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Vegan Outreach Enewsletter  •  January 10, 2002

 

Happy New Year!

Because of the length of the excerpts from Milk and Breast Cancer, this Vegan Spam contains only one item. The entire article can also be found online, along with references.

Although the article is long, it is quite informative for anyone interested in what the research looking at the link between dairy products and breast cancer has produced to date.

 

Vegan Society briefing paper on Milk and Breast Cancer

Stephen Walsh, November 2001.

Summary

In August and September 2001 there were claims in the press that drinking milk could reduce the risk of breast cancer. Dairy Council nutrition manager Anita Wells said, "To discover that life-long milk drinkers have a reduced risk of breast cancer is an exciting step forward." "Good health starts with dairy," said Greg Miller, executive vice president of nutrition and scientific affairs for the National Dairy Council.

These claims were triggered by a prospective study of premenopausal women in Norway, published on 15 September 2001 in the International Journal of Cancer. This study found a significantly reduced risk of breast cancer in women who reported high milk consumption both as adults and as children compared with women who reported low milk consumption both as adults and as children. The risk of breast cancer among the high milk group appeared to be about half the risk in the low milk group.

If this had been one of many studies finding similar results, the claim of a protective effect would have some credibility. However, considering other prospective studies on breast cancer and dairy products shows that this is not so. One study found a significant increase in risk with milk consumption and another found a significant increase in risk with cheese consumption. A Finnish study agreed with the recent Norwegian study that there was a protective association with milk products. Two other studies found no significant association, with one finding a tendency to a beneficial effect and the other finding a tendency to an adverse effect. The authors of the most recent study state, "The contradicting results may indicate that any association between milk and breast cancer is not a strong one." This seems a fair conclusion. Indeed, if a true effect exists there is no reason to assume it will prove to be beneficial. The suggestion that consuming milk reduces breast cancer risk is therefore unjustified.

Some reports latch on to a constituent of dairy fat - conjugated linoleic acid (CLA) - as a likely explanation for the suggested beneficial effect, suggesting that milk fat might benefit health. Neither of the two prospective studies which found a beneficial association of milk with breast cancer showed a link with milk fat. The more likely explanation for any beneficial effect, if one exists, is the calcium and vitamin D content of the milk. Even if CLA showed some benefits, it must be recognised that it is only a small part of dairy fat and that breast cancer is only one aspect of health. Milk fat is highly saturated and promotes elevated cholesterol, which is strongly linked with heart disease. Milk fat is therefore a very unlikely choice as a health food. Calcium and vitamin D can of course be obtained from many sources other than dairy products.

Consuming milk increases levels of a growth hormone, IGF-1, in the body. Increased IGF-1 levels are strongly implicated in prostate cancer, colorectal cancer, premenopausal breast cancer and lung cancer. The effect of milk on IGF-1 may be due to absorption of IGF-1 from the milk or may simply be due to the high protein and zinc content of the milk.

For those concerned with their risk of breast cancer and with their overall health there are useful recommendations to consider. Weight gain in adult life increases breast cancer risk. Physical activity helps in maintaining a steady weight and also has additional benefits in reducing risk. Olive oil provides a healthful alternative to saturated fats. Alcohol consumption increases risk of breast cancer, even at moderate levels, but this increase in risk can be effectively countered by high folate intakes. As moderate alcohol intake is associated with reduced risk of heart disease, increasing folate intake may be a better option than cutting out alcohol altogether. Folate is found in green leafy vegetables and many other plant foods and can also be obtained from folic acid supplements. Adequate B12 may be needed to get the full benefit of folate. Adequate calcium and vitamin D may also be beneficial.

There is a lot we can do to take control of our health for the better, including reducing risk of breast cancer. Green leafy vegetables, olive oil and physical activity can all be expected to be beneficial. Drinking cow’s milk doesn’t appear on the list.

Good information supports health. Pass it around.

 

Is there an association between milk and breast cancer?

What is the evidence on milk and breast cancer?

If we compare breast cancer rates and milk consumption between different countries, we find that countries consuming large amounts of dairy products have higher rates of breast cancer than those consuming low amounts of dairy products [1]. As noted above, this association may well be non-causal and we need to consider other evidence.

Results from individual case control and prospective studies are very diverse so it is essential to look at the studies as a whole. Picking studies to suit a conclusion is easily done but has no value in the search for truth.

