Breast Cancer

Breast cancer

One in nine women in the UK will develop breast cancer at some point in their lives. In 2003 nearly 40,000 new cases were diagnosed, representing a third of all cancers in women, and in the same year around 10,500 women died from this disease. Between 1971 and 2003, the incidence rates of breast cancer have increased by 80 per cent (National Statistics, 2005). Figure 5.0 shows that while the incidence of breast cancer has risen sharply, mortality from breast cancer has remained fairly constant over the same period thanks largely to improved diagnostic methods and more efficient treatment.

Much has been made of the link between genes and breast cancer. The genes BRCA1 and BRCA2 have received the most attention since they were first discovered in 1994 and 1995 respectively. There are two other very rare genes which probably only account for less than two per cent of all breast cancers: the P53 gene and the AT (ataxia telangiectasia) gene. These recent discoveries linking genetics to cancer has given rise to a certain degree of genetic fatalism. However, current estimates are that only about five per cent of breast cancers are due to abnormal genes (BACUP, 2005). This means that the vast majority of cancers (95 per cent) are not caused by abnormal genes. Secondly, it is important to remember that having an abnormal gene does not mean that a person will definitely develop breast cancer, but does mean they are considerably more at risk of developing the condition than someone who does not have one of the abnormal genes (BACUP, 2005).

The age-adjusted incidence rates per 100,000 for breast cancer differ markedly from one country to another. For example, Uruguay has a very high rate at 114.9, followed by 92.1 in the US and 87.1 in Israel. Much lower rates are seen in Korea at just 12.7, 20.0 in Mali and 16.1 in Thailand (Ganmaa and Sato, 2005). In response to this discrepancy, an increasing amount of attention is now focusing on the links between diet and breast cancer, particularly the relationship between the consumption of cow’s milk and dairy products and breast cancer.

Studying cancer incidence among particular groups of people can provide useful insights into the links between diet and disease. Researchers from the London School of Hygiene and Tropical Medicine recently reported breast cancer incidence is substantially lower, and survival rates higher, in South Asians living in the UK than other women (Farooq and Coleman, 2005). No data on diet was collected but the authors of this study suggested that differences in diet and lifestyle could explain the different rates observed. Earlier research published in the British Journal of Cancer also showed that South Asian women living in the UK are less likely to be diagnosed with breast cancer than other women, but found that the risk varied according to their specific ethnic subgroup. This research showed that Muslim women from India and Pakistan are almost twice as likely to develop breast cancer as Gujarati Hindu women. This study did examine the diet and found that the Gujarati Hindu women were more likely to be vegetarian and therefore had more fibre in their diet due to their higher intake of fruit and vegetables (McCormack et al., 2004). There are several mechanisms by which the diet might influence breast cancer risk. One possible mechanism is through an effect on hormones: increasing the amount of fibre in the diet may reduce breast cancer risk by altering the levels of female hormones (oestrogens) circulating in the blood (Gerber, 1998).

A number of studies show that women with breast cancer tend to have higher levels of circulating oestrogens. Prospective studies follow groups of people over time. Generally these people are alike in many but not all ways (for example, young women who smoke and young women who do not). The prospective cohort study will then look for a link between their behaviour and a particular outcome (such as lung cancer). A prospective study conducted on the island of Guernsey examined serum levels of the oestrogen hormone oestradiol in samples taken from 61 postmenopausal women who developed breast cancer an average of 7.8 years after blood collection. Compared to 179 age-matched controls, oestradiol levels were 29 per cent higher in women who later developed breast cancer (Thomas et al., 1997). Another prospective study (this time from the US), compared oestrogen levels in 156 postmenopausal women who developed breast cancer, after blood collection, with two age-matched controls for each cancer patient. Results showed increased levels of the hormones oestradiol, oestrone, oestrone sulphate and dehydroepiandrosterone sulphate in women who subsequently developed breast cancer thus providing strong evidence for a causal relationship between postmenopausal oestrogen levels and the risk of breast cancer (Hankinson et al., 1998). A review of studies carried out over a 10 year period in the Department of Clinical Chemistry at the University of Helsinki in Finland suggested that the Western diet (characterised by milk and meat products) increases levels of these types of hormones and concluded that the hormone pattern found in connection with a Western-type diet is prevailing in breast cancer patients (Adlercreutz, 1990). 

