Alcohol is a risk factor for breast cancer in women.

A study of more than one million middle-aged British women concluded that each daily alcoholic beverage increases the incidence of breast cancer by 11 cases per 1000 women. This means that among a group of 1000 women who have one drink per day, they will have 11 extra cases of breast cancer when compared to a group of women who drink less than one alcoholic beverage per week; a group of 1000 women who have four drinks per day will have an extra 44 cases of breast cancer compared to non-drinkers. One or two drinks each day increases the relative risk to 150% of normal, and six drinks per day increases the risk to 330% of normal. Approximately 6% of breast cancers reported in the UK are due to women drinking alcohol.

The primary mechanism through which alcohol causes breast cancer is increased estrogen levels.

Fat intake

Dietary influences have been examined since decades with conflicting results and so far failed to confirm any significant dependency. One recent study suggests that low-fat diets may significantly decrease the risk of breast cancer as well as the recurrence of breast cancer. Another study showed no contribution of dietary fat intake on the incidence of breast cancer in over 300,000 women. A randomized controlled study of the consequences of a low-fat diet, the Women's Health Initiative, failed to show a statistically significant reduction in breast cancer incidence in the group assigned to a low-fat diet, although the authors did find evidence of a benefit in the subgoup of women who followed the low-fat diet in a strict manner. A prospective cohort study, the Nurses' Health Study II, found increased breast cancer incidence in premenopausal women only, with higher intake of animal fat, but not vegetable fat. Taken as a whole, these results point to a possible association between dietary fat intake and breast cancer incidence, though these interactions are hard to measure in large groups of women.

Specific dietary fatty acids

Although many claims have been made in popular literature there is no solid evidence linking specific fats to breast cancer.

A study published in 2001 found higher levels of monounsaturated fatty acids MUFAs (especially oleic acid) in the erythrocyte membranes of postmenopausal women who developed breast cancer.

That same study discussed that a diet high in MUFAs is not the major determinant of erythrocyte membrane MUFAs, where most oleic acid in mammalian tissue is derived from the saturated stearic acid residue. Where key conversion is controlled by the Delta9-desaturase, which also regulates the transformation of the other common saturated fatty acids (SFAs) (myristic and palmitic). The study discussed that fat content of the diet has an important effect on Delta9-d activity, while high levels of SFAs increase Delta9-d activity by twofold to threefold, whereas polyunsaturated fatty acids (PUFAs) decrease.

That conclusion was partially contradicted by a latter study, which showed a direct relation between very high consumption of omega-6 (PUFAs) and breast cancer in postmenopausal women.

Phytoestrogens

Phytoestrogens#Health Risks and Benefits

Phytoestrogens have been extensively studied in animal and human in-vitro and epidemiological studies. Research failed to establish any noticeable benefit and some phytoestrogens may present a breast cancer risk.

The literature support the following conclusions:

1. Plant estrogen intake in early adolescence may protect against breast cancer later in life.

2. The potential risks of isoflavones on breast tissue in women at high risk for breast cancer is still unclear.

Vitamin D

Vitamin D is related to reduced risk of breast cancer and disease prognosis. A 2011 study done at the University of Rochester Medical Center found that low vitamin D levels among women with breast cancer correlate with more aggressive tumors and poorer prognosis. The study associated sub-optimal vitamin D levels with poor scores on every major biological marker that helps physicians predict a patient’s breast cancer outcome. The lead researcher stated, “Based on these results, doctors should strongly consider monitoring vitamin D levels among breast cancer patients and correcting them as needed.

Brassica vegetables

In a study published in the Journal of the American Medical Association, biomedical investigators found that Brassicas vegetable intake (broccoli, cauliflower, cabbage, kale and Brussels sprouts) was inversely related to breast cancer development. The relative risk among women in the highest decile of Brassica vegetable consumption (median, 1.5 servings per day) compared to the lowest decile (virtually no consumption) was 0.58. That is, women who consumed around 1.5 servings of Brassica vegetables per day had 42% less risk of developing breast cancer than those who consumed virtually none.

