Breastfeeding and breast cancer: a protective effect?

Research conducted by the PNNS working groups concluded that [4]: 

"The reduction in breast cancer risk through breastfeeding is considered convincing, both before and after the menopause."

ANSES also concludes that regarding breastfeeding, "the level of evidence is convincing for breast cancer" [5].
 

A meta-analysis (i.e. an article that brings together and examines several scientific studies) combined data from nearly 100 articles that studied the combination of between breastfeeding and the risk of breast cancer [6]. The authors report that having previously breastfed was associated with a 22% reduction in the risk of breast carcinoma. 
 

But if parity is removed from the equation (the number of pregnancies), this risk falls to just 8%.

Breastfeeding does have an effect, but a much more modest one.
 

The duration of breastfeeding appears to be an important factor : the reduction in risk appears to be greater the longer the duration of breastfeeding. Studies on the duration of breastfeeding report that breastfeeding for less than 6 months, for 6-12 months, and for more than 12 months were associated with a reduction in the risk of breast carcinoma of 7%, 9%, and 23% respectively. 

A study analysed the results of 47 epidemiological studies in 30 countries, including more than 50,000 women with breast cancer and nearly 100,000 unaffected women. The researchers were able to conclude after adjusting for factors including the number of pregnancies, that the risk of being affected by invasive breast cancer decreased by 4.3% for every 12 months of breastfeeding [7]. An effect, then, but a relatively modest one.

The protective role of breastfeeding appears modest and may be linked to specific subgroups of women: premenopausal women, BRCA1 mutation carriers, etc. 
 

ESPECIALLY BEFORE THE MENOPAUSE 

Breastfeeding appears to be most protective against cancers occurring before the menopause, though studies show conflicting results.
 

ESPECIALLY IN CASES OF FAMILY HISTORY

It is important to take family history of breast cancer into account when assessing risk. Indeed, 5 to 10% of breast cancers fall within the context of a hereditary predisposition, linked to a mutation in a predisposition gene [8]. The most commonly implicated genes are: 

BRCA1 (for BReast Cancer 1: breast cancer gene 1) 

BRCA2 (for BReast Cancer 2: breast cancer gene 2). 

A woman carrying a predisposition gene has a 50 to 70% risk of developing breast cancer. 

OTHER RISK FACTORS FOR BREAST CANCER 

Finally, it should not be forgotten that other risk factors exist, such as obesity, alcohol consumption, smoking and exposure to ionising radiation [11].

A meta-analysis involving 21,941 patients with breast cancer and 864,177 controls (i.e. women who did not develop breast cancer) was conducted [12]. In total, 11 studies evaluated the association between breastfeeding and the risk of developing breast cancer according to tumour subtype. 

Hormone-dependent cancers
 

Some breast cancers are sensitive to natural female hormones: oestrogens and progesterone. Breast cancer cells have receptors on the surface of the cell membrane that can "capture" specific hormones [13].
 

Tumours are distinguished by: 

- Oestrogen receptor positive (ER+). The cells of this type of breast cancer have receptors that allow them to use the hormone oestrogen to grow. 

- Progesterone receptor positive (PR+). This type of breast cancer is sensitive to progesterone, and the cells have receptors that allow them to use this hormone to grow. 

- Positive hormonal status (HR+). Hormonal status is described as positive if the cell presents one or both receptors (ER/PR).

- Negative hormonal status (HR-). Hormonal status is described as negative when no hormone receptors are detected. These patients will not be eligible for "anti-hormonal" treatment. 

In a meta-analysis of women with HR+ tumour status, a 23% reduction, (therefore significant) in the risk of developing a breast cancer subtype was observed in patients who had breastfed [14].

Another meta-analysis of 10 studies observed that each birth reduced the risk of HR+ cancer (with ER+ and PR+) by 11% [15]. 
 

HER2 cancer

In all breast cancers, the expression of the HER2 oncogene is assessed. 15 to 20% of breast cancers present an amplification of the HER2 oncogene, promoting tumour growth.  
 

No correlation between the presence of this factor and breastfeeding has been described. 

Characteristics of triple-negative breast cancer
 

Triple-negative breast cancers [16] do not have hormone receptors and do not overproduce the HER2 protein. 

A 2016 meta-analysis reports that there is a 21% reduction in the risk of developing a triple-negative breast cancer subtype in women who had previously breastfed [17].  

However, the results of studies on the effect of breastfeeding on triple-negative breast cancer are contradictory. Some studies find a reduction in risk associated with breastfeeding whilst others find no link between the two. Most studies showing a link included both pre-menopausal and post-menopausal women, without distinguishing by menopausal status, which could influence the results obtained [18]. 