While some research has identified dietary factors that reduce the risk of breast cancer, such as fibre, other studies have identified dietary factors that increase the risk, such as dietary fat. Case-control studies use a group of people with a particular characteristic (for example older women with lung cancer). This particular group is selected and information collected (for example, history of smoking), then a control group is selected from a similar population (older women without lung cancer) to see if they smoked or not, then a conclusion is drawn (smoking does or does not increase risk of lung cancer). A combined analysis of 12 case-control studies designed to examine diet and breast cancer risk found a positive association between fat intake and this disease. The reviewers estimated that the percentage of breast cancers that might be prevented by dietary modification in the North American population was 24 per cent for postmenopausal women and 16 per cent for premenopausal women (Howe et al., 1990).

In 1999 researchers at the Department of Preventive Medicine at the University of Southern California Medical School in Los Angeles published a review of 13 dietary fat intervention studies that were conducted to investigate the effect of fat intake on oestrogen levels. The results showed decreasing dietary fat intake (to between 10 and 25 per cent of the total energy intake) reduced serum oestradiol levels by between 2.7 and 10.3 per cent. It was concluded that dietary fat reduction can result in a lowering of serum oestradiol levels and that such a dietary modification may offer an approach to breast cancer prevention (Wu at al., 1999).

However, other studies of fat intake and the incidence of breast cancer have yielded conflicting results. The discrepancy in results may reflect the difficulties of accurately recording fat intake. Dr Sheila Bingham of the Dunn Human Nutrition Unit in Cambridge has developed a data-collection method which may overcome these problems. Bingham used food frequency questionnaire methods with a detailed seven-day food diary in over 13,000 women between 1993 and 1997. The study concluded that those who ate the most animal saturated fat (found mainly in whole milk, butter, meat, cakes and biscuits) were almost twice as likely to develop breast cancer as those who ate the least. It was also concluded that previous studies may have failed to establish this link because of imprecise methods (Bingham et al., 2003).

In a subsequent prospective cohort study involving over 90,000 premenopausal women, researchers from Harvard Medical School confirmed that animal fat intake was associated with an elevated risk of breast cancer. Red meat and high-fat dairy foods such as whole milk, cream, ice-cream, butter, cream cheese and cheese were the major contributors of animal fat in this cohort of relatively young women. Interestingly, this research did not find any clear association between vegetable fat and breast cancer risk; the increased risk was only associated with animal fat intake. It has been suggested that a high-fat diet increases the risk of breast cancer by elevating concentrations of oestrogen. However, the author of this study, Dr Eunyoung Cho, suggests that if this were true a diet high in animal fat and a diet high in vegetable fat should both lead to higher rates of cancer, and that was not the case in this study. Cho suspects that some other component such as the hormones in cow’s milk might play a role in increasing the risk of breast cancer (Cho et al., 2003). 

Such conclusions have led many research groups to focus on the endogenous hormonal content of milk (hormones produced by the cow and excreted in the milk), which has not been widely discussed. The milk produced now is very different from that produced 100 years ago; modern dairy cows are frequently impregnated while still producing milk (Webster, 2005). Two-thirds of milk in the UK is taken from pregnant cows with the remainder coming from cows that have recently given birth. This means that the hormone (oestrogen, progesterone and androgen precursor) content of milk varies widely. It is the high levels of hormones in milk that have been linked to the development of hormone-dependent cancers such as ovarian and breast cancer.

In a review of the relationship between breast cancer incidence and food intake among the populations of 40 different countries, a positive correlation was seen between the consumption of meat, milk and cheese and the incidence of breast (and ovarian) cancer. Meat was most closely correlated with breast cancer incidence, followed by cow’s milk and cheese. By contrast, cereals and pulses were negatively correlated with the incidence of breast cancer. This review concluded that the increased consumption of animal foods may have adverse effects on the development of hormone-dependent cancers. Among dietary risk factors of particular concern were milk and dairy products, because so much of the milk we drink today is produced from pregnant cows, in which oestrogen and progesterone levels are markedly elevated (Ganmaa and Sato, 2005).

