Friday January 13 2012
The link between meat and pancreatic cancer is not certain
BBC News has reported a possible link between eating processed meat, such as bacon or sausages, and pancreatic cancer. Apparently, consuming only an extra 50g a day – about one sausage – can raise your risk of this rare cancer by 19%.
The news is based on research that combined results from 11 earlier studies that included over 2 million people. It did not find a link between red meat consumption and cancer overall, but suggested that eating an extra 50g of processed meat a day increased the risk of pancreatic cancer by 19%. It is important to note that pancreatic cancer is rare, so even a 19% increase in risk would mean that a person’s lifetime risk of pancreatic cancer would still be very small.
While the review was well conducted, dietary research has inherent limitations. For example, it must rely on people to estimate what they eat, and researchers must account for the influence of other factors such as physical activity, smoking and drinking.
The causes of pancreatic cancer are not fully understood, although it is speculated that age, smoking, certain medical conditions such as diabetes, family history of cancer, excess weight and dietary factors and could all be involved. This study, which focused on meat intake, suggests that processed meats could be involved, although it cannot conclusively prove that this is the case. That said, there is evidence that red meat consumption can contribute to other types of cancer, particularly bowel cancer, so the public has been advised to limit daily intake of red and processed meat.
Where did the story come from?
This study was carried out by researchers from the Karolinska Institute in Sweden. It was funded by the Swedish Cancer Foundation and through a Research Fellow grant from the Karolinska Institute. The study was published in the peer-reviewed British Journal of Cancer.
Though BBC News did not discuss the wider limitations of this research, its reporting was generally accurate. Importantly, it reported that the association is only “suggested” rather than definite, and clarified that a person’s overall (or absolute) risk of pancreatic cancer is low. The Daily Mirror did not comment on the absolute risk but did say that other risk factors, such as smoking, are likely to have a greater effect. The Daily Express did not mention these important points.
What kind of research was this?
This systematic review and meta-analysis aimed to combine all available studies that investigated whether consumption of red and processed meat is associated with the risk of pancreatic cancer.
A systematic review searches for all available literature on a topic, and is the best way of identifying and analysing all relevant observational studies that assess the link between an exposure and an outcome. In this case, the exposure was a particular food and the outcome was cancer. Such reviews have inherent limitations as they often have to combine the results of studies that have very different methods, follow-up and observation of outcomes.
In particular, there is room for inaccuracy when combining observational studies that report on food consumption. Participants may have difficulty accurately estimating their intake of certain foods, their consumption of those foods may not remain the same over time. It may also be difficult to verify that the pattern of food consumption predated the outcome of interest, such as the development of cancer. The individual studies would also need to ensure that they accounted for other potential factors (confounding factors) that could affect both diet and cancer risk, such as physical activity, smoking and alcohol. This means it can be difficult to prove causation.
What did the research involve?
The researchers looked through scientific and medical databases for studies published up to November 2011, in addition to carrying out a manual search through the reference lists of the identified studies. They looked for prospective cohort or case-control studies that included the terms “meat” or “foods” and “pancreatic cancer” or “pancreatic neoplasm”. To be included in the meta-analysis that combined the results of the studies, the studies must have looked at the outcomes of pancreatic cancer incidence (new cases) or mortality, and have calculated the risk of this outcome in relation to the subjects’ consumption of red and/or processed meat. The researchers used standard statistical methods to combine the results of the studies in the meta-analysis.
What were the basic results?
The search identified 13 prospective studies, 11 of which had data suitable for combining in the meta-analysis. Of these 11 studies, six were carried out in the United States, four in Europe, and one in Japan. Men and women were included in six studies, while three included only women, and two only men. The sample sizes ranged from 17,633 to 1,102,308, and the number of pancreatic cancer cases that occurred in the studies varied from 57 to 3,751. Altogether, the studies involved 2,307,787 participants and 6,643 pancreatic cancer cases.
