There is now consistent and substantial evidence that the higher the milk consumption of a country, the greater their breast and prostate cancer risk. The highest risk of cancer death is found in Switzerland, Norway, Iceland and Sweden, countries that are among the biggest consumers of milk. In stark contrast, in most Asian countries the risk is minimal.  In such countries, where the diet consists mainly of wholegrains, vegetables, fruits, tofu, soya milk and other soya products – and where milk is not a normal part of the diet – people are generally healthier, and breast and prostate cancers are much rarer than in the US and Europe.
The problem with milk
The connection between milk an increased risk of cancer has been known for some time. Back in 1937, a group of 4,999 children in the UK took part in a long-term study recording their dietary habits year on year. Some 65 years later, those with a high dairy intake during childhood were found to have tripled their odds of having colorectal cancer.  There was a weaker association with prostate cancer risk and no association, in this study, with increased breast cancer risk. According to the National Cancer Institute (NCI), 19 out of 23 studies have shown a positive association between dairy intake and prostate cancer: “This is one of the most consistent dietary predictors for prostate cancer in the published literature… In these studies, men with the highest dairy intakes had approximately double the risk of total prostate cancer, and up to a fourfold increase in risk of metastatic or fatal prostate cancer relative to low consumers.” 
Another large study involving 63,550 people in Britain, known as the EPIC study (European Prospective Investigation into Cancer and Nutrition), found no clear association between milk consumption and breast cancer, but it did find a trend towards a higher risk for premenopausal women partial to butter, and also in those with high intakes of processed meats.  To date it appears that countries that are virtually milk-free have the lowest risk, and that, among Western countries, low milk intake means a low risk of colorectal cancer and prostate cancer in particular.
What’s in milk? Why would milk increase risk? Milk contains 38 different hormones and growth promoters. After all, that’s its job: to make cells grow. But one in particular is attracting a lot of attention. It’s called insulin-like growth factor, or IGF-1. It’s a naturally occurring hormone, found both in cow’s milk, breast milk and your blood. The more milk you drink, the higher your level. What this hormone does is stimulate growth. Blood levels of IGF-1 peak during adolescence, stimulating development of breasts in girls or the prostate in boys, then levels rapidly drop off as you get older. Not so if you keep guzzling milk and cheese. Milk not only contains IGF-1, a small part of which is absorbed into your blood, but it also stimulates the body to produce more of its own. It simply does what it’s meant to do – stimulate growth. It also stops overgrowing cells from committing suicide, a process called apoptosis. While you are a rapidly growing baby this is good news. But when the only overgrowing cells are cancer cells, this is especially bad news, because IGF-1 has also been found to directly stimulate the growth of cancer cells, with high levels being linked to an increased risk of breast, prostate, colon and lung cancer. [5a][5b]
Having a high IGF-1 level as a pre-menopausal woman just about doubles your risk of cancer overall. 
Studies on high IGF-1 levels
A Harvard University study showed that men who had the highest levels of IGF-1 had more than four times the risk of prostate cancer compared with those who had the lowest levels.  Two other major Harvard studies have shown that milk-drinking men have 30 to 60% greater prostate cancer risk than men who generally avoid dairy products. [8a][8b] In one of these, involving more than 20,000 male doctors (known as the Physicians’ Health Study), those who consumed more than two dairy servings daily had a 34% higher risk of developing prostate cancer than men who consumed little or no dairy products. 
According to Professor Jeff Holly from Bristol University’s Faculty of Medicine – one of the world’s leading experts in IGF – “Those in the top quarter for blood IGF-1 levels have approximately a three to fourfold increase in risk of breast, prostate or colorectal cancer. This level of increased risk is in the same order as the risk of having cardiovascular disease from a high level of cholesterol.” Professor Holly’s research, and that of others at Harvard and Montreal, show that a non-milk drinking 30-year-old might have an IGF level of 130ng/ml, while a high-dairy consumer might have a level of 200ng/ml, and that’s more than enough to dramatically increase your risk. The evidence is compelling and any scientist who denies this is simply not up to speed. Holly doesn’t drink milk and actively discourages anyone with a diagnosis of these cancers to have any dairy produce.
