Colorectal (bowel) cancer
Colorectal cancer is the second most common cancer in England and the third most common cause of cancer death (after lung and prostate cancer in men, and lung and breast cancer in women). Between 1971 and 2009 the incidence of colorectal cancer increased by 33 per cent for men and 14 per cent for women. In 2009 there were 18,538 new cases for men and 15,066 for women. While the incidence of colorectal cancer has increased, mortality rates have halved for women between 1971 and 2010 and have decreased by 38 per cent for men during this time. In 2010, there were 15,708 deaths from bowel cancer in the UK: 8,574 (55 per cent) in men and 7,134 (45 per cent) in women (Office for National Statistics, 2012). Colorectal cancer occurs when the process of cell renewal in the bowel goes wrong. Abnormal cells can form polyps (small growths) which may develop into cancer. Risk factors for colorectal cancer include obesity, alcohol, smoking and poor diet.
A large body of evidence suggests a diet high in red and processed meat (such as smoked meat, ham, bacon, sausages, pâté and tinned meat) can increase the risk of colorectal cancer. In 2005, a large prospective study from the EPIC group investigated the role of diet in colorectal cancer. They followed 478,040 men and women from 10 European countries between 1992 and 1998. Information on diet and lifestyle was collected and after nearly five years, 1,329 cases of colorectal cancer were recorded. Results showed that colorectal cancer risk is linked to a high consumption of red and processed meat (Norat et al., 2005). Several mechanisms by which red and processed meat may cause colorectal cancer have been suggested. The type of iron (haem iron) found in meat, but not plant foods, may cause changes in cells that lead to cancer (Tapel et al., 2007). Other compounds found in red and processed meats called N-nitroso compounds, heterocyclic amines and polycyclic aromatic hydrocarbons may be responsible for the link with cancer (Lewin et al., 2006; Cross et al., 2007; Genkinger and Koushik, 2007).
In November 2007, The World Cancer Research Fund launched the report Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. It was the most comprehensive report to date ever published on the link between cancer and lifestyle (WCRF/AICR, 2007). Their recommendation to eat less red meat (such as beef, pork and lamb) and avoid processed meat became headline news on a global scale. In more detail, they said: To reduce your cancer risk, eat no more than 500 grams (cooked weight) per week of red meat, like beef, pork and lamb, and avoid processed meats such as ham, bacon, salami, hot dogs and some sausages. The report warned that eating 150 grams of processed meat a day (the equivalent of two sausages and three rashers of bacon) increases bowel cancer risk by 63 per cent and that 50 grams a day (one sausage) increases the risk by about 20 per cent. The evidence that processed meat is a cause of bowel cancer is so strong that the WCRF recommends that people should avoid eating it altogether. However, less than a third of people in Britain are aware that eating processed meat such as bacon and ham increases risk of cancer (WCRF, 2009). 10 per cent of bowel cancers cases in the UK could be prevented through reducing the amount of processed meat we eat. The Department of Health advises people who eat more than 90 grams (cooked weight) of red and processed meat per day to cut down on their intake (NHS Choices, 2012e).
While red and processed meat is linked to an increased risk of colorectal cancer, there is good evidence that a diet high in fibre and low in saturated fat can help reduce the risk (NHS Choices, 2012e). Several mechanisms by which fibre may offer a protective effect have been suggested: the formation of short-chain fatty acids from fermentation by colonic bacteria; the reduction of secondary bile acid production; the reduction in intestinal transit time and increase of faecal bulk; and a reduction in insulin resistance (Murphy et al., 2012).