All 11 studies had examined the link between red meat consumption and pancreatic cancer and found no overall significant association between the two. However, when they separated their analyses for men and women, the researchers found a significant association between red meat consumption and pancreatic cancer in men (relative risk [RR] 1.29, 95% confidence interval [CI] 1.08 to 1.53; five studies). No significant association was found in women (RR 0.93, 95% CI 0.74 to 1.16; six studies). For these calculations, there were no significant differences (heterogeneity) between study results (significant heterogeneity would mean that we would have less confidence that the individual studies were suitable for combining in a meta-analysis).
Seven studies had examined the link between consumption of processed meat and pancreatic cancer and found that, overall, an increase in processed meat consumption of 50g a day was associated with a 19% increased risk of cancer (RR 1.19, 95% CI 1.04 to 1.36; no significant heterogeneity between the study results). This time, however, when they performed separate analyses for men (three studies) and women (four studies), the researchers found no significant association between processed meat consumption and cancer for either sex.
How did the researchers interpret the results?
The researchers concluded that processed meat consumption is linked to pancreatic cancer risk, while red meat consumption was associated with an increased risk of pancreatic cancer in men only. They say that further prospective studies are needed to confirm these findings.
This systematic review combined the results of 11 studies that looked at associations between red and processed meat consumption and the risk of pancreatic cancer. The study has strength in its large size (featuring over 2 million participants) and the fact that all studies assessed the participants’ food consumption and looked at whether this was linked to later development of cancer. Systematic reviews often have inherent limitations because they must combine studies that may have very different methods, follow-up and observation of outcomes. However, in this case, we can have some confidence in that the individual studies generally gave similar types of results.
However, there are some important points to note when drawing any conclusions from this study:
- The individual studies variably accounted for other potential confounders that could affect both diet and cancer risk. For example, most studies adjusted for age, sex and smoking, but others were variable in their adjustment for factors such as physical activity, alcohol, educational level, BMI and other dietary factors. This means it can be difficult to prove a causative link and to say that this specific dietary factor increases cancer risk.
- With any study that relies on reporting of food consumption, there is likely to be some inaccuracy when an individual estimates their intake of certain foods, and their consumption may not remain the same over time.
- The individual studies had reported meat consumption using different methods. For example, some studies reported combined intake of red meat and processed meat as a single measure, some reported the two separately, and others specified intake of pork, beef, lamb, ham, sausages etc. Therefore, combining the studies and looking at “red meat” or “processed meat” is likely to include different types of meat in these categories depending on the study.
- There was no overall association between red meat and pancreatic cancer, and the researchers only found a risk for men. There was an overall association between processed meat and cancer, but no separate risk for men and women. Therefore, interpreting whether there is a true risk – and with what food in particular – is difficult.
- Though a 19% increased risk of pancreatic cancer was associated with processed meat, this is only a “relative risk increase”. It suggests that people who consume high levels of processed meat are 19% more likely to get pancreatic cancer than if they did not eat it. Any individual’s lifetime risk of pancreatic cancer (their absolute risk) is likely to be small. Therefore, even if the figure of 19% is accurate, it is a 19% increase on a small risk of this cancer.
The causes of pancreatic cancer are not firmly established, but factors considered to increase the risk including age, smoking, certain medical conditions such as diabetes, family history of cancer, excess weight and obesity, and – as this study has investigated – potential dietary factors. The reason why any individual develops cancer is always hard to say. For pancreatic cancer, it may be due to a combination of these factors, or none at all. Although the review was well conducted, it cannot say that by cutting out red or processed meat, you will definitely reduce your risk of pancreatic cancer. Further prospective studies are needed to investigate the association.
Although the link between processed meat and pancreatic cancer is not conclusive, eating high levels of red and processed meat has other health implications. These foods are often high in saturated fat and salt. Due to their calorific content and a suspected link to bowel cancer, the Department of Health recommends that people who eat more than 90 grams of red or processed meat a day should reduce their intake to 70 grams a day.