Country diet

A significant environmental effect is likely responsible for the different rates of breast cancer incidence between countries with different dietary customs. Researchers have long measured that breast cancer rates in an immigrant population soon come to resemble the rates of the host country after a few generations. The reason for this is speculated to be immigrant uptake of the host country diet. The prototypical example of this phenomenon is the changing rate of breast cancer after the arrival of Japanese immigrants to America.

Mushrooms

In 2009, a case-control study of the eating habits of 2,018 women suggested that women who consumed mushrooms had an approximately 50% lower incidence of breast cancer. Women who consumed mushrooms and green tea had a 90% lower incidence of breast cancer. A case control study of 362 Korean women also reported an association between mushroom consumption and decreased risk of breast cancer.

Iodine deficiency

The protective effects of iodine on breast cancer have been postulated from epidemiologic evidence and described in animal models.

Obesity

Gaining weight after menopause can increase a woman's risk. A recent study found that putting on 9.9 kg (22 lbs) after menopause increased the risk of developing breast cancer by 18%.

Hormones

Persistently increased blood levels of estrogen are associated with an increased risk of breast cancer, as are increased levels of the androgens androstenedione and testosterone (which can be directly converted by aromatase to the estrogens estrone and estradiol, respectively). Increased blood levels of progesterone are associated with a decreased risk of breast cancer in premenopausal women. A number of circumstances which increase exposure to endogenous estrogens including not having children, delaying first childbirth, not breastfeeding, early menarche (the first menstrual period) and late menopause are suspected of increasing lifetime risk for developing breast cancer.

However, not only sex hormones, but also insulin levels are positively associated with the risk of breast cancer.

Pregnancy, childbearing and breastfeeding

Lower age of first childbirth, compared to the average age of 24, having more children (about 7% lowered risk per child), and breastfeeding (4.3% per breastfeeding year, with an average relative risk around 0.7) have all been correlated to lowered breast cancer risk in large studies. Women who give birth and breast-feed by the age of 20 may have even greater protection. In contrast, for instance, having the first live birth after age 30 doubles the risk compared to having first live birth at age less than 25. Never having children triples the risk.

Hormonal contraception

Hormonal contraceptives may produce a slight increase in the risk of breast cancer diagnosis among current and recent users, but this appears to be a short-term effect. In 1996 the largest collaborative reanalysis of individual data on over 150,000 women in 54 studies of breast cancer found a relative risk (RR) of 1.24 of breast cancer diagnosis among current combined oral contraceptive pill users; 10 or more years after stopping, no difference was seen. Further, the cancers diagnosed in women who had ever used hormonal contraceptives were less advanced than those in nonusers, raising the possibility that the small excess among users was due to increased detection. The relative risk of breast cancer diagnosis associated with current and recent use of hormonal contraceptives did not appear to vary with family history of breast cancer. Some studies have suggested that women who began using hormonal contraceptives before the age of 20 or before their first full-term pregnancy are at increased risk for breast cancer, but it is not clear how much of the risk stems from early age at first use, and how much stems from use before the first full-term pregnancy.

Hormone replacement therapy

Data exist from both observational and randomized clinical trials regarding the association between menopausal hormone replacement therapy (menopausal HRT) and breast cancer. The largest meta-analysis (1997) of data from 51 observational studies, indicated a relative risk of breast cancer of 1.35 for women who had used HRT for 5 or more years after menopause. The estrogen-plus-progestin arm of the Women's Health Initiative (WHI), a randomized controlled trial, which randomized more than 16,000 postmenopausal women to receive combined hormone therapy or placebo, was halted early (2002) because health risks exceeded benefits. One of the adverse outcomes prompting closure was a significant increase in both total and invasive breast cancers (hazard ratio = 1.24) in women randomized to receive estrogen and progestin for an average of 5 years. HRT-related breast cancers had adverse prognostic characteristics (more advanced stages and larger tumors) compared with cancers occurring in the placebo group, and HRT was also associated with a substantial increase in abnormal mammograms. Short-term use of hormones for treatment of menopausal symptoms appears to confer little or no breast cancer risk. A correlation was found between the use of hormonal contraceptives and subsequent reliance on hormone replacement therapy.