’t need to eat bacon everyday and I’m willing to treat processed meats as a ‘sometimes food’ and have a couple of servings a week.

There is some evidence that diets high in fibre can mitigate the risks of processed meats (see here) so I want to eat more vegetables, beans and fruits.

Other risk factors for colorectal cancer are obesity, a sedentary life style and alcohol so avoid fattier foods, exercise and drink less.

Really we’re back to where we always get to when thinking about diet and health.

Eat a well balanced diet with plenty of fresh fruits, vegetables, nuts, and grains, enjoy the odd steak or charcuterie board and we’ll probably be fine; we’ll certainly be healthier and better able to enjoy whatever amount of life we end up getting.