22nd August 2019

Do Dairy Products Increase The Risk Of Breast Cancer?

By Dr Justine Butler, Senior Health Researcher & Writer, Viva!Health

When Viva!Health first published its report on diet and breast cancer in 2006, the lifetime risk for women in the UK was one in nine. Now it is one in eight. That means one in every eight women in the UK will develop breast cancer at some point in their lives. Over 55,000 women in the UK are diagnosed with it every year.  

Breast cancer occurs in men too, around 400 men are diagnosed every year in the UK, and the number is rising. Men tend to have larger tumours which have spread further by the time they seek help. That said, breast cancer remains a rare disease among men.  

Thankfully, survival rates have improved, thanks to improved diagnostic methods and more efficient treatment, but incident rates have soared. There has been a steep and steady increase in the number of cases diagnosed in women of all ages since the 1970s. So, while you might be more likely to survive breast cancer, you are also more likely to suffer from it too.

Much is 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 the mid-1990s. The discoveries linking genes to cancer has given rise to a degree of genetic fatalism. However, only around five to 10 per cent of breast cancers are due to genes – the vast majority (90-95 percent) being linked to lifestyle and/or environmental factors.

These factors include age – the risk increases as you get older – alcohol, obesity, early puberty, late menopause, late age at first childbirth, hormone replacement therapy (HRT) and the contraceptive pill.

Factors that may lower the risk include younger age at first pregnancy, breastfeeding, late puberty, early menopause and physical activity.

The contribution of environmental and lifestyle factors (excluding reproductive factors) to breast cancer risk was calculated by a group from Harvard School of Public Health. They concluded that one in five of all breast cancer deaths worldwide were attributable to alcohol, being overweight or obese and physical inactivity. This proportion was even higher (27 per cent) in high-income countries. That's nearly a third of all breast cancer cases being attributed to avoidable risk factors.

Breast cancer incidence rates vary greatly worldwide, with age-standardised rates of around 100 women per 100,000 in North America, Europe, Australia and New Zealand. According to the International Agency for Research on Cancer, Belgium had the highest rate of breast cancer in 2012 with 112 cases per 100,000, while in the UK the rate was 95 per 100,000. Much lower rates are seen in parts of Africa and Asia, with just 22 per 100,000 in China and 19 per 100,000 in Nepal.

Migration studies show that people who move from an area of low incidence, to one of high incidence, adopt the rate of where they live after a number of years. This demonstrates that the variation between different countries is not due to genes and that environmental and lifestyle factors must be involved. Because of this, an increasing amount of attention has focused on the links between diet and breast cancer – particularly the role of cow’s milk and dairy products.

Researchers from the London School of Hygiene and Tropical Medicine found that breast cancer incidence was much lower, and survival rates higher, in South Asians living in the UK than other women. They suspected that differences in diet and lifestyle might explain the different rates observed.

Research published in the British Journal of Cancer also found 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. Muslim women from India and Pakistan were almost twice as likely to develop breast cancer as Gujarati Hindu women – who were more likely to be vegetarian and have more fibre in their diet from fruit and vegetables.

Another dietary study looked at the links between plant foods, fibre and risk of breast cancer in over 11,000 postmenopausal women in the Malmö Diet and Cancer cohort in Sweden (among whom 342 cases of breast cancer were recorded). They also found that diets high in fibre and low in fat were linked to a lower risk of postmenopausal breast cancer. They suggested that increasing fibre in the diet may reduce breast cancer risk by lowering the levels of female hormones (oestrogens) circulating in the blood.

A number of studies show that women with breast cancer tend to have higher levels of circulating oestrogens. One study looked at oestrogen levels in blood samples taken from 61 postmenopausal women from the island of Guernsey, who later developed breast cancer. Compared to age-matched controls, hormone levels were 29 percent higher in the women who went on to develop breast cancer.

Another study, from the US, compared oestrogen levels in 156 postmenopausal women who later developed breast cancer, with age-matched controls. Results showed higher levels of oestrogen hormones in those who subsequently developed breast cancer, thus providing evidence for a causal relationship between hormone levels and breast cancer.