Boyd et al. 1993 [2] review studies of dietary fat and breast cancer risk up to February 1993.

7 case control studies considered milk intake.

  • 5 found that high milk intakes were associated with significantly increased risk of breast cancer.
  • 2 found no significant effect.

5 case control studies considered cheese. 3 of the results on cheese came from studies also reporting results on milk (above), and 2 came from separate studies.

  • 3 found a significant adverse effect,
  • 1 found a significant protective effect and
  • 1 found no significant effect.

Only one prospective study considered milk and cheese and found no significant effect for milk (relative risk (RR) 1.03) and a significant adverse effect for cheese.

The summary statistics from this review showed a modest, but significant, adverse effect of both milk and cheese on breast cancer risk (RR =1.17 for each).

The above review did not include the paper by Ursin et al. 1990 [3]. This paper was a prospective study of milk consumption and cancer risk in Norway. Individuals consuming two or more glasses of milk a day vs less than one glass a day showed a relative risk of 1.48 for breast cancer, but this was not statistically significant. Overall cancer rates showed a non-significant relative risk of 0.99. The results were not adjusted for standard breast cancer risk factors.

So up to until 1993, the clear balance of evidence was for a modest adverse effect of milk and cheese on breast cancer risk. The evidence was, however, dominated by case control studies and therefore subject to selection and recall bias.

Toniolo et al. 1994 [4] present a prospective study from New York. This study showed an almost significant protective association of high dairy intake (RR=0.59 upper quintile vs lower quintile) with breast cancer. Milk and other dairy products were not separated. Adjustments were made for standard breast cancer risk factors.

Gaard et al. 1995 [5] present another Norwegian prospective study. Adjustments were made for age, energy, smoking, height and BMI but not for other standard breast cancer risk factors. This study found a significant elevated risk with higher consumption of whole milk (RR=2.91 upper vs lower quintile). The corresponding RR for all forms of milk combined was 1.71, but this was not statistically significant.

Knekt et al. 1996 [6] present a Finnish prospective study. This study had relatively few participants, 4697, but follow-up was over 25 years so the total years at risk were high. Adjustments were made for many risk factors, but not for height or for age at menarche, first childbirth or menopause. The adjusted relative risk for the highest compared with the lowest tertile of the cohort by milk intake was 0.49 and was significant. Adjustments for some other foods did not eliminate the association. Calcium showed an almost identical relative risk to milk, but milk fat showed a weaker and non-significant association. Other nutrients were noted not to show a significant association. It has been suggested that fermented milk may have particular benefits in relation to cancer. There was no association between breast cancer and fermented milk consumption in this study. The association observed was with ordinary milk.

Hjartaker et al. 2001 [7] present a further Norwegian prospective study. This study was limited to premenopausal women. There were 48,844 participants and follow-up was for about 6 years. Results were presented in two forms: with age adjustment only and with adjustment for age and other risk factors, but not for height, age at menopause or energy intake. The dietary questionnaire was insufficiently comprehensive to evaluate energy intake. Women with prior cancer diagnosis were excluded as required to avoid recall bias and elimination of cases arising within one year of the start of the study was shown not to alter the conclusions. Association with adult milk consumption was significant only in the youngest age group (34-39). The negative association tended to be stronger for low-fat milk than for skimmed or whole milk, but was not significant for any of these. Associations with milk consumption as a child were also not significant. Milk fat intake showed a much weaker association with breast cancer risk than overall milk intake did, suggesting that fat was not the key component of any effect of milk. The effect of calcium was not tested.

Only by using a combined measure of childhood and adult milk intake was a statistically significant protective association found and this was only just significant (RR=0.51 "high" milk intake vs "low" milk intake, with full adjustment for known risk factors). 11% of the overall group was in the low milk category. This category had 36 cases of breast cancer against an expected 29, based on the average risk for the whole group. 7% of the group was in the high milk category. This category had 13 cases of breast cancer against an expected 20, based on the average risk. There was little difference between the age adjusted and fully adjusted relative risks, indicating that any interactions between milk consumption and known risk factors, such as age at menarche, did not have a large effect on the observed risk.

 

Reducing breast cancer risk and promoting health

In contrast to the statements from dairy industry representatives, milk does not seem to be the answer to breast cancer. Calcium and vitamin D may have a beneficial role. However, even these do not rank in the established dietary recommendations for reducing breast cancer incidence.

