Before looking at the health effects of smoked and cured foods let's consider what we are talking about.
- Smoked foods are those which have been exposed to burning plants materials such as wood in order to preserve or flavour them. Although smoke is an anti-microbial and anti-oxidant it does not penetrate far into the inside of most foods. As such it is more commonly used as a flavouring than as a preservative. However, it can help reduce the speed of deterioration of foods by sealing meats and fish, and preventing new bacteria entering into them from outside.
- Cured foods are those to which salt, nitrites, nitrates or even sugar have been added to flavour or increase shelf life. Salt is the most commonly used agent in curing. In fact salting was the only way of preserving meat up until the 20th century and was vital to the success of long sea voyages before modern times. The salt creates an environment in which water is sucked out of microorganisms, either killing them or slowing their growth. Nitrates and nitrites kill bacteria, and also add a distinctive reddish brown colour to many products. Sugar is sometimes used to offset too harsh a flavour given by the salt.
Foods that are commonly treated.
Smoking and curing are often used in conjuction, as smoking a meat can reduce the amount of salt required to eliminate the bacteria.
- Sausages are most often made from pork, but sometimes from beef or vegetarian components. Some sausages are smoked and some not. Smoked sausages are nearly always smoked with nitrite.
- Ham, from the hind leg of a pig, is cured in salt and sugar. In many cases nitrites or nitrates are added for preservation. Ham is often smoked for flavour.
- Bacon is often from the back or side of the pig. Streaky bacon is made from pork bellys. Bacon is salted and then dried, boiled or smoked.
- Tofu is made from coagulated soy-milk, the coagulating agents often containing calcium. Tofu is occasionally smoked, in which case it has a light brownish colour.
- Salmon that is smoked is often in thin slices, although not always, and is usually served cold.
- Mackerel is often smoked and in this form also often sprinkled with peppercorns.
- Herring (kippers), breakfast staple of the Scots, is nearly always smoked.
- Haddock such as Arbroath smokies. Any yellow coloured haddock is normally smoked.
- Gravlax is salmon treated with salt, but not not smoked.
- Bacon is usually salted with sodium nitrite. It is then either dried, smoked or boiled. Fatback or streaky bacon contains a lot more fat and loin or back bacon is the bacon with a fatty rind and then a meaty part.
- Kipper are herrings that are split, salted and then smoked. Cold smoked kippers need to be cooked. Other types can be eaten straight away.
- Sausages such as salami and hot dogs contain nitrates and nitrites.
- Biltong and jerky are generally salted, and have sugar added also along with spices. They don't normally contain nitrates or nitrites.
The health risks.
Smoked and cured products do appear to have some capacity to cause cancer, but what substances in these foods are responsible?
Firstly let's consider smoked products which produce polycyclic aromatic hydrocarbon compounds which are carcinogenic.
- Benzo(a)pyrenes, benxo(a)anthracenes, benzo(b)fluoranthene and di-benzo(a,h)anthracene are examples of toxic components of wood smoking. The amount of these compounds in smoked products gives a rough guide to their potential to be carcinogenic.
Secondly let's consider the common agents used in curing: nitrates, nitrites and their common by-product, nitrosamines. It is the nitrosamines that appear to be linked with cancer incidence most strongly1.
- Nitrates NO3 such as saltpetre, E252, are used in curing, both as a colour and as a preservative. They are more powerful anti-microbial agents than ordinary table salt, and imbue a pinkish colour to meat. Salt cured meat can appear a grey colour. Nitrates are neccessary for the synthesis of proteins for plant growth. It is not surprising then that green leafy vegetables, and many root vegetables contain high levels of nitrates. Also drinking water can be contaminated with high levels of nitrates. Nitrates on their own are unlikely to cause us harm, however they can turn into other more damaging compounds in our guts.
- Nitrites NO2 also used in curing are often produced by soil bacteria from ammonia. Ammonia is a waste product of soil bacteria and fungi, and sometimes added to soil as a fertilizer. It is also produced by plants and animals. Nitrites can appear in our digestive tracts after bacteria convert ammonium or nitrates into nitrite. It can also appear when we eat foods using the common preservative, sodium nitrite, E250, that is used in curing processes. High levels of nitrite can react with haemoglobin in the blood to cause a reduction in oxygen levels leading to so called "blue baby syndrome". It affects infants more than adults because they have a lower stomach acidity.