So, what factors increase hormones levels in the body? A review of 13 studies identified a number of factors, including obesity and alcohol consumption – both known risk factors for breast cancer – can increase the levels of hormones circulating in the body. This raises the concern that foods that alter hormone levels may also increase the risk for breast cancer.

A review of studies carried out over 10 years 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.

Researchers at the Department of Preventive Medicine at the University of Southern California Medical School in Los Angeles published a review of 13 studies looking at the effect of dietary fat on oestrogen levels. The results showed that low-fat diets reduced oestrogen levels in the blood and they concluded that lowering your fat intake may help prevent breast cancer in some women. Cow's milk and dairy products, like cheese and butter, are a major source of saturated fat.

Another study investigated breast cancer risk in postmenopausal French women consuming two different types of diet. The 'alcohol/Western' diet included processed meat, ham, offal, French fries, sandwiches, rice and pasta, potatoes, pulses, pizza and pies, canned fish, eggs, alcoholic drinks, cakes, mayonnaise, butter and cream. The 'healthy Mediterranean' diet included a high intake of vegetables and fruits, fish and crustaceans, olives and sunflower oil. Results showed those eating the Western diet had a 20 per cent higher risk of breast cancer while those consuming the Mediterranean diet had a 15 percent lower risk.

Identifying the type of diet that can increase or reduce the risk of cancer is just part of the puzzle. Pinpointing which particular components of that diet that are responsible is a matter of considerable complexity. While some research has identified dietary factors that reduce the risk of breast cancer, such as fibre, other studies have attempted to identify dietary factors that increase the risk, such as dietary fat.

A review of 12 studies looking at diet and breast cancer risk did find a link between dietary fat and the disease. They estimated that between 16 to 24 per cent of breast cancers could be prevented by dietary modification in the North American population. This is a significant number of cancers that could be prevented simply by changing diet.

Researchers from the Dunn Human Nutrition Unit in Cambridge used food frequency questionnaires with a detailed seven-day food diary in over 13,000 women between 1993 and 1997. They also found 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.

In another study, involving over 90,000 premenopausal women, researchers from Harvard Medical School also found that animal fat intake was associated with a higher 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 group of relatively young women. This research did not find any clear association between vegetable fat and breast cancer risk – the increased risk was associated only with animal fat.

It has been suggested that a high-fat diet increases the risk of breast cancer by raising levels of oestrogen. However, lead author of this study, Dr Eunyoung Cho, points out that if this were the case, diets high in vegetable and animal fat should both lead to higher rates of cancer – that was not the case. 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.  

Just like human breast milk, cow’s milk is packed with hormones and growth factors. They are there to help the young infant grow and develop. Commercial milk is now very different from that produced 100 years ago as modern dairy cows are impregnated while still producing milk and two thirds of milk in the UK is taken from pregnant cows with the remainder coming from cows that have recently given birth. This drives up the hormone content of milk.

In a review of diet and breast cancer in 40 different countries, a link was found between meat, milk and cheese and a higher risk of the disease. By contrast, cereals and pulses were linked to a lower incidence of breast cancer. They concluded that high intakes 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 taken from pregnant cows, in which hormone levels are markedly high.

Researchers from Princeton University in New Jersey suggested that milk may promote breast cancer by the action of the growth factor IGF-1, which stimulates the growth of human breast cancer cells in the laboratory. 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. It’s not just that milk contains the growth factor IGF-1 (and the possible effect of this has been questioned), but what concerns some scientists is that cow's milk is known to increase IGF-1 levels in human blood by driving up our IGF-1 production by the liver.

In either scenario, the net effect is the same – cow’s milk increases IGF-1 levels and higher IGF-1 levels are linked to cancers of the colon, prostate and breast.

Cow's milk is a perfect food for a rapidly growing calf but that doesn’t mean it is good for human babies – or adults! If you want to improve your health by making just one change to your diet, Viva!Health recommends you ditch dairy.

Viva!Health is a part of the vegan charity Viva!. We monitor scientific research linking diet to health and provide accurate information on which you can make informed choices about the food you eat. 

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