The protective role of a whole grain plant-based diet containing plenty of fruit and vegetables (and therefore fibre) is well-documented. Two large-scale studies (both published in the Lancet) examined the relationship between diet and colorectal cancer; both confirmed that as dietary fibre intake increases, the risk of colorectal cancer decreases. In the first of these two studies, a research team from the National Cancer Institute in the US compared fibre intake of 3,591 people with at least one bowel adenoma or polyp (a benign growth that may or may not transform to cancer), with that of 33,971 people without polyps. They found that the participants in the top 20 per cent for dietary fibre intake had 27 per cent lower risk of adenoma than people in the lowest 20 per cent (representing a difference in fibre intake of 24 grams per day). It was concluded that dietary fibre, particularly from grains, cereals and fruits, was associated with a decreased risk of colorectal adenoma (Peters et al., 2003). In the second study, (the largest prospective study published at that time on fibre in colorectal cancer prevention) researchers from the EPIC group prospectively examined the association between dietary fibre intake and incidence of colorectal cancer in 519,978 individuals aged between 25 and 70 years-old, recruited from 10 different European countries. Participants completed a dietary questionnaire between 1992 and 1998 and were followed up for cancer incidence on average 4.5 years later. From this group, 1,065 cases of colorectal cancer were reported. Again, people with the highest fibre intake (35 grams per day) had a 40 per cent lower risk of colorectal cancer compared to those with the lowest intake (15 grams per day). They concluded that in populations with low average intake of dietary fibre, an approximate doubling of total fibre intake from foods could reduce the risk of colorectal cancer by 40 per cent (Bingham et al., 2003a). These studies provide convincing evidence that increasing the amount of whole grains and fruit and vegetables in the diet reduces the risk of colorectal cancer. A further EPIC report, in which an even larger number of cases (1,721 cases) were included, confirmed the original results showing an even stronger protective association between fibre intake in food and risk of colorectal cancer (Bingham et al., 2005). In the most recent EPIC study, 4,517 colorectal cancer cases were documented amongst the 477,312 participants (Murphy et al., 2012). After 11 years of follow-up, this analysis of EPIC data also confirmed the protective role of dietary fibre in colorectal cancer.
Not all studies report a positive effect of fibre; some have found that fibre has little or no effect on colorectal cancer risk (Pietinen et al., 1999; Fuchs et
al., 1999; Terry et al., 2001). It should be noted that these studies only looked at populations from single countries and may have looked at ranges of fibre that were too low. For example, Americans eat very little fibre on average. So a large study that focused on Americans would not be able to see the benefits of the high levels of fibre that, for example, an Italian person would eat (Cancer Research UK, 2009). Taken together, the WCRF and EPIC research (which looks at multiple countries) and numerous other studies (Jacobs et al., 1998; Peters et al., 2003; Nomura et al., 2007; Wakai et al., 2007) confirm the protective role of dietary fibre intake in colorectal cancer. These results strengthen the evidence for the recommendation of increasing the consumption of fibre rich foods for colorectal cancer prevention.
Studies looking at the links between dairy foods and colorectal cancer have produced mixed results. Some prospective studies have reported a lower colorectal cancer risk associated with dairy products and calcium. In 2004, a pooled analysis of 10 cohort studies from North America and Europe concluded that the consumption of dairy milk (but not other dairy foods) and calcium were related to a lower risk of colorectal cancer (Cho et al., 2004). The inverse association between calcium intake and colorectal cancer was only statistically significant among those with the highest vitamin D intake. This may be either because vitamin D enhances calcium absorption, or because vitamin D itself may decrease colorectal cancer risk (Garland, 1999). More recently, an updated meta-analysis from the WCRF Continuous Update Project also found that milk and total dairy products (but not cheese or other dairy products), are associated with a reduction in colorectal cancer risk (Aune et al., 2012).
The principal anti-carcinogenic component in cow’s milk and dairy products is believed to be calcium (Murphy et al., 2013). One study looking at dairy foods and calcium intakes in relation to cancer in the National Institutes of Health (NIH)-AARP (formerly known as the American Association of Retired Persons) Diet and Health Study found that during an average of seven years of follow-up, dairy food and calcium intakes were inversely associated with cancers of the digestive system. A decreased risk was particularly pronounced in colorectal cancer. Interestingly in this study, supplemental calcium intake was also inversely associated with colorectal cancer risk. They concluded that calcium intake is associated with a lower risk of total cancer and cancers of the digestive system, especially colorectal cancer (Park et al., 2009). A meta-analysis of 60 epidemiological studies including 26,335 colorectal cases also found that the risk reduction associated with calcium was similar for dietary and supplemental sources (Huncharek et al., 2009). So in these studies, it would appear to be the calcium rather than some unidentified component of dairy that lowered the risk.
