Draft Vegan Society briefing paper on Milk and Breast Cancer
(subject to approval by the Vegan Society Council on November 25)

By Stephen Walsh, November 13, 2001

Summary

During September and October this year, there has been a spate of claims in the press that drinking milk will reduce the risk of breast cancer. These claims were triggered by a prospective study of premenopausal women in Norway, published on September 15, 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 to 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 50% of the risk in the low milk group.

If this study 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 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.

In a bizarre twist, some reports latch on to a constituent of dairy fat - conjugated linoleic acid (CLA) - as a likely explanation for the beneficial effect, suggesting that milk fat might benefit health. Neither of the two prospective studies that 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 to heart disease. Milk fat is therefore a most unlikely choice as a health food. Calcium and vitamin D can of course be obtained from many sources other than dairy products.

For those concerned with 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. All these recommendations can be expected to promote health in other respects and are entirely consistent with a vegan diet.

Acknowledgements

This briefing paper was greatly improved through critical comments from members of the International Vegetarian Union Science group (IVU-SCI).

What the dairy industry would like you to believe

The dairy industry propaganda machine sprang into action recently to promote the idea that drinking milk reduces risk of breast cancer.

Anti-cancer link's all white for milk, South Wales Evening Post, August 30, 2001
"FARMERS across South West Wales have welcomed new evidence that women who drink three glasses of milk a day are less likely to develop breast cancer.
Members of the Farmers' Union of Wales say the news is a boost for their White Stuff campaign to give free milk to all Welsh primary school pupils.
The FUW has urged all primary schools in Wales to take advantage of the scheme introduced last February, in which the Assembly and the EC pay the cost of school milk for pupils aged between five and seven. …
The Dairy Council's nutrition manager Anita Wells said: "To discover that life-long milk drinkers have a reduced risk of breast cancer is an exciting step forward."

Milk may stop breast cancer, The New York Post, September 5, 2001
The new anti-breast cancer slogan could be: Got milk?
A new study reveals that drinking milk in childhood and as an adult can protect women against the disease.
Women who drank more than three glasses of milk per day had half the risk of breast cancer, compared with women not drinking milk.
Researchers in Norway came to their conclusions after studying the health histories of 48,844 women, ages 34 to 39.
Milk's protective effect is attributed to a cancer-fighting substance called conjugated linoleic acid, which is found in milk fat.
Numerous studies have shown that the acid has a positive effect in fighting off breast cancer. The results of the study were published in the September issue of the International Journal of Cancer.

October is National Breast Cancer Awareness Month; Drinking Milk May Reduce the Risk of Breast Cancer, Business Wire, September 27, 2001, (National Dairy Council/Milk Processors Education Program press release)
A new study published in the September 15th issue of the International Journal of Cancer found that women who drink milk on a daily basis are less likely to develop breast cancer than those who drink little or no milk….
CLA or conjugated linoleic acid - which is found in milk fat - has been gaining a lot of attention lately as a potential cancer prevention agent. A recent Finnish study found that women with breast cancer had significantly lower levels of CLA in their diets and blood compared to women without cancer….
The results of the Norwegian Women and Cancer Study are in line with earlier research results showing the protective effect of milk on breast cancer. A study published in the British Journal of Cancer in 1996 showed that the women who consumed the most milk had less than half the risk of breast cancer compared to women consuming the least milk.
"Good health starts with dairy," said Greg Miller, Ph.D., F.A.C.N., executive vice president of nutrition and scientific affairs for the National Dairy Council.

In fact, the evidence for an association between milk and breast cancer is weak and conflicting and does not justify a claim for a protective effect. There are many ways of reducing breast cancer risk while promoting overall health that are properly justified and do not involve any harm to other animals. The rest of this paper justifies these observations in detail.

Is there an association between milk and breast cancer?

How are associations between diet and disease established?

Before looking specifically at milk and breast cancer, it is useful to consider some general methods by which researchers have attempted to establish dietary causes for human cancers by studying people. Readers already familiar with nutritional epidemiology can skip this section without loss, as it is standard material.

A common form of study involves comparing variations in disease rates in different countries with variations in diet. This will often throw up associations that are unlikely to be due simply to chance but there are many factors that vary between countries and most apparent associations are not causal. It is particularly difficult to differentiate between the effects of the many distinct changes that occur with economic development. These typically include increased consumption of animal products and fat, adequate calorie intake throughout life, less physical activity, access to effective sanitation and medical services and having fewer children at a later age. For this reason, inter-country comparison is regarded as a weak form of evidence. A strength of inter-country comparisons is that characteristics such as average diet are well defined and vary over a wide range.

