Tuesday December 11 2007
Red meat: delicious but dangerous?
“Red meat 'can raise the risk of cancer by 25 per cent'”, according to the headline in the Daily Mail. It adds that “one in 10 cases of both lung and bowel cancer could be prevented if people cut down on beef, lamb, pork, sausages, ham and bacon”.
The newspaper reports are based on a study that looked at the relationship between diet and risk of cancer in almost 500,000 retired Americans. The relationship between what we eat, and the risk of different types of cancer is complex. This study found that increased consumption of red or processed meat is associated with an increased risk of bowel and lung cancer. This study echoes the recent report by the World Cancer Research Fund that recommended that people limit their intake of red meat and avoid processed meat.
Where did the story come from?
Dr Amanda Cross and colleagues from the National Cancer Institute, and the AARP (American Association of Retired Persons) carried out this research. The study was funded in part by the National Institutes of Health and the National Cancer Institute; data on cancer incidence was collected by centres in the individual states. It was published in the peer-reviewed medical journal: PLoS Medicine.
What kind of scientific study was this?
This study was part of a prospective cohort study – the National Institutes of Health-AARP Diet and Health study – looking at the effects of diet on mortality from 1995 to 2005. The researchers enrolled more than 500,000 people aged 50–71 who were members of AARP. People completed a questionnaire about themselves at enrolment, recording any health related problems they had. Anyone who did not return their questionnaires, who had other people fill in their questionnaires, who already had cancer, or end-stage kidney disease, or reported a very high or very low energy intake in their diet were excluded from these analyses. This left 494,036 people for analysis in this study.
The study participants answered a questionnaire about their diets (the Diet History Questionnaire), and gave information about what foods they ate, how much of these foods they ate, and how often. Based on their replies, the researchers ranked people according to how much red and processed meat they ate. The red meat category included all types of beef, lamb, and pork (including processed forms of these meats and meats included in dishes such as stews). The processed meats category included bacon, any sausages and hot dogs (including those made from poultry), luncheon meats, ham, and “cold cuts” (red and white meat). Results were adjusted to take into account the fact that people eat different total amounts of food.
The researchers followed up these people over 10 years, and identified those who developed cancer using state cancer registries. From national registries they found out whether they had died, and from what cause They then compared the rates of different types of cancer in people whose consumption of red and processed meats was in the highest 20 per cent, with that in people whose consumption of red and processed meats was in the lowest 20 per cent. In their analyses, researchers allowed for factors that might affect the results – including family history of cancer, smoking, age, gender, race, education, body mass index, physical activity, alcohol consumption, and fruit and vegetable consumption.
What were the results of the study?
There were 53,396 new cases of cancer during an average of nearly seven years of follow up. People who ate the most red meat (those in the top 20 per cent of consumption) were at significantly greater risk of developing cancer of the oesophagus (cancer of the gullet), bowel, liver or lung compared with people who ate the least. There was also a trend towards an increased risk of laryngeal cancer with higher red meat consumption, but this difference was not statistically significant.
Men, but not women, who ate the most red meat were at increased risk of pancreatic cancer. Higher red meat consumption was associated with a reduced risk of endometrial cancer (cancer of the lining of the womb). There was no association between red meat consumption and the rates of stomach, bladder, breast, ovarian, or prostate cancers, or leukaemia, lymphoma or melanoma.
People who ate the greatest amount of processed meat were at significantly greater risk of developing cancer of the bowel or lung. Men who ate the highest amounts of processed meat were at increased risk of pancreatic cancer, but not women. There was also a trend towards an increased risk of bladder cancer and myeloma with higher processed meat consumption, but these differences were small and not statistically significant. Higher processed meat consumption was associated with a reduced risk of leukaemia and melanoma. There was no association between processed meat consumption and the rates of stomach, liver, laryngeal, breast, ovarian, or prostate cancers, or lymphoma. These results were not altered by adjusting for smoking.
What interpretations did the researchers draw from these results?
Researchers concluded that consumption of red or processed meats is associated with an increased risk of lung and bowel cancer. Consumption of red meat was also associated with an increased risk of oesophageal and liver cancer. They suggest, “A decrease in the consumption of red and processed meat could reduce the incidence of cancer at multiple sites.”
What does the NHS Knowledge Service make of this study?
This was a large study, which does gain reliability by the prospective way in which the data was collected. However, there are a few points to bear in mind when interpreting this study:
- As with all studies of this kind, it is difficult to draw definite conclusions about whether what is being studied (in this case eating red and processed meat) definitely causes or prevents the outcome seen (cancer in this case). This is because it is impossible to eliminate the possibility that other factors that were not investigated could also be responsible. This is particularly the case when there is no obvious biological reason why an exposure might cause an outcome, for example, it is not clear how consumption of red and processed meat might cause lung cancer or prevent endometrial cancer. In the case of lung cancer, the authors acknowledge that although they attempted to control for smoking in their analyses, they may not have fully removed its effect on the results. In addition, people who eat diets high in red or processed meats may also have other dietary habits that may affect their risk of cancer, such as a high fat intake or low intake of fibre.
- People’s diets were assessed by questionnaire when they enrolled. Although the researchers tried to make sure people remembered their food intake correctly by checking their answers against two 24-hour food diaries, people may still have had inaccurate recall of what they ate. In addition, people’s diets may have changed over the follow up period, which could also affect results.
- This study only included relatively healthy people, who did not have a history of cancer or kidney disease, and the majority of whom were white. These results may therefore not be representative of the potential effects of a diet high in red or processed meats in people from different ethnic backgrounds, or people who are less healthy.
The relationship between what we eat, and risk of different types of cancer is complex. However, this study does add to the evidence suggesting that eating less red and processed meat may be better for us.
Sir Muir Gray adds...
As societies become richer they consume more and more meat, then the need for a balanced diet is recognised and the trend is back to fish or vegetable protein.