Breast cancer

Background

Breast cancer is the most common type of cancer for women in Australia and New Zealand. In Australia, over 19,866 women were diagnosed with breast cancer in 2021, and 3,102 died from breast cancer. In New Zealand, on average 3,000 women are diagnosed each year and nine women will hear the news today that they have breast cancer.

The incidence of breast cancer in men is about 0.8% the rate in women. There are approximately 30 types of breast cancer, with 85% of the invasive tumors being invasive ductal carcinoma. Other histologic types are invasive lobular, medullary, mucinous, micropapillary, inflammatory, and tubular carcinomas.

Australia and New Zealand both have a high breast cancer incidence rates [Figure 1], 94.5 per 100,000 and 92.6 per 100,000 respectively.

Lifestyle factors, especially the standard Western diet, appear to play a major role in the development and progression of breast cancer. Important contributors (shared by what is often considered to be the male cancer equivalent, prostate cancer) are the effect on hormone levels and the impact on overweight and obesity for postmenopausal women. Breast cancer rates are substantially lower in women in countries that do not traditionally consume the standard Western diet.

Higher incidence of breast cancer is also attributed to consumption of alcohol, exposure to chemicals, changing reproductive patterns, such as earlier age at menarche, later age at first childbirth, lower parity and shorter duration of breastfeeding.

Nutritional Considerations

The following information is kindly reproduced with the permission of Dr Neal Barnard, Physicians Committee For Responsible Medicine.[1] Dr Barnard is an International Advisor for Doctors For Nutrition.

Nutritional factors may reduce both the risk of developing cancer and the likelihood of cancer progression and mortality after diagnosis. Both are discussed below.

Nutrition and Risk Reduction

Western diets high in meat, high-fat dairy products, fat (particularly saturated and omega-6 fatty acids), processed foods, and simple sugars, while simultaneously low in fruits, vegetables, legumes, whole grains, and fibre, are linked to higher breast cancer risk.[2],[3] Breast cancer is less prevalent in countries where simple plant-based foods are staples.[4],[5],[6] In North America it is has been demonstrated that the incidence increases successively in 1st-and 2nd-generation immigrants in proportion to the degree to which they adopt a Western diet and lifestyle.[7],[8],[9]

This risk may be explained in part by the increase in estrogen activity resulting from the conversion of androgens to estrogens in adipose tissue and the increase in circulating estrogens resulting from a fatty, low-fibre diet.[10],[11] In controlled studies, high-fibre, low-fat diets have been shown to significantly decrease estradiol, estrone, and testosterone concentrations.[12],[13],[14],[15]

Dietary factors may also influence the age of menarche, which can increase lifetime estrogen exposure.[16]

Animal fat and animal protein intake are linked to elevated levels of insulin-like growth factor-1 (IGF-1).[17] As a potent growth promoter, IGF-1 may be associated with other established risk factors for breast cancer (e.g., breast density), and women with high circulating levels of IGF-1 have 38% more estrogen-driven breast cancers than those with low IGF-1.[18],[19]

Specific dietary factors and dietary patterns under investigation for a potentially helpful role are described below.[20],[21]

Healthy body weight:

For postmenopausal women an elevated BMI is associated with a significantly higher risk of breast cancer.[24],[25],[26] Elevated estrogen levels, presumably due to peripheral aromatization of androstenedione to estrone (and, to a lesser extent, testosterone to estradiol) in adipose tissue, may help explain this increased risk.[27]

Avoiding alcohol:

Alcohol consumption increases breast cancer risk.[28] For premenopausal women, each glass of wine consumed per day increases the chances of developing breast cancer by about 7%. Two glasses a day increase a woman’s risk by 14%, and so on. The same is true for other alcoholic beverages. A 350ml bottle or can of beer, a 150ml glass of wine, and a 45ml shot of liquor all have about the same alcohol content. For older women, the effect is amplified. For each drink a postmenopausal woman consumes daily, her risk of breast cancer increases by about 13%.[22] Another meta-analysis found that relative to non-drinkers and occasional drinkers, heavy drinkers had a 61% higher risk of breast cancer.[29]

Limiting or avoiding meat:

A number of studies, including the Nurses’ Health Study II and the UK Women’s Cohort Study, have found significant associations between meat intake and breast cancer risk, with a 64% greater risk for postmenopausal breast cancer when women who consumed processed meat (bacon, sausage, ham, deli meats) were compared with those who did not.[30],[31] The NIH-AARP study involving nearly 200,000 women found a 25% higher risk for breast cancer in those eating the most red meat, compared with those eating the least.[32] When all published studies, which vary in quality, are combined in meta-analyses, results have been less straightforward.[33],[34],[35],[36]

Minimizing meat intake appears to be especially important for teens, as mammary cells develop and divide rapidly during adolescence. A study of the Nurses’ Health Study II cohort found that women who ate the most red meat in adolescence had a 42% higher risk of premenopausal, although not postmenopausal, breast cancer later in life, relative to those who ate the least meat.[37]

It is not yet clear whether these associations reflect the effect of meat-based diets on hormone concentrations, the presence of carcinogens (e.g., heterocyclic amines, polycyclic aromatic hydrocarbons), or other factors. Conversely, plant-derived foods appear to reduce breast cancer risk (see below).

