Colorectal Cancer

Colorectal (bowel) cancer

Cancers of the colon and rectum account for around one in every eight newly diagnosed cancers in the UK and one in every nine deaths from cancer (National Statistics, 2005a). In the UK it is the third most common cancer in men, and the second most common cancer in women (Cancer Research UK, 2005). 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 poor diet, obesity, alcohol and smoking.

Although the causes of colorectal cancer are not known, it is thought that there may be a link with a diet high in animal fats and protein and low in fibre (NHS, 2006). To reduce the risk of developing colorectal cancer, the Government recommends a healthy, balanced diet including plenty of fresh fruit and vegetables (NHS, 2006). It is also important to take regular physical exercise, maintain a healthy weight and avoid alcohol and smoking.

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) have 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 even larger study, researchers from the European Prospective Investigation into Cancer and Nutrition (EPIC) 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. 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). 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.

While it has been demonstrated that dietary fibre can protect again colorectal cancer, evidence suggests that animal foods (animal fat and animal protein) may be associated with increased colorectal cancer risk. In another EPIC study, researchers prospectively followed 478,040 men and women from 10 European countries that were free of cancer between 1992 and 1998. Information on diet and lifestyle was collected and after a mean follow-up of 4.8 years, 1,329 cases of colorectal cancer were documented. An investigation of the relationship between intakes of red and processed meat, poultry and fish revealed that colorectal cancer risk was positively associated with intake of red and processed meat (Norat et al., 2005).

In a recent study, the association between the consumption of dairy foods and calcium and colorectal cancer risk was assessed in a pooled analysis of 10 cohort studies from North America and Europe (Cho et al., 2004). In this study the authors concluded that the consumption of milk and calcium were related to a lower risk of colorectal cancer. However, the inverse association between calcium (and by inference, dairy) 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 incidence (Garland, 1999). In contrast to these findings, most prospective studies show only a moderate and not statistically significant decrease in the risk of colorectal cancer with increased dietary calcium intake (Ma et al., 2001).

Furthermore, 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). For example, 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). It may be that 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.