Vegan Society Briefing

Milk and Breast Cancer (4 of 7)

Stephen Walsh, November 2001.

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.

The dairy industry sources quoted above also refer to a Finnish case control study, Aro et al. 2000 [8]. This study made use of both dietary questionnaires and evaluation of fatty acid levels in blood samples. Protective associations are found with consumption of cheese and with levels of conjugated linoleic acid, myristic acid and trans-vaccenic acid in the blood. All these fatty acid levels are primarily associated with recent dairy fat intake. There is no particular reason to regard this case control study as carrying more weight than the many other case control studies discussed earlier, which generally indicated an adverse effect of milk and cheese.

The trigger for the dairy industry campaign appears to be a result of modest statistical significance for which there is equally strong contradictory evidence. As Hjartaker et al. [7] note, "The contradicting results may indicate that any association between milk and breast cancer is not a strong one". Indeed, any effect that may exist could prove to be adverse rather than beneficial.

Of the components of milk suggested as potentially beneficial (calcium, vitamin D, and CLA), only CLA is fairly specific to milk and beef, while calcium and vitamin D have many non-animal sources. CLA forms a small part of dairy fat. Dairy fat is highly saturated and therefore has a strong effect in increasing cholesterol. The ill effect of elevated cholesterol on mortality from heart disease and on overall mortality is well established. The main studies above that were suggestive of a beneficial effect of milk on breast cancer ([6], [7]) also suggested that if such an effect existed it was unlikely to be dependent on milk fat. In [6] calcium showed a clear protective association with breast cancer risk. There is independent evidence that vitamin D (present naturally in milk and also added to milk in many countries) from diet and sunlight has a role to play in breast cancer ([9], [10]). As sunlight is insufficient to produce vitamin D for almost 6 months of the year in Scandinavia, this may be a reason why the most positive results on milk and breast cancer have come from Scandinavia. However, calcium and vitamin D can be efficiently obtained by other means than dairy products, so it was therefore convenient for the dairy and beef suppliers to emphasise CLA.

This concludes the direct review of evidence on milk and breast cancer. However, before discussing recommendations for reducing breast cancer risk, it is important to consider other evidence for a role of milk in increasing a major risk factor for premenopausal breast cancer, colorectal cancer, lung cancer and prostate cancer.

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