Another source of evidence is provided by case control studies. In such studies, a group of people with established disease ("cases") are compared with a group of people who are free from the disease ("controls"). Statistical analysis is then used to identify characteristics that differentiate the cases and controls and the results are expressed as a relative risk (RR) for the disease associated with such characteristics. The results will be given in terms of the value of the relative risk comparing two groups. Typically the overall group will be divided into thirds (tertiles), quarters (quartiles) or fifths (quintiles) after ordering the group from lowest to highest values some characteristic. The relative risk will compare the group with the highest values of that characteristic with the group with the lowest values, e.g. upper quintile (fifth of group with highest values for characteristic) vs lower quintile. The probability that the relative risk differed from 1 purely by chance will also be reported. If the probability of a relative risk being different to 1 by chance is less than 0.05 that association is usually described as statistically "significant". The choice of 0.05 as a threshold is a compromise between getting an excessive number of false associations due to chance and missing a real relationship, and is to some extent arbitrary. For example, a relative risk of 1.5 between the upper quintile and the lower quintile with a probability of 0.003 would suggest a moderate and clearly statistically significant association. The RR of 1.5 means that the upper quintile show 50% greater risk of the disease being studied than the lower quintile. A relative risk of 5 with a probability of 0.04 would indicate a strong effect with modest statistical significance. A relative risk of 10 with a probability of 0.3 means little as the observed relative risk could easily be a chance observation, that is the result is not significant.

There are two major weaknesses in case control studies looking at diet. Firstly, the controls may not be representative of the general population being studied (selection bias). Often controls show a greater interest in health than most people, making them more likely to volunteer, and they may therefore show an unusually high consumption of foods believed to be healthy. This can lead to a false conclusion that these "healthy" foods make development of the disease less likely, where in fact the controls were simply unrepresentative of the disease free population. Secondly, cases' current diet may be altered as a consequence of the disease and their recollection of past diet may also be changed. A simple example of this is that people with hip fractures tend to underestimate past milk consumption. This underestimation makes case control studies likely to conclude that milk consumption is protective against fractures, regardless of the true effect. Because of these biases, case control studies are viewed with considerable scepticism. However, case control studies have the potential strength that because relatively few people are involved it is possible to carry out very thorough evaluations of diet. This can make them more likely to detect a true effect than studies that rely entirely on questionnaires to evaluate diet and may therefore show a higher measurement error that will tend to obscure real effects.

Prospective studies overcome the above biases of case control studies. In a prospective study a sample of the population (cohort) who are free from the diseases of interest at the start of the study is examined. As there are no cases at this stage selection and recall biases do not come into play. To provide increased confidence that pre-existing disease is not affecting the initial observations it is common practice to eliminate cases arising within a few years of the start of the study from the subsequent analysis. If a significant relative risk is observed in a prospective study that has followed these precautions then it is more likely to be a true association than one obtained from a case control study. A weakness of prospective studies is the difficulty of accurately characterising the population studied without excessive cost as such studies must include large numbers of people to ensure a reasonable number of cases will arise in the course of the study. Inaccurate characterisation may lead to true effects being missed.

Both case control and prospective studies are subject to confounding - where an apparent effect from one characteristic is actually due to another related characteristic. If certain characteristics are themselves statistically associated then it can be difficult to separate out the causal characteristic even if all the potentially relevant characteristics are measured. If a relevant characteristic is not measured or not analysed then a false association may arise. A key example of this is that intake of most foods is strongly associated with total energy intake, which in turn may be associated with obesity or physical activity. It is therefore standard practice to use statistical methods to adjust for energy intake so that this effect is removed from the analysis. Age must also be adjusted for, as it is an important factor in every disease. Other known risk factors for a particular disease should be measured and adjusted for. Risk of breast cancer rises with earlier age of menarche, later age at first live birth, later age at menopause, increased BMI (weight in kg divided by height in metres squared) in postmenopausal women, and increased height. It would be normal to present results after adjustment for these factors. As some of these factors are themselves dependent on diet it can be useful to present results with adjustment for age and energy only, as well as fully adjusted results. If the partially and fully adjusted risks are very different then particularly careful analysis is required to try to separate the influence of diet.

What is the evidence on milk and breast cancer?

If we compare breast cancer rates and milk consumption between different countries we find that there is a clear association - the countries consuming large amounts of dairy have higher rates of breast cancer than the countries 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 [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. The other two found no significant effect. 5 case control studies considered cheese. 3 found a significant adverse effect, 1 found a significant protective effect and 1 found no significant effect. 3 of the results on cheese came from studies also reporting results on milk (above), and 2 came from separate studies. Only 1 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). 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.

The above review missed the paper by Ursin et al, 1990 [3]. This paper was a prospective study of milk consumption in Norway and cancer risk. Individuals consuming two or more glasses of milk a day vs less than 1 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.