Avoiding dairy products:

A possible role for dairy consumption in breast cancer risk has been suggested by international comparisons showing strong correlations between dairy intake and breast cancer risk and by the marked increase in breast cancer incidence in Japan following that country’s massive increase in dairy consumption toward the end of the 20th century.[38],[39] In the Adventist Health Study-2, including 52,795 North American women, higher dairy milk intake was associated with higher risk of breast cancer, with a 50% increased risk among those with the highest, compared with lowest, milk intake. The results were similar for full-fat and reduced-fat milk products. Soy milk consumption was not associated with increased risk. Biological plausibility comes from the presence of estrogens in cow’s milk products, which, although modest in comparison with endogenous production, nonetheless appears to be sufficient to alter a variety of physiological processes.[40]

Emphasizing vegetables:

Vegetables have bioactive components, including folate and carotenoids, which may confer protection against breast cancer. Folate may be especially important in women who consume alcohol.[41] Foods that contain folate (leafy green vegetables, legumes, oranges) have been found more effective than folic acid supplements, perhaps due to the presence of other protective factors (e.g., fibre, vitamin C, phytochemicals). The European Prospective Investigation into Cancer and Nutrition (EPIC) study concluded that higher vegetable intakes were related to a 13% lower breast cancer risk compared with the lowest level of consumption, an effect that was more demonstrable for ER-/PR- cancers than ER+/PR+ ones.[42] These effects may be due to vegetables high in carotenoids, higher concentrations of which were associated in the Nurses’ Health Study with 18-28% lower risk of breast cancer and inversely associated with breast cancer recurrence and mortality from this disease, over a follow-up period of 20 years.[43]

Consuming soy products:

Consumption of legumes, particularly soy products, that are high in isoflavones and lignans is also associated with lower risk for breast cancer, an effect that is greater if soy foods are consumed in abundance during adolescence.[44] A meta-analysis of soy intake and breast cancer risk found a roughly 35% lower risk for ER+/PR+ cancers and a roughly 40% lower risk for ER-/PR- types, as well as significant decreases in breast cancer recurrence and mortality in high versus low soy consumers. These effects have been attributed in part to inhibition of vascular endothelial growth factor (VEGF), pro-apoptotic effects, inhibition of tyrosine kinase, induction of tumor suppressor proteins, and down-regulation of HER2, among other mechanisms.[45] These findings are particularly important given the popular myth that soy products may increase breast cancer risk; evidence strongly suggests a significant preventive effect. In this regard, it is noteworthy that, in contrast to estradiol which binds to both estrogen receptor-α and estrogen receptor-β, soy isoflavones have greater affinity for estrogen receptor-β.[46]

Further evidence of the benefits of legumes was noted in the Nurses’ Health Study II, in which eating beans or lentils twice per week was associated with a 24% lower risk, compared with consuming those foods less than once per month.[47] Higher intake of plant protein and nuts during adolescence has also been associated with significantly lower breast cancer risk during adulthood when compared with lower intake.[48]

Reducing fat:

Many studies have examined the effect of fat intake on breast cancer risk. The greatest risk appears to come from saturated fat and animal fat.[49],[50],[51],[52] Vegetable oils, such as olive oil, have not been shown to increase breast cancer risk.[53],[54],[55],[56] High-fat diets in general (not just saturated fat), however, may promote weight gain, and this weight gain elevates postmenopausal breast cancer risk, as noted above.[57]

The Women’s Health Initiative Dietary Modification Trial, which included 48,835 women who were cancer-free at baseline, tested a diet that aimed to reduce fat intake to 20% of energy and to increase vegetable and fruit consumption. Participants were consuming more fat than the US average at study baseline. The actual fat intake achieved by study participants averaged 24% of energy at 1 year and drifted upward toward baseline values by the intervention’s end after a median of 8.5 years. Even so, after 17.7 years of follow-up, deaths after breast cancer were 15% lower in the dietary intervention group.[58]

Increasing fibre:

Dietary fibre intake is inversely associated with the risk for breast cancer.[59] Dietary fibre interrupts the enterohepatic circulation of estrogen by binding unconjugated estrogens in the gastrointestinal tract.[60] High-fibre, low-fat diets reduce serum estradiol, which is known to be associated with breast cancer risk.[61] Fibre is abundant in plants and absent from animal products.

High-fibre diets help keep blood glucose levels within normal limits and lower the risk for adult-onset diabetes, both of which have been related to increased breast cancer risk.[62]

Managing blood glucose:

Elevated fasting glucose levels are associated with breast cancer risk in non-diabetic women.[7],[63] Postmenopausal women with diabetes have been shown to have a slightly greater risk for breast cancer, compared with those who did not have diabetes.[64] Fat buildup in muscle and liver cells contributes to insulin resistance and the risk of diabetes.

Vitamin D adequacy:

In a pooled analysis of 2 studies examining vitamin D and risk of breast cancer, women whose serum 25(OH)D levels were in the lowest quintile (< 13 ng/dL) had twice the odds of developing breast cancer as those whose levels were in the highest quintile (≥ 52 ng/dL). An inverse dose-response relationship was detected, suggesting that the lower the circulating vitamin D level, the higher the risk of breast cancer.[65]

Physicians Committee for Responsible Medicine: breast cancer topic summary, resource kit based on a four-pronged lifestyle approach to reduce risk, and links to further articles.