Although we do not know whether a high milk intake could initiate cancer, we can be pretty confident that a high milk intake, by increasing IGF-1 and possibly other growth promoters, speeds up the growth of pre-existing cancer cells.
Dairy and ovarian cancer
A high dairy intake is also linked to increased ovarian cancer risk. This link, however, is thought to be more due to the way that the milk sugar, lactose, breaks down in the body. Lactose breaks down into another sugar called galactose, which appears to be able to damage the ovary. A review in 2006 found that for every 10g of lactose consumed (the amount in one glass of milk), ovarian cancer risk increased by 13%.  I don’t mean to scare you unnecessarily, unless it might save your life, and I don’t mean to put you off ever touching the white stuff. But I do recommend two things.
Firstly, give yourself a dairy-free week. You may find relief from some niggling symptoms you have suffered from in the past. If you find your indigestion or bloating stops, the headaches you have suffered from have stopped, your energy increases or your sniffs and snuffles clear up, get yourself tested for dairy intolerance. There’s a simple Food intolerance is not the same as a food allergy. Food intolerances tend to cause digestive symptoms such as bloating, cramps and diarrhoea, but are… test available from yorktest.com you can do at home for this.
Secondly, add up all the milk you have in teas, coffees and cereals, plus servings of yoghurt and cheese, and if it’s over half a pint a day, cut back. Try rice milk, oat milk or soya milk instead. Milk is a food designed for baby cows, but not for you.
- If you have cancer, especially any hormone-related cancer such as breast or prostate cancer, or colorectal cancer, I recommend the complete avoidance of dairy products.
- If you don’t have cancer, keep your intake of dairy products low; that is, below half a pint a day and ideally less than two pints a week. If your aim is to minimise your chances of prostate or colorectal cancer, it would make sense to be largely dairy-free.
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2. J.C. van der Pols, et al., ‘Childhood dairy intake and adult cancer risk: 65-y follow-up of the Boyd Orr cohort’, American Journal of Clinical Nutrition, 2007 Dec.;86(6):1722–9.
3. J.L. Stanford, et al., ‘Prostate cancer trends 1973–1995’, SEER Program, National Cancer Institute. NIH Pub. No. 99–4543. Bethesda, MD, 1999.
4. P.V. Krogh, ‘Meat, eggs, dairy products, and risk of breast cancer in the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort’, American Journal of Clinical Nutrition, 2009 Jun 2; [Epub ahead of print].
5a. D. LeRoith and C.T. Roberts, Jr., ‘The insulin-like growth factor system and cancer’, Cancer Letters, 2003 June 10;195(2):127–37.
5b. S.E. Hankinson, et al., ‘Circulating concentrations of insulin-like growth factor-I and risk of breast cancer’, Lancet, 1998 May 9;351(9113):1393–6.
6. M.H. Wu, et al., ‘Relationships between critical period of estrogen exposure and circulating levels of insulin-like growth factor-I (IGF-I) in breast cancer: Evidence from a case-control study’, International Journal of Cancer, 2009 Jul 7; [Epub ahead of print].
7. J.M. Chan, et al., ‘Plasma insulin-like growth factor-I and prostate cancer risk: a prospective study’, Science, 1998 Jan. 23;279(5350):563–6.
8a. E. Giovannucci, et al., ‘What it does: Promotes a healthy heart, clots blood, promotes healthy nerves, contracts muscles, improves skin, bone and dental health, relieves aching muscles and bones,… and Fructose is a simple sugar that occurs naturally in fruits, vegetables and their juices, as well as in honey…. intake in relation to risk of prostate cancer’, Cancer Research, 1998 Feb. 1;58(3):442–7.
8b. J.M. Chan, et al., ‘Dairy products, calcium, and prostate cancer risk in the Physicians’ Health Study’, American Journal of Clinical Nutrition, 2001 Oct.;74(4):549–54.
9. J.M. Chan, et al., ‘Dairy products, calcium, and prostate cancer risk in the Physicians’ Health Study’, American Journal of Clinical Nutrition, 2001 Oct.;74(4):549–54.
10. S.C. Larsson, et al., ‘Milk, milk products and lactose intake and ovarian can- cer risk: A meta-analysis of epidemiological studies’, International Journal of Cancer, 2006 Jan. 15;118(2):431–41.