Red-meat-free diet not proven to reduce overall bowel cancer risk

Tuesday April 3 2018

"Cutting out red meat significantly reduces people's risk of bowel cancer, study finds," is the somewhat misleading headline from the Mail Online.

The news website was reporting on a new UK study that aimed to assess whether different diets are associated with cancers of the colon and rectum (bowel cancer) in women.

Bowel cancer (colorectal cancer) is the second most common cancer in women worldwide. Previous studies have linked red meat consumption with a higher risk of developing bowel cancer. The International Agency for Research on Cancer has classified red meat as "probably carcinogenic [able to cause to cancer] to humans".

However, it's less clear whether vegetarian and low-meat diets are associated with a lower risk of developing bowel cancer.

Despite the Mail Online's headline, this study did not find that a diet free of red meat "significantly reduces people's risk of bowel cancer". An association was only found for distal colon cancer – where cancer develops in the last section of the bowel – and the number of women getting this type of cancer was small, meaning it could have been a chance finding.

However, current UK guidelines on red meat have not changed: it's recommended that people eat no more than 70g of red or processed meat a day. That's roughly equivalent to 1 lamb chop or 3 slices of ham.

Where did the story come from?

The study was led by researchers from the University of Leeds and funded by the World Cancer Research Fund (WCRF). The authors also received individual funding from several institutions.

The study was published in the peer-reviewed International Journal of Cancer on an open-access basis and is free to read online.

The Mail Online's headline was inaccurate, as the researchers made clear there was no statistically significant link between a red-meat-free diet and lower risk of overall bowel cancer. However, the actual report was a fair representation of the study, as it made it clear the link only applied to distal colon cancer.

What kind of research was this?

This was an analysis of a large cohort of UK women to assess whether different diets were associated with bowel cancer.

Large observational studies that follow people through time, like this one did, are very useful when studying the link between a possible exposure and outcome.

However, people chose what diet they ate rather than being randomly assigned to groups, and the study design didn't allow for the complete exclusion of other potentially influential factors such as physical activity, smoking or alcohol.

What did the research involve?

The UK Women's Cohort Study recruited women using a WCRF direct-mail survey between 1995 and 1998. A total of 35,372 women, aged 35 to 69, returned a questionnaire that briefly asked about dietary preferences. This allowed researchers to identify non-red-meat eaters.

The women were then asked to complete a longer, 217-item, self-administered food frequency questionnaire that indicated how often different types of food were consumed over the previous 12 months.

Using this information, 4 commonly reported eating patterns were identified and categorised:

  • Red meat eaters – consumed red meat at least once a week and sometimes also poultry or fish
  • Poultry eaters – consumed poultry at least once a week and sometimes also fish, but not red meat
  • Fish eaters – consumed fish at least once a week, but no meat
  • "Vegetarians" – consumed red meat, poultry or fish less than once a week

Red meat was defined as beef, pork, lamb, offal and processed meats.

A subsequent bowel cancer diagnosis was confirmed via linkage to NHS Digital medical records.

What were the basic results?

After excluding women with incomplete data at the start of the study and those with a previous history of cancer, 32,147 participants were included in the final analysis. Of these:

  • 65% (20,848) were classified as red meat eaters
  • 19% (6,259) as vegetarians
  • 13% (4,141) as fish eaters
  • 3% (899) as poultry eaters

At follow-up, 462 individuals had been diagnosed with bowel cancer.

There was no statistically significant difference in risk of overall bowel cancers when comparing grouped diets free of red meat with diets containing red meat (hazard ratio [HR] 0.86, 95% confidence interval [CI] 0.66 to 1.12). Nor was there any difference in risk when looking specifically at either cancer of the colon (HR 0.77, 95% CI 0.56 to 1.05) or cancer of the rectum (HR 1.04, 95% CI 0.66 to 1.63).

Further analysis suggested there may be a reduced risk of cancer of the last section of the bowel (distal colon cancer) for grouped diets free of red meat (HR 0.56, 95% CI 0.34 to 0.95). However, this was based on only 119 people who developed distal bowel cancer, 101 of whom ate red meat and 18 of whom did not.

How did the researchers interpret the results?

The researchers said: "In summary, grouped and independently analysed red-meat-free diets showed a non-significantly decreased risk of [colorectal cancer] compared to red meat eaters. Only exploratory subsite analysis showed a significant risk reduction for distal colon cancer in red-meat-free dietary patterns.

"These results indicate that protective associations of red-meat-free diets on colorectal cancers merit further investigation in a larger study with larger numbers of cases."

Conclusion

This is a valuable study that analysed a large cohort of UK women to see if not eating red meat might lower the risk of bowel cancer, given the previously suggested link with red meat consumption.

However, there was no overall difference in the chances of developing bowel cancer when comparing people who ate red meat with those who didn't.

And while they did find a reduced risk of distal colon cancer, this was based on just 119 people, increasing the possibility it could have been a chance finding.

Even though there was no clear evidence that red-meat-free diets lowered the risk of bowel cancer, that doesn't mean all the previous research was wrong and that red meat is not linked with cancer risk.

We can't be sure whether the women stuck strictly to the type of diet they were grouped into or what quantity of meat the meat eaters ate.

The fact that the red meat category included processed meat also complicates things because processed meat has been established as even more likely to be a possible carcinogen than red meat.

The researchers adjusted their analysis to take into account any potential differences in levels of physical activity, body mass index (BMI), smoking, family history of cancer and socioeconomic status, which may have influenced the results. However, they may not have fully accounted for these confounding factors.

They also didn't take into account other health and lifestyle factors that may have differed between the groups and affected bowel cancer risk, such as alcohol consumption.

Finally, the participants were all women, and they were also healthier than the general population, with a lower average BMI and lower smoking rates. It's therefore difficult to know whether the findings are applicable to the UK population as a whole.

Our advice would be to stick to the current UK guidelines on red and processed meat consumption: try to eat no more than 70g a day, or 490g over the course of a week, and have several meat-free days a week.

Analysis by Bazian
Edited by NHS Choices