- Nitrosamines are produced when nitrates react with amino acids, either during the curing process or in the digestive system. Nitrosamines have been clearly linked with cancers of the gut. This tells us that it is the form in which we eat nitrates and nitrites that is important. Processed meats, and other protein containing foods all contain amino acids (proteins are made up of strings of amino acids). It is this combination of protein and nitrates that is likely to increase our risk of cancer.
Thirdly let's consider salt, the most commonly used preservative in historical times and still today.
- Salt has been linked with gastric cancer in a number of studies2. However, most of these are epidemiological in nature. That means they test large population groups which can't be fully controlled. They try to account for confounding factors, but are restricted by the bias and lack of foresight of those conducting the test. For instance, a food questionnaire to assess salt intake, would need to assess also the intake of nitrates as well to work out whether it was nitrates or salt that was behind any increased incidence of cancer. That is one example, but there are countless more, which is why epidemiological studies can never be used to prove the cause of any phenomena, only to suggest possible causes. Having said that the study identified in reference 2 does suggest some mechanisms by which salt could cause cancer. One was by damaging gastric mucus, the other by interacting with H Pylori bacteria. I'm not convinced by what I've seen so far, and would suggest that salt alone is unlikely to be a primary cause of cancers of the digestive tract.
The million dollar question.
So I've told you all about smoked and cured products and the damage they may cause, but what you really need to know is: "what is your risk?". For instance if you have bacon for breakfast every day of the week, what is the chance it will make you ill or kill you ". The diseases that have been identified in some reports as being made more likely by smoked and processed foods include cancers of the bowel, especially colorectal cancer and cardiovascular disease.
The chance you will die from colorectal cancer is about 3%. That is 3 people out of every 100 in the UK are expected to die from it. The question is, by how much do smoked and cured foods increase your risk? Well a 2005 article in the Journal of the American Medical Association identified that your risk could rise by about 1% if you moved from the bottom third of consumers to the top third. However the same report said that when adjusting for obesity and cigarette smoking the association disappeared. That does not actually mean that there is no risk from smoked and cured products however, as if a disporportionate number of obese cigarette smokers are eating processed meat products then we are still unsure if it is obesity, cigarettes or smoked products that cause colorectal cancer. My best guess is that the risk is less than the 1% identified in this study, but that there is still a risk of colorectal cancer from smoked and cured products. A smaller risk I believe would also apply to stomach cancer.
The chance of dying from cardiovascular disease (CVD) in the UK, is much greater (nearly 10 times) than that of dying from colorectal cancer. It is about 32%, and therefore reports of increased risk could be significant. A recent study4 from the Archives of Internal Medicine, that was widely reported in the news5, stated that the risk of dying from cardiovascular disease had risen by about 20% for people who consumed one more portion per day of unprocessed or processed red meat. That equates to an increased risk of death from CVD of 6% (20% of 32%!). However, I suspect strongly that these figures are grossly misleading due to the inherent flaws of the epidemiological study that reported them.
The problem I have with the data for CVD is the lack of a clear causative factor. With regard to red meat, many suggest saturated fat as a factor. However, that does not hold up to scrutiny from many other studies that I identified in a previous article. Salt intake could be a factor, but again when you look at the detail it seems unlikely that salt alone would increase heart disease risk for most people. The data on nitrosamines from dietary sources causing CVD is limited and I suspect its role is not that great. All in all my take is that is smoked and cured foods do not increase your risk of dying from heart disease by very much, probably by less than 0.5%. As such not more than the danger of dying from colorectal cancer as a result of eating cured and smoked products.
So my advice is that eating fresh red meat is fine. I'd suggest you could eat from 1-6 portions a week. I really don't think it has many negative health consequences and provides a great source of iron and zinc as well as protein. Eating processed red meat that has been cured or smoked, is likely to cause some damage to health, but the amount of damage, I would argue, doesn't preclude its consumption in moderation. I would suggest limiting intake of these products to a maximum of 3 times per week.
I'd put it like this: If you were in a room with 99 other people from the UK, possibly 1 of you would die early from eating smoked and cured foods. The chances are that you will get away with eating processed meat on a daily basis, but it all depends on your atitude to risk!