Meat and fish

There is strong and consistent evidence that eating red and processed meat increases bowel cancer risk, but more evidence is needed to clarify whether eating fish (and perhaps which types of fish) reduces bowel cancer risk. The World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) Colorectal Cancer Report 2010 concluded that “The evidence that red meat [and] processed meat… are causes of colorectal cancer is convincing” but that “Evidence for foods containing fish is less consistent and no conclusion could be drawn”.

At least four meta-analyses have shown a 17-30% increased risk of bowel cancer in relation to 100-120g/day of red meat and a 9-50% increased risk of bowel cancer in relation to 25-50g/day of processed meat. A pooled analysis of UK case-control studies found no effect of 50g/day red or processed meat, but the number of participants was relatively small (less than 600 bowel cancer patients) and the amount of red/processed meat they consumed was relatively low. In most studies, red meat is defined as beef, veal, pork, mutton and lamb (fresh or frozen); and processed meat is defined as meat preserved in any way other then freezing, including ham, bacon, sausages, pate and tinned meat.

In 2011, it was estimated that around 21% of bowel cancers in the UK in 2010 were linked to consumption of red and processed meat. Red meats contain haem iron, and there is limited evidence that iron consumption is associated with higher bowel cancer risk.

A 2007 meta-analysis found a 3% decrease in bowel cancer incidence for every 100g fish eaten per week, but this effect was not statistically significant. A meta-analysis two years later, and more recent large cohort studies in Japan and America, have found no association between fish consumption and bowel cancer incidence. However, there may be a modest risk reduction for rectal cancer, according to a more recent meta-analysis.

Sugar

It remains unclear whether dietary sugar intake is associated with bowel cancer risk. The WCRF/AICR 2010 Report concluded that “Evidence suggesting that consumption of foods containing sugar is a cause of colorectal cancer is limited”.

Two meta-analyses have explored dietary sugar intake measured as glycaemic index or glycaemic load and colon cancer risk. These analyses were conducted within a year of one another and with almost identical groups of source studies, however the first found an 18-26% risk increase for the highest versus lowest sugar intake categories, whilst the second found no association between sugar intake and colon cancer risk. The first analysis adjusted for study design whilst the second did not, and the second included a large cohort study which observed no association between sugar intake and colon cancer risk. A more recent pooled analysis which explored the effect of sugar-sweetened carbonated soft drinks found no effect on colon cancer incidence.

Fruit and vegetables

The WCRF/AICR 2010 Report concluded that “The evidence that non-starchy vegetables [and] fruits ... protect against this cancer is limited”, because though the evidence base is substantial, findings are inconsistent.

A 2011 meta analysis, pooling data from over 1.5 million participants, found that colon cancer risk was reduced by 2% for every 100g/day of vegetables consumed. This analysis also found that increasing fruit or vegetable intake from very low levels up to about 100-200g/day served to reduce bowel cancer risk by about 10%, but there was little further reduction in risk with higher intakes. Most studies in this analysis adjusted for other lifestyle factors relevant to risk of bowel cancer and associated with levels of fruit and vegetable intake (e.g. physical activity, smoking, bodyweight, alcohol intake and red and processed meat intake), and there were no significant differences in findings between studies which did and did not adjust for these factors.

However, a 2003 review by the International Agency for Research on Cancer (IARC) concluded that the risk reduction from fruit and vegetable consumption was so modest that confounding could not be ruled out as an explanation for the observed association.

The WCRF/AICR 2010 Report concluded that “Consumption of garlic… probably protects against this cancer”. Epidemiological evidence for a protective effect of garlic is limited but some large trials and studies with animals point to a benefit. A 2009 systematic review concluded that there is “very limited credible evidence for a relation between garlic consumption and reduced colon cancer risk” as most epidemiological studies have found no significant effects; however a 2007 systematic review concluded that garlic probably protects against bowel cancer, based on a randomised controlled trial of garlic extract in bowel cancer patients, and studies with animals. Pooled epidemiological evidence shows that eating 5-6 cloves of raw or cooked garlic per week reduces the risk of bowel cancer by around 30%, though this analysis excluded the largest study that showed no association, which means the results may not be reliable. Most studies have looked at the effect of dietary garlic rather than garlic supplements.