The hypothesis that breastfeeding causally protects against triple-negative breast cancer is one that is not yet supported by biological evidence, and it is possible that breastfeeding may in fact have no effect on the occurrence of triple-negative breast cancer [19]. 

From a biological standpoint, there are several explanations for why breastfeeding appears to reduce the risk of breast cancer and seems to be highly beneficial for women's health. The most likely of these are the hormonal changes that occur during pregnancy and lactation. 
 

These include:

- the reduction and elimination of oestrogens through breast fluid, 

lower oestrogen levels during breastfeeding (due to the period of amenorrhoea that breastfeeding can cause, although this effect was not found in the American nurses' study; furthermore, triple-negative and BRCA1 cancers are generally not hormone-sensitive)

- the excretion of carcinogenic agents through breast tissue during breastfeeding (although there is very limited data on this subject)

- and physical changes to mammary epithelial cells, which tend to differentiate and thus delay the risk [20], thereby protecting them from environmental carcinogens. This is currently the most favoured hypothesis.

On the other hand, breastfeeding requires energy for milk production, and the mobilisation of fat and glucose reserves, and reduces insulin concentrations in women. Chronically elevated insulin levels can increase concentrations of Insulin Growth Factor-1, which is an established risk factor for breast cancer [21].

TNF-related apoptosis inducing ligand (TRAIL) plays a key role in the immune system as well as in controlling the balance between apoptosis (cell death, important for limiting the development or proliferation of cancer cells) and proliferation in various organs and tissues. Levels of soluble TRAIL in colostrum and mature human milk are at least 400 and 100 times higher, respectively, than those detected in human serum [22].

Independently of breastfeeding, some studies have highlighted the probable role of growth factors (destruction of mammary cells and inflammation) in breast cancer. This may explain the poorer prognosis of breast cancers diagnosed during weaning [23]. Another hypothesis is that women may fail to detect the signs during this period, when the breasts can be very engorged. It is therefore worth being vigilant during weaning.

A Swedish study of 592 women observed a higher risk of relapse in cases of breastfeeding the first child for more than 12 months, as well as in cases of excessive milk production during breastfeeding of the first child for more than 12 months. [24] The possible role of androgen and prolactin receptors may be at play in this mechanism.

The exact mechanism by which elevated circulating prolactin levels lead to an increased risk of breast cancer is not fully understood. Prolactin may enhance the survival of breast cancer cells by stimulating the generation of new cancer cells and reducing cell death. It may also increase cell motility and promote the spread of cancer [25]. Animal and cell culture studies suggest that prolactin may be involved in mammary oncogenesis [26].

Breastfeeding may have a protective effect, but please note: 
 

It is possible to develop breast cancer while breastfeeding.

You may have breastfed your children for years and still develop breast cancer.

So check your breasts every month, whether breastfeeding or not, pregnant or not! 

Self-examination is the simple way for you to know your "normal state". And don't hesitate to seek advice at the slightest doubt — it could save your life.

The Academy of Breastfeeding Medicine (United States) Protocols set out the following recommendations [27]: 

Breastfeeding women should not forgo routine breast cancer screening because they are breastfeeding. The decision to screen breastfeeding women should be individualised and based on personal breast cancer risk throughout life.

All radiological procedures used for breast cancer screening are safe during breastfeeding. Feeding your baby or expressing milk before a breast imaging examination is recommended in order to reduce density and improve the sensitivity of the scan.
 

The Institut du Sein Henri Hartmann notes that "in the case of a double mastectomy (total removal of both breasts), breastfeeding will not be possible. In the case of a unilateral mastectomy or lumpectomy, the new mother can use the non-operated (contralateral) breast to breastfeed her child safely. In this case, it should be noted that the mother will generally produce enough milk to breastfeed her child using her one intact breast."

The data on cancer risk should be interpreted with caution. These studies show correlations between a reduced risk and breastfeeding, but they do not allow us to conclude that breastfeeding prevents all risk of developing these cancers. Furthermore, there are many types of cancer (hormone-dependent or otherwise, etc.) and they depend on numerous variables, so further in-depth research would be needed to assess the true effect of breastfeeding and its precise mechanisms. 

The protective effect of breastfeeding on breast cancer is modest and would concern only certain cancers, particularly those occurring before the menopause. 

We are talking here about a possible protective factor, not a 100% guarantee. Breastfeeding does not guarantee that you will not develop breast cancer, just as not breastfeeding does not mean you will get cancer!

In short, you do not choose to breastfeed in order to be protected from breast cancer!

As Professor Eisinger puts it, "among 1,000 women who breastfeed their children, 937 will gain no benefit from it because they would never have developed breast cancer, 27 will develop breast cancer despite breastfeeding, and 36 will avoid breast cancer thanks to breastfeeding". [28]