Toniolo et al, 1994 [4] present a prospective study from New York. This study showed an almost significant protective association of high dairy (milk and other dairy products were not separated) intake (RR=0.59 upper quintile vs lower quintile) with breast cancer. Adjustments were made for height, BMI, age at menarche, age at first term birth, number of full-term pregnancies, family history of breast cancer and several other factors.

Gaard et al, 1995 [5] present another Norwegian prospective study. Adjustments were made for age, energy, smoking, height and BMI but not for reproductive history or age at menarche, first child-birth or menopause. 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 energy, age, smoking, BMI, number of childbirths, occupation and geographic area, but not height or age at menarche, first childbirth or menopause. The adjusted relative risk for the highest compared to 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. There was no association between breast cancer and fermented milk consumption. 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, maternal history of breast cancer, age at menarche, number of children, age at first birth, use of oral contraceptives, BMI, years of education, level of physical activity and alcohol consumption, but not for height, age at menopause, or energy intake. The dietary questionnaire was insufficiently comprehensive to evaluate energy consumption. 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 fat 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). Only about 11% of the overall group was in the low milk category (36 cases of breast cancer against an expected 29, based on the group average risk) and about 7% of the group was in the high milk category (13 cases of breast cancer against an expected 20, based on the group 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 as carrying more weight than the many other case controls discussed earlier, that 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 contrary 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 such effect as does exist could prove to be adverse rather than beneficial. In attempting to use the Hjartaker study to promote the use of milk, the dairy industry shows that its concern is with its customers' wallets and not with their health.

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 milk and it is therefore opportune for the dairy and beef suppliers to emphasise CLA.

Calcium is the best thing milk has going for it. It may be no coincidence that proposals for mandatory calcium fortification of basic foodstuffs such as bread in the USA are not being pursued despite calls for this from leading researchers on the role of calcium [11].

Reducing breast cancer risk and promoting health

Milk is not the answer to breast cancer. Calcium and vitamin D may have a role to play. However, even these do not rank in the established dietary recommendations for reducing breast cancer incidence.

Walter Willett, a nutritional epidemiologist with Harvard School of Public Health and the American Institute for Cancer Research makes the following recommendations for avoiding breast cancer [12]. Avoid weight gain in adult life. Limit alcohol intake to moderate amounts and ensure you get sufficient folate (or supplementary folic acid) to avoid the excess risk associated with even moderate intakes of alcohol. Replace saturated fat with olive oil.

There is no mention of milk, though high fat dairy products are implicitly rejected in favour of olive oil, as they are a major source of saturated 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 hence reduce risk of heart disease. 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.

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.

Willett takes a strongly sceptical stance towards soy products due to evidence that they can have estrogenic (cancer promoting) effects on breast cancer tissue in postmenopausal women. He suggests, "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 (1 cup of soya milk or 100g (4oz) of tofu) per day, consumed with apparent lack of adverse effects. This consumption is however from an early age and this may be relevant. Overall it seems prudent not to significantly exceed Japanese levels of consumption.

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 [13]. No details of the particular diet characteristics of this group are given, but this observation adds to reasons for caution over very low fat diets.

A review paper by Timothy Key and Naomi Allen [14] provides an alternative summary of risk factors, concluding that obesity in post-menopausal 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 [15]. 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, 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, there is stronger evidence in relation to heart disease, diabetes and diverticular disease and that 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, he 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. Good information supports health. Pass it around.

Appendix: IGF-1, cancer and milk

CLA, calcium and vitamin D have been hypothesised to give rise to a beneficial effect of diary products. Insulin-like growth factor 1 (IGF-1) has been hypothesised to give rise to an adverse effect. High levels of IGF-1 in blood, or more specifically, a high ratio of IGF-1 to IGFBP-3 (Insulin-like growth factor binding protein 3), have been associated with notably increased risk of premenopausal breast cancer, prostate cancer, colorectal cancer and lung cancer. All cow's milk contains some IGF-1 and milk from cows treated with BST contains increased amounts. Adding dairy products to the diet causes an increase in IGF-1 levels, though this may well be due to the protein content of the milk rather than the IGF-1 content. This leads to the hypothesis that dairy products may increase risk of cancer, including breast cancer, by increasing circulating levels of IGF-1 in the body. Until very recently no study had looked at the effect of dairy products on the IGF-1 to IGFBP-3 ratio or the interaction of potential beneficial effects from calcium or vitamin-D with potential adverse effects due to elevated IGF-1.

