2.5.1 Alcohol
The WCRF/AICR 2011 report concluded that there was convincing evidence that alcoholic drinks increased the risk of CRC in men and probable evidence of the same association in women (WCRF/AICR, 2011); this was confirmed in the latest CUP report where it was concluded that consumption of over 30 grams of alcohol per day was a convincing cause of CRC (WCRF/AICR, 2017). In a meta-analysis of sixteen prospective cohort studies on the relationship between alcohol intake and CRC cancer, including over 6300 patients with CRC, a weekly intake of 100g alcohol was associated with a 15% increased risk, with no significant differences for colon and rectal cancer (Moskal et al., 2007). In another pooled analysis of primary data from 8 cohort studies in 5 countries from North America and Europe, alcohol intake of approximately 490 000 participants was assessed at baseline using a FFQ and followed up a minimum of 6 and maximum of 16 years. An increased risk of both colon and rectal cancer was associated only with consumption of over 2 drinks/day (Cho et al., 2004). This was confirmed by results of a dose-response meta-analysis published in 2011, summarising the evidence from 27 cohort and 34 case-control studies that provided strong evidence for an association between drinking over 1 alcoholic drink / day and CRC risk, with stronger RRs reported for men and in Asian populations when compared to non-/occasional drinkers (Fedirko et al., 2011). In 2010, 11.6% of all CRC cases in the UK were
attributed to alcohol consumption: 15.5% of all male cases and 6.9% of all female cases (Parkin et al., 2011). Thus, notwithstanding the fact that epidemiological evidence supports positive associations between alcohol consumption and CRC risk, findings with respect to sex, the dose-response association and geographical region warrant further investigation. Nevertheless, from the evidence to date, it is sensible to
recommend that people decrease their alcohol intake, especially if their current level is high, to prevent CRC.
2.5.2 BMI and abdominal fatness
Overweight and obesity are risk factors for CRC; the WCRF/AICR classified body fatness, as marked by BMI, waist circumference and waist: hip ratio as being
review of prospective studies including over 9 million people, the RR of CRC incidence for obese individuals vs. those in the normal category of BMI was 1.33 (95% CI = 1.25, 1.42), whilst the RR for individuals in the highest vs. the lowest category for waist circumference (WC) was 1.46 (95% CI = 1.33, 1.60) (Ma et al., 2013). Thus both general and central obesity were positively associated with risk of CRC. When the studies were stratified by anatomical site, it was evident that a higher BMI and a higher WC
increased the risk of both proximal and distal colon cancer, as well as of rectal cancer (Ma et al., 2013). The association for BMI was stronger for men than for women, with a 47% increased risk in obese vs. normal men, to a 15% increased risk in obese vs. normal women (Ma et al., 2013).
Notwithstanding, Robsahm and colleagues reported a more pronounced association for the distal colon with BMI, with a RR of 1.59 (95% CI = 1.34, 1.89) when compared to the proximal colon and rectum, with a RR of 1.24 (95% CI = 1.08, 1.42) and 1.23 (95% CI = 1.02, 1.48) respectively (Robsahm et al., 2013). They however reported such differences as being minor and added that it is unlikely that the biological mechanisms in place vary in their impact on the different colorectal sites.
A quantitative analysis from 56 observational studies including almost 94 000 cases showed the association of BMI with CRC is stronger in premenopausal women when compared to postmenopausal women. Even women with a BMI in the ‘normal’ range of 23.0 to 24.9 kg m-2 had an increased risk of CRC compared to women with a BMI of
< 23.0 kg m-2 (Ning et al., 2010). In a systematic review and meta-analysis of
observational studies looking at adult weight gain and occurrence and recurrence of colorectal adenomas, even a small amount of weight gain was associated with a higher adenoma occurrence (Schlesinger et al., 2017). The authors argued that in view of the fact that adenomas are precursors of most carcinomas, weight control in adulthood may have a role in the early CRC prevention. Although based on the above studies, obesity, in particular visceral adiposity appears to play a role in CRC, the mechanisms by which obesity increases risk of CRC are still not well understood. The several possibilities that have been hypothesised are discussed in section 2.6.
2.5.3 Physical activity
In the WCRF/AICR 2011 report, the evidence for physical activity reducing the risk of CRC was listed as convincing (WCRF/AICR, 2011). Following the report, two meta- analyses were published supporting the role of physical activity in decreasing both proximal and distal colon cancer (Boyle et al., 2012; Robsahm et al., 2013), but not in decreasing rectal cancer (Robsahm et al., 2013). An approximate 33% decreased risk of colon cancer was reported by Robsahm and colleagues for those with the highest level of physical activity when compared to the least physically active. The magnitude of the inverse association was the same for both distal and proximal colon cancer with physical activity (Robsahm et al., 2013). This difference in association by anatomical site could be indicative of different mechanisms in the development of colon and rectal cancer. The 2017 CUP confirmed the findings of the previous 2011 CUP stating there was convincing evidence to show that physical activity reduced the risk of colon cancer, but no conclusion could be drawn on rectal cancer (WCRF/AICR, 2017).
The risk reduction in CRC as a result of physical activity could be due to several mechanisms. Firstly, there is evidence to show that the risk of adenomas decreases with physical activity, with an approximate 16% decrease risk (RR=0.84, 95% CI = 0.77, 0.92) reported, and a similar inverse association in both sexes (Wolin et al., 2011). Adenomas could progress into cancerous tumours, as outlined in section 2.2.1.
Physical activity leads to more regular bowel movements, thus decreasing transit time and the contact time of harmful substances in undigested food with the intestinal lumen. Furthermore, it also reduces the levels of insulin, hormones and other growth factors that could stimulate tumour growth, and potentially alters the level of
prostaglandins thus reducing inflammation l