Reproductive factors are associated with the risk of breast cancer

Full-term pregnancies reduce the risk of breast cancer; this protection is greater the younger one is when first giving birth. Breastfeeding further reduces breast cancer risk, with parous women who also breastfeed their children benefiting from an additional 10% reduction in risk. Early age at first menstruation and late age at menopause increase the probability of developing breast cancer (Collaborative Group on Hormonal Factors in Breast Cancer, 2002).

Exogenous hormones increase breast cancer risk

Users of oral contraceptives that combine estrogens and progestogen experience a small increase in risk of breast cancer during the years of usage. Since the excess risk is present only during use, it does not persist after a few years of cessation, and since the users are young women, it is estimated that the associated excess risk has a small impact on the overall incidence of the disease (IARC, 2007).

The other important source of exogenous hormones is HRT in post-menopausal women. Several large epidemiological studies have shown that HRT increases the risk of breast cancer in users. The excess risk declines after cessation and reaches that of never users about 5 to 9 years later. The relative risks of being diagnosed breast cancer while taking combined HRT was estimated at 1.2 compared with never users. The relative risk was back to 1.0 (indicating no excess risk) 10 years after use ceased (Collaborative Group on Hormonal Factors in Breast Cancer, 1997). The excess risk for users is relatively small but it applies to the age groups in which breast cancer incidence rates are high. The additional cases caused may therefore be significant (IARC, 2007).

Alcohol consumption and overweight increase the risk for breast cancer

Wide differences in nutritional habits between high-risk (e.g. Europe) and low-risk populations (e.g. Japan and India) have supported the idea that some components of diet could explain differences in breast cancer incidence. However, the recent expert review on Nutrition and Cancer by the World Cancer Research Fund (WCRF/AICR, 2007) identified only two nutritional factors unequivocally linked to the risk of breast cancer: alcohol consumption and overweight.

The WCRF/AICR review estimated a 10% increase in risk for every 10 g of ethanol consumed per day. In postmenopausal women, the incidence of breast cancer increases by 13% for every increase of 5 BMI units. Additionally, postmenopausal women who are obese (BMI> 30) have a 30% greater probability to develop the disease compared to those with normal weight. Conversely, overweight in pre-menopause confers a 15% reduced incidence for every increase by 5 BMI units. It should be noted that breast cancer incidence increases with age so that two thirds of the cases occur at postmenopausal ages. Therefore, the additional cases related to overweight in postmenopausal women, outweighs those prevented in younger women.

Regular intense physical activity probably contributes to reducing the risk of breast cancer; it certainly helps maintain a healthy body weight. The WCRF/AICR concludes, however, that the evidence on any relationship between other food items/groups (cereals, vegetables, fruits, meat, etc.), macronutrients (proteins, carbohydrates, fats and oils, etc.) or a variety of micro-nutrients and anti-oxidants and breast cancer is inconclusive.

Host factors, including genetic susceptibility, are important

Family history:

Women who have had a first-degree family member (parents, siblings or children) diagnosed with breast cancer, are nearly two times as likely to develop breast cancer themselves. If two or more first-degree family members have developed breast cancer, the risk is increased by three to four times (Collaborative Group on Hormonal Factors in Breast Cancer, 2001). The occurrence of several cases in a family, diagnosed at a young age or in both breasts may indicate the presence of a genetic predisposition in the family and therefore imply a high risk of the disease.

Genetic susceptibility:

40% to 85% of women carriers of certain mutations in the BRCA1 or BRCA2 genes develop breast cancer during their life. Luckily genetic susceptibility is rare in most populations. Therefore, these conditions explain only about 5% of breast cancer cases (Ferla et al., 2007).

Multiple tumours of the breast:

Women who have had breast cancer are three to four times more likely to develop cancer in their other breast. This means that 15 to 20% of women who survive breast cancer are diagnosed as having breast cancer for a second time within the following 20 years.

Benign breast tumours:

Benign breast tumours do not spread to other organs; they are removed surgically and have no clinical consequences. Women who have had a benign lump removed have an increased risk of developing a malignant cancer later, in particular when the benign tumour had certain microscopic features called Atypical Hyperplasia (AH). 4-8% of benign lumps, depending on the woman’s age, present AH features (Dupont & Page, 1985; Carter et al., 1988; Hartmann et al., 2005).