Yesterday, the headlines screamed out from around the world that a new study from Oxford University found that there is a 20% higher chance of developing colorectal cancer from eating as little as one strip of bacon per day. This sounds terrible, but is it true? To understand what this study is reporting, it is important to understand the difference between relative risk and absolute risk.
It was known before this study, that regardless what people eat, there is approximately a 5% chance of developing colorectal cancer in a person’s lifetime (whether they eat bacon every day or not). This is known as the absolute risk.
A 5% likelihood means that for every 100 people, 5 will get colorectal cancer regardless what they eat.
Illustrated, this looks as follows;
The study reported that there is a 20% higher chance of developing colorectal cancer by eating as little as one strip of bacon per day.
This means that compared to not eating bacon daily, eating it daily results in one more person per 100 people developing colorectal cancer in their lifetime.
This is known as relative risk and illustrated that looks as follows:
Headlines as click-bate
It wasn’t only the American media that reported this, Canadian new outlet CTV did also.
Each individual person’s increased risk of getting colorectal cancer by eating as little as 1 strip of bacon per day is NOT 20%! Their increased absolute risk of getting colorectal cancer (*based on this study) is 0.08%.
*this study was an epidemiological study, not a clinical study and can only show if there is an association between two factors and cannot make any conclusions about cause. The difference is explained below.
The study found that for every 10,000 people who ate 21g a day of red and processed meat, 40 were diagnosed with colorectal (bowel) cancer, and a single slice (or rasher) of bacon is ~23g.
i.e. 40 / 10,000 = 0.4%
The study also found that for every 10,000 people who ate 76g a day of red and processed meat, 48 were diagnosed with colorectal (bowel) cancer.
i.e. 48/10,000 = 0.48%
The actual chance of a person getting colorectal cancer (i.e. absolute risk) from eating bacon daily is the difference between these two numbers; i.e. 0.4% – 0.48% = 0.08%
Association is not Causation
This was an epidemiological study based on population data, and was not a clinical study.
Epidemiological studies are the study of diseases in populations and are helpful for researchers to know which areas warrant clinical studies.
It is important to know that epidemiological studies cannot attribute “cause” of disease or death. When an epidemiological study finds an “association” between two factors such as bacon and higher colon cancer rates — this does NOT mean that eating bacon ’causes’ heart disease.
Based on this study, all that can be said is that there was an increase in the association between eating bacon and absolute rates of colon cancer of 0.08%.
Not so impressive now, is it?
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Kathryn E Bradbury, Neil Murphy, Timothy J Key, Diet and colorectal cancer in UK Biobank: a prospective study, International Journal of Epidemiology, , dyz064, https://doi.org/10.1093/ije/dyz064