Walter Willett, a nutritional epidemiologist with the Harvard School of Public Health and the American Institute for Cancer Research makes the following recommendations for avoiding breast cancer [15].

  • Avoid weight gain in adult life.
  • Limit alcohol intake to moderate amounts.
  • Ensure sufficient folate (or supplementary folic acid) intake to avoid the excess risk associated with even moderate amounts of alcohol.
  • Replace saturated fat with olive oil.

Willett does not mention milk, though high-fat dairy products are implicitly rejected in favour of olive oil as they are a major source of saturated fat. Willett does not suggest a direct adverse role for saturated fat in breast cancer, but rather a benefit to health from replacing it with olive oil, a particularly healthful source of monounsaturated fat. The recommendation of olive oil as opposed to rapeseed/canola oil or other oils high in monounsaturated fats is deliberate: there is specific evidence for a benefit from olive oil over and above its major constituent fats. The recommendation to substitute olive oil for saturated fat appears to be based on evidence for positive benefits of olive oil in relation to breast cancer and the fact that substituting olive oil for saturated fat will improve blood cholesterol levels and thus reduce risk of heart disease.

Willett does not recommend completely eliminating alcohol, as moderate intakes of alcohol appear to have beneficial effects on heart disease. He notes that the adverse effects of alcohol in promoting breast cancer appear to be completely eliminated by high folate intake, allowing the benefit to be obtained without the damage. Folate is particularly well supplied by green leafy vegetables but is present in many other plant foods as well. In supplements it usually takes the form of folic acid. Some of the effects of folate are dependent on the presence of adequate vitamin B12, so vegans should ensure an adequate B12 intake (about 3 micrograms per day) to ensure that they get the full benefit of folate.

Willett takes a sceptical stance towards soy products due to recent evidence that they can stimulate proliferation of breast tissue, potentially increasing the risk of breast cancer. The overall effect of increased soy intake is more likely to be adverse in postmenopausal women than in premenopausal women due to interactions with oestrogen. He therefore suggests that "soy products should be used in moderation if started during midlife, perhaps no more than a few times a week, until further data are available". Typical Japanese and Chinese consumption is about 10g of soy protein (about 300ml (half a pint) of soya milk or 100g (4oz) of tofu) per day, consumed with apparent lack of adverse effects. Indeed, both these populations show low levels of breast cancer. However, their consumption of soy is from an early age and this is likely to be relevant. Overall it seems prudent not to exceed Japanese levels of consumption significantly, particularly if soy consumption is started late in life.

One of the more surprising observations is the comment that in the largest prospective study on dietary fat and breast cancer to date (pooling results from multiple studies) women consuming less than 15% fat showed double the risk of breast cancer [16]. No details of the particular diet characteristics of this group are given, but this observation adds to reasons for caution about very low fat diets.

A review paper by Timothy Key and Naomi Allen [17] provides an alternative summary of risk factors for breast cancer, concluding that obesity in postmenopausal women and alcohol consumption are the only well established diet-related risk factors for breast cancer. They conclude, "Current dietary advice should be to avoid obesity, limit alcohol intake and to maintain a varied diet."

Willett makes some more general observations on diet and cancer in [18]. He emphasises the need to make choices that are beneficial for both cancer and heart disease - we can only eat one diet.

Willett does not regard the evidence against meat as clear enough to make a strong recommendation regarding breast cancer. However, he notes that the evidence against red meat in relation to colorectal cancer is much stronger.

While the evidence for fibre and whole grains in relation to cancer is weak, Willett comments that there is stronger evidence in relation to heart disease, diabetes and diverticular disease and this justifies a recommendation that increased cereal fibre consumption is likely to be beneficial for health.

Willett also emphasises that physical activity reduces risk of colorectal and breast cancer directly, as well as by reducing weight.

Finally, Willett estimates that remaining lean (BMI < 25), taking at least 30 minutes a day of moderate physical exercise, avoiding excessive alcohol consumption, getting plenty of folate, not smoking, and consuming red meat less than three times a week could reduce colorectal cancer by 70%.

There is a lot we can do to take control of our health for the better, including reducing risk of breast cancer. Green leafy vegetables, olive oil and physical activity can all be expected to be beneficial. Drinking cow’s milk doesn’t appear on the list.

 

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Vegan Outreach is a 501(c)(3) nonprofit organization dedicated to reducing the suffering of farmed animals by promoting informed, ethical eating.

All donations are tax-deductible.

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