A study published on September 15, 2001, does just that [16]. A prospective study was carried out on milk intake and colorectal cancer, including evaluation of the effect of milk intake on IGF-1 and IGFBP-3. Low fat milk, calcium from milk and calcium from dairy foods showed a statistically significant 10-16% increase in IGF-1 and a 6-7.5% increase in the IGF-1:IGFBP-3 ratio comparing the upper tertile in terms of milk consumption to the lower tertile. Poultry and fish showed similar associations but these did not reach statistical significance. Other forms of dairy products including milk, hard cheese and ice cream also did not show significant associations. The authors of the study leave as an open question whether the increase in IGF-1 in response to low fat milk was due to conventional dietary factors, particularly protein, or to absorption of IGF-1 from the milk. I have written to the authors to try to clarify this question as their analysis of this issue appears incomplete.

Intake of low fat milk showed a moderate, but non-significant, protective association with colorectal cancer and elevated IGF-1/IGFBP-3 showed a statistically significant adverse effect (RR=1.8 upper tertile to lower tertile). Among the upper tertile of IGF-1/IGFBP-3, intake of low-fat milk showed a significant protective association with colorectal cancer (RR=0.78 for moderate milk drinkers and 0.39 for frequent milk drinkers). In men with lower IGF-1:IGFBP-3 ratio no effect of low-fat milk consumption was observed. Among frequent milk drinkers there was no association between IGF-1:IGFBP-3 ratio and colorectal cancer risk (RR=1.03), while in non-drinkers of low-fat milk there was a relative risk of 3.05 for those in the highest tertile by IGF-1:IGFBP-3 ratio.

The authors conclude, "this prospective study shows a protective effect of dietary calcium on colorectal cancer among men with a high IGF-1/IGFBP-3 despite the moderate positive influence of milk or dairy food intake on circulating IGF-1 levels." No doubt the dairy industry will try to paint this study as another great triumph, but I see a different message. Calcium is beneficial. Elevated IGF-1 is not. For breast cancer the evidence suggests the various effects of milk are in approximate balance, though an adverse effect cannot be ruled out. For prostate cancer the net effect of milk appears to be notably adverse. In [17] Willett notes "one of the most consistent observations has been an association between the consumption of dairy products and prostate cancer incidence or mortality." Get a good calcium intake from something other than dairy and avoid excessive protein intake (one gram of protein per kilogram of body weight per day is adequate). You have nothing to lose.

References

[1] Meera Jain, Nutrition Research, 1998; 18:905-307, Dairy foods, dairy fats and cancer: a review of the epidemiological evidence

[2] Boyd et al, British Journal of Cancer, 1993, 68:627-636, A meta-analysis of studies of dietary fat and breast cancer risk.

[3] Ursin et al, British Journal of Cancer, 1990, 61:454-459, Milk consumption and cancer incidence: a Norwegian prospective study

[4] Toniolo et al, Epidemiology, 1994, 5: 391-397, Consumption of meat, animal products, protein and fat and risk of breast cancer: a prospective cohort study in New York

[5] Gaard et al, International Journal of Cancer, 1995; 63: 13-17, Dietary fat and the risk of breast cancer: a prospective study of 25,892 Norwegian women

[6] Knekt et al, British Journal of Cancer, 1996; 73: 687-691, Intake of dairy products and the risk of breast cancer

[7] Hjartaker et al, International Journal of Cancer, 2001; 93:888-893, Childhood and adult milk consumption and risk of premenopausal breast cancer in a cohort of 48,844 women - the Norwegian women and cancer study.

[8] Aro et al, Nutrition and Cancer, 2000; 38:151-157, Inverse association between dietary and serum conjugated linoleic acid and risk of breast cancer in postmenopausal women

[9] John et al, Cancer Epidemiology, Biomarkers and Prevention, 1999; 8:399-406, Vitamin D and Breast Cancer Risk: The NHANES I Epidemiologic Follow-up study

[10] Lipkin et al, Journal of the American College of Nutrition, 1999; 18:392S-397S, Vitamin D, Calcium and Prevention of Breast Cancer: a review.

[11] Robert Heaney, Journal of Clinical Endocrinology and metabolism, 2000; 85:3009-10, Editorial: More evidence and still no action

[12] Walter Willett, Journal of Internal Medicine, 2001; 249:395-411, Diet and Breast Cancer

[13] Hunter et al, New England Journal of Medicine, 1996; 334: 356-361, Cohort studies of fat intake and the risk of breast cancer - a pooled analysis

[14] Key et al, The Breast, 2001; 10 (Supplement 3): 9-13, Nutrition and breast cancer

[15] Walter Willett, Cancer epidemiology, biomarkers and prevention, 2001; 10: 3-8, Diet and Cancer: One View at the Start of the Millennium

[16] Ma et al, Journal of the National Cancer Institute, 2001; 93:1330-Milk Intake, Circulating Levels of Insulin-Like growth Factor-I and Risk of Colorectal Cancer in Men

[17] Walter Willett, American Journal of Clinical Nutrition, 1999; 70:434S-438S, Convergence of philosophy and science: the Third International Congress on Vegetarian Nutrition