However, the EPIC group found that their inverse associations were limited to dairy sources of calcium. They investigated intakes of milk (whole-fat, semi-skimmed and skimmed), yoghurt, cheese and dietary calcium with colorectal cancer risk amongst 477,122 men and women. During 11 years of follow-up, 4,513 incident cases of colorectal cancer occurred. Results showed that higher intakes of all dairy products and dietary calcium (from dairy sources only) were associated with a modest (seven per cent) reduction in colorectal cancer risk (Murphy et al., 2013). They suggest that a possible explanation for the lack of a protective effect of non-dairy calcium could be that plant sources of calcium (the main contributors to nondairy calcium intake amongst EPIC participants) contain oxalate and phytate (phytic acid) which inhibit calcium absorption. Furthermore, it should be noted that dietary calcium has been consistently associated with an increased risk of prostate cancer risk. Within EPIC, a 300 mg per day intake of dietary calcium was previously associated with a nine per cent increased risk of prostate cancer (Allen et al., 2008) and the WCRF/AICR 2007 report judged it a probable cause of the disease (WCRF/AICR, 2007). So it would seem clear that recommending dairy to men to lower their risk of colorectal cancer would not be a sensible option. Indeed, obtaining a good supply of calcium from non-oxalate vegetables and other plant-based foods (see below) is the healthier option for all people.
It has been suggested that the high-fat content of some dairy products may negate their protective effect against certain cancers. However the EPIC study found no difference in how high or low-fat dairy products affected colorectal cancer risk (Murphy et al., 2013). Other constituents of dairy products may contribute to the protective role observed. For example lactoferrin, vitamin D in fortified dairy products and certain fatty acids, such as butyric acid, have been linked with having possible beneficial roles against colorectal cancer (Murphy et al., 2013). Also it should be noted that in the EPIC cohort the lowest dairy consumers had the highest proportion of smokers and the highest dairy consumers were more physically active, had lower BMIs, had lower intakes of alcohol, higher intakes of fibre and had achieved a higher level of education. Because we have been led to believe that milk is a health food, it may be that people who use dairy may be doing other 'healthy' activities which could be masking the negative effects of their dairy consumption, even making it look positively healthy. More work is required to tease out these complex relationships.
As with breast cancer, there are growing concerns that the consumption of cow’s milk raises levels of IGF-1 in the blood (either directly or indirectly) and higher IGF-1 levels are a risk factor for colorectal cancer. In fact, circulating IGF-1 levels are not just related to future colorectal cancer risk but may also predict cancer progression (Renehan et al., 2001). In a study of 204 healthy men and women aged 55 to 85 years, three servings of non-fat milk per day over 12 weeks increased blood serum levels of IGF-1 by 10 per cent (Heaney, 1999). Because elevated levels of IGF-1 are associated with increased risk of colorectal cancer (Ma et al., 1999; Giovannucci et al., 2000; Kaaks et al., 2000), an increase in IGF-1 attributable to the consumption of milk could potentially counter any protective effect conferred by dietary calcium (and vitamin D in US fortified milk).
Taken together, the research suggests that plant foods provide a safer and healthier source of calcium than dairy products. Plant-based sources of calcium, including non-oxalate dark green leafy vegetables, dried fruits, nuts, seeds and pulses as well as fortified foods such as calcium-set tofu (soya bean curd) and calcium-enriched soya milk, provide a safer source of calcium. Vitamin D can be either obtained from the diet or synthesised in the skin following exposure to sunlight.
To lower your risk of colorectal cancer it is important to eat a healthy plant-based diet rich in fibre and low in fat, take regular physical exercise, maintain a healthy weight and avoid excessive alcohol consumption and avoid smoking.