The last time I talked to someone about what I study, they asked me if meat really causes cancer. Unfortunately, the science is clear: meat consumption is a cancer risk factor – just ask IARC. The International Agency for Research on Cancer is considered the authority on cancer risk factors. IARC regularly evaluates and updates their monographs on cancer risk factors - long reports written by scientists that come together at the IARC headquarters in Lyon, France with expertise in various areas surrounding the risk factor or chemical in question.
In 2015, IARC determine that red meat consumption is a likely risk factor for cancer based on several different animal studies, and epidemiologic studies conducted in large populations. Processed meat was classified as a definite carcinogen based on the strict criteria that IARC uses to group carcinogens (things that can cause cancer).
A few months ago, I came across this news article, which only served to confirm the findings of previous studies on meat consumption. This study comes from a large, ongoing investigation in the UK (1).
What Were They Studying?
The researchers were interested in investigating the relationship between diet and colorectal cancer. Colorectal cancer is strongly linked to different, modifiable lifestyle behaviours like smoking, alcohol consumption, physical activity, and diet. There have been several studies previously done that have assessed the role of diet on colorectal cancer risk. These studies are called meta-analyses, and they combine the results from individual studies to come to conclusions. These meta-analyses have consistently pointed to an increased risk of colorectal cancer from meat consumption. The researchers pointed out that previous studies on this topic have typically been conducted in the 1990s, and that eating patterns have changed in the UK since, emphasizing the need for an updated investigation.
How Did They Study It?
Researchers used a large, ongoing study called the UK Biobank. This study has used a mix of questionnaire data and biological samples to collect information on individuals in the UK over several years. Using this information, the researchers were able to study relationships between meat consumption and colorectal cancer. A key aspect of this study is that it was prospective, meaning that individuals did not know their colorectal cancer diagnosis when they were filling out dietary information. The investigators used self-reported measures of diet. This is commonly done in diet studies as it is difficult to monitor food consumption details in large groups overtime.
What Did They Find?
Researchers reported hazard ratios in their paper, which are a common method of determining a relative risk (see my post “What are the odds?”). The word relative is used because the risk in the exposed group is typically compared to an unexposed or “control” group. The study reported a 20% increased risk for colorectal cancer among those consuming red and processed meat 4 or more times a week compared to those consuming red meat less than two times a week.
Interestingly, no increased risks were found based on consumption of other animal products such as poultry, fish, cheese, and dairy milk.
How Can We Interpret These Results?
The relative risk of 20% can be interpreted to mean that those that regularly consume red and processed meats 4 or more times a week have a 20% increased risk of colorectal cancer compared to those that consume these meats less than two times a week. It is important to consider that the relative risk can only be compared to the referent group, which in this case was composed of people that may still eat red or processed meat occasionally. However, a similar increase in risk has been found when comparing vegetarians to meat eaters, which suggests no increased risks from occasional consumption of red meat (less than two times a week) (2,3).
Increases in risk were similar for red meat and processed meats when consumption was evaluated separately, and although those results were not statistically significant, they are consistent with the overall results presented.
In a supplemental table, the authors reported that although the relative risk for red and processed meat consumption four or more times a day compared to less than two was 1.39% in men, there was no increased risk found in women. Although this study did not find an increased risk when assessing women in the study specifically, several previous studies have pointed to smaller, but consistent increases in risk in women (4). Based on this study, it appears that the relationship between red and processed meat consumption and colorectal cancer risk is not the same in men and women. Future studies should better highlight and address these differences.
Based on these different results for men and women, the overall message from The Guardian article was somewhat misleading, as it did not discuss this sex-specific difference.
Were There Any Issues With Their Analysis or Interpretation?
The study appears to have been well-conducted. Dietary assessments are always associated with some measurement issues as they typically rely on self-reported measures. It is possible that individuals under-reported their red meat and processed meat consumption. However, because participants could not have known if they would develop colorectal cancer in the future, their dietary responses could not have been affected by that knowledge. The study even took into account reported changes in individual dietary patterns overtime.
The researchers could have applied better exclusion criteria. Exclusion criteria are used in epidemiology studies to create a more uniform study base. In this case, the researchers did not exclude individuals with conditions such as inflammatory bowel disease. This means that some of the colorectal cancer cases that were diagnosed in the study could have been due to inflammatory bowel disease, and not red or processed meat consumption. This could have had a small impact on the exact risk estimates presented, but is unlikely to change the overall results.
Final Thoughts
This study adds to our knowledge about the relationship between red and processed meat consumption and colorectal cancer risk due to the different consumption levels that were studied. Although an increased risk for colorectal cancer in women was not found in the paper, we know from previous studies that red and processed meat can also increase risk of colorectal cancer in women.
A different, Canadian study recently reported that 5.3%, or 1,193 colorectal cancer cases in Canada per year are due to red meat consumption, and 4.3% or 967 are due to processed meat consumption. These statistics allows for a direct interpretation of the impact of red and processed meat consumption on colorectal cancer in Canada.
Currently, the Canadian Cancer Society recommends limiting red meat consumption to less than three times a week, and avoiding processed meats whenever possible (5). Canadians currently consume about 3.2 servings of red meat per week, and 1.2 servings of processed meat per week (6). The UK study found a non-significant increase in risk in those that consume red or processed meats 2-3 times a week, which is consistent with this recommendation.
If Canadians reduced red meat consumption by one serving a week, we could prevent 820 cases of colorectal cancer in 2042. Reducing processed meat consumption by one serving per week could prevent 1,699 colorectal cancer cases in 2042. The year 2042 was chosen to represent a meaningful target year for policy interventions, and incorporates increasing trends in colorectal cancer diagnosis (6).
In epidemiology, several studies are used together to make informed decisions about cancer risk factors, instead of relying on just one study. In my opinion, the science is clear on this one!
References
1. Bradbury, K. E., Murphy, N., & Key, T. J. (2019). Diet and colorectal cancer in UK Biobank: a prospective study. International journal of epidemiology.
2. Orlich, M. J., Singh, P. N., Sabaté, J., Fan, J., Sveen, L., Bennett, H., ... & Herring, R. P. (2015). Vegetarian dietary patterns and the risk of colorectal cancers. JAMA internal medicine, 175(5), 767-776.
3. Sanjoaquin, M. A., Appleby, P. N., Thorogood, M., Mann, J. I., & Key, T. J. (2004). Nutrition, lifestyle and colorectal cancer incidence: a prospective investigation of 10 998 vegetarians and non-vegetarians in the United Kingdom. British journal of cancer, 90(1), 118.
4. Larsson, S. C., & Wolk, A. (2006). Meat consumption and risk of colorectal cancer: a meta‐analysis of prospective studies. International journal of cancer, 119(11), 2657-2664.
5. Canadian Cancer Society (2019). Why you should limit red meat and avoid processed meat-Prevention and Screening. Retrieved August 9th, from: https://www.cancer.ca/en/prevention-and-screening/reduce-cancer-risk/make-healthy-choices/eat-well/limit-red-meat-and-avoid-processed-meat/?region=bc.
6. Ruan, Y., Poirier, A. E., Hebert, L. A., Grevers, X., Walter, S. D., Villeneuve, P. J., ... & ComPARe Study Team. (2019). Estimates of the current and future burden of cancer attributable to red and processed meat consumption in Canada. Preventive medicine, 122, 31-39.