In addition to animal fat and various chemical contaminants, cow’s milk and dairy products contain hormones and growth factors, which have been implicated in the proliferation of human breast cancer cells. In a review of the evidence linking dairy consumption with breast cancer risk, researchers from Princeton University in New Jersey concluded that milk may promote breast cancer by the action of the growth factor IGF-1, which has been shown to stimulate the growth of human breast cancer cells in the laboratory (Outwater et al., 1997). In another review, examining the role of IGF-1 in cancer development, Yu and Rohan state that IGFs play a critical role in regulating cell growth and death. This function has led to speculation about their involvement in cancer development. Laboratory experiments demonstrate the ability of IGFs to stimulate growth of a wide range of cancer cells and to suppress cell death or apoptosis (Yu and Rohan, 2000). The concern here is that if IGF-1 can cause human cancer cells to grow in a Petri dish in the laboratory, they might have a cancer-inducing effect when consumed in the diet. IGF-1 is present in all milk and is not destroyed during pasteurisation. Dr J.L. Outwater of the Physicians Committee For Responsible Medicine (PCRM) in Washington, DC, warns that IGF-1 may be absorbed across the gut and cautions that regular milk ingestion after weaning may produce enough IGF-1 in mammary tissue to encourage cell division thus increasing the risk of cancer (Outwater et al., 1997).

In her book Your Life in Your Hands, Professor Jane Plant CBE, the chief scientist of the British Geological Survey, describes a very personal and moving story of how she overcame breast cancer by excluding all dairy products from her diet (Plant, 2000). Plant was diagnosed with breast cancer in 1987. She had five recurrences of the disease and by 1993 the cancer had spread to her lymphatic system. She could feel the lump on her neck, and was told that she had just three months to live, six if she was lucky. However, Plant was determined to use her scientific training to find a solution to this ‘problem’. She began researching breast cancer in other cultures and found a much lower incidence in China. The data showed that in rural China breast cancer affects just one in 10,000 women compared to one in 10 British women (now one in nine). However, Plant observed that among wealthy Chinese women with a more Western lifestyle (for example in Malaysia and Singapore), the rate of breast cancer is similar to that in the West. Furthermore, epidemiological evidence shows that when Chinese women move to the West, within one or two generations their rates of breast cancer incidence and mortality increase to match those of their host country. This suggested that diet and lifestyle (rather than genetics) must be a major determinant of cancer risk.

Plant decided to investigate the role of diet in breast cancer risk. She examined the results of the China-Cornell-Oxford project on nutrition, environment and health (Campbell and Junshi, 1994). This project was based on national surveys conducted between 1983 and 1984 in China. The project was a collaboration between T. Colin Campbell at Cornell University in the US, Chen Junshi from the Chinese Academy of Preventative Medicine, in Beijing, China, Li Junyao at the Chinese Academy of Medical Sciences, Beijing, and Richard Peto from Oxford University in the UK. The project revealed some surprising insights into diet and health. For example, it showed that people in China tend to consume more calories per day that people in the US, but only 14 per cent of these calories come from fat compared to a massive 36 per cent in the West. This coupled to the fact that Chinese people tend to be more physically active than people in the West, is why obesity affects far more people in the West than in China. However, Plant’s diet had not been particularly high in fat; indeed she describes it as very low in fat and high in fibre. Then Plant had a revelation: the Chinese don’t eat dairy produce. Plant had been eating yogurt and skimmed organic milk up until this time, but within days of ceasing all dairy, the lump on her neck began to shrink. The tumour decreased and eventually disappeared, leading her to the conviction that there is a causal link between the consumption of dairy products and breast cancer. Although Plant received chemotherapy during this time, it did not appear to be working and so convinced was her cancer specialist that it was the change in diet that saved her life, he now refers to cancer mortality maps in his lectures and recommends a dairy-free diet to his breast cancer patients.  

Plant eventually defeated cancer by eliminating dairy products from her diet, replacing them with healthy alternatives and making some lifestyle changes. Plant advises that if you do only one thing to cut your risk of breast cancer, make the change from dairy to soya (Plant, 2000). Providing breast cancer patients with sound dietary advice could greatly increase survival rates. Taken together, these observations show that a plant-based diet can reduce many of the risk factors associated with breast cancer.