Original cartoon by Alex Martin
Source: Original cartoon by Alex Martin

It is now widely accepted that breastfeeding a baby has advantages over bottle-feeding, notably for infant brain development. Although evidence comes largely from epidemiological surveys and is hence mainly circumstantial, numerous studies have revealed small but significant differences between breast-fed and bottle-fed babies. (See my blog post Breast is Best for Growing Brains,  January 11, 2014.) And there are many other advantages. But it has been far less widely reported that breast-feeding also has real health benefits for the mother. These range from more rapid recovery after birth through a reduced risk of cardiovascular disease and on to a lower probability of certain cancers later in life.

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Breast-feeding a newborn baby
Source: Author Produnis (own work, 2007) Wikimedia Commons: File licensed under the Creative Commons Attribution-Share Alike 3.0 Unported.

Healing the womb

Breast-feeding helps to restore the mother’s general physical condition after birth, especially by ensuring faster recovery of the womb. It aids in reducing blood loss by increasing the tempo of womb contractions. While breast-feeding an infant during the first few days after birth, women commonly experience after-pains in the womb. Starting out from this knowledge, Selina Chua and colleagues conducted tests on a small sample of eleven new mothers, who served as their own controls. Effects of breast-feeding and nipple stimulation on womb activity were compared to baseline levels. Chua’s team found that the rate of womb contractions almost doubled during breast-feeding. The rate of contraction was also boosted to a lesser extent by nipple stimulation alone.

In developing countries, bleeding after birth is a major cause of maternal death, so this finding has important practical consequences. In fact, in a 1993 press release for World Breastfeeding Week the World Health organization stated that, if suitable medical supplies are lacking, breast-feeding or nipple stimulation may be safe, effective and economical means of reducing blood loss after birth.

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Illustration of atherosclerosis: (a) Diagram showing build-up of plaque; (b) histological section of an artery with plaque.
Source: Author: OpenStax College. Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/ Wikimedia Commons: File is licensed under the Creative Commons Attribution 3.0 Unported

Breast-feeding and cardiovascular disease

Heart disease is the leading cause of death in women in the USA. It is therefore important that breast-feeding reportedly reduces the risk over the long term that mothers will suffer from blockages in the arteries of the heart and in the circulatory system generally (cardiovascular disease). In 2009, Eleanor Schwarz and colleagues published results from investigating almost 140,000 postmenopausal women (median age: 63 years) with at least one reported live birth. To examine the relationship between risk factors for cardiovascular disease and duration of breast-feeding, Schwarz and colleagues used statistical models allowing for effects of several possible confounding factors such as age, number of births (parity) and income. Data for obesity, high blood pressure (hypertension), self-reported diabetes, high blood fat (notably cholesterol and triglycerides) and cardiovascular disease were analysed. Compared to women who had never breast-fed, those reporting a lifetime total of more than 12 months lactation were significantly less likely to have hypertension (38.6% versus 42.1%), diabetes (4.3% versus 5.3%), high blood fat (12.3% versus 14.8%) and cardiovascular disease appearing after menopause (9.1% versus 9.9%). However, they were not less likely to be obese.

Adapted from a figure in Gunderson et al. (2015).
Relationship between the risk of atherosclerosis and the total duration of breast-feeding (lactation). A clear reduction is noticeable for breast-feeding with a total duration exceeding 6 months
Source: Adapted from a figure in Gunderson et al. (2015).

In 2015, a team led by Erica Gunderson reported on a similar study of breast-feeding in relation to hardening of the arteries (atherosclerosis), which is the main cause of heart attacks, stroke and peripheral vascular disease. They examined data from a multi-center prospective study in the USA: Coronary Artery Risk Development in Young Adults. In that study, women were initially examined in 1985-1986 when aged 18-30 years (baseline evaluation) and then re-examined 20 years later in 2005-2006. Gunderson and colleagues selected 846 women who had no heart disease or diabetes at baseline and subsequently had one or more births. For each woman, total duration of breast-feeding after baseline examination was calculated across all births, and atherosclerosis was assessed from the thickness of the wall of the common carotid artery using ultrasound. For comparisons between breast-feeding categories, statistical methods were applied to exclude confounding effects of pre-pregnancy obesity, cardiac status, parity and other risk factors. Duration of breast-feeding was found to have an inverse relationship with carotid wall thickness that remained significant after excluding confounding effects.

Breast-feeding and cancer

Health benefits of breastfeeding for the mother extend far beyond a lower incidence of heart disease. They apparently include protection against certain cancers, notably breast cancer. Reports from the 1920s suggested that human breasts that had never been used to feed an infant were more likely to become cancerous. In one ingenious approach to this question, in a 1977 paper Roy Ing and colleagues drew information from the unusual custom of women inhabiting fishing villages in Hong Kong, who suckled infants only from the right breast. Radiotherapeutic records for the period 1958-1975 were searched and breast-cancer patients were interviewed to compile details of their lactation history. For the sample as a whole, there was no difference between frequencies of cancers on left and right sides. But comparisons of postmenopausal patients who had nursed exclusively from one breast with those who had never breast-fed, whether they had given birth or not, revealed a highly significant increase in the risk of cancer for the unsuckled breast.

Adapted from a figure in Layde et al. (1989).
Estimated change in the risk of breast cancer with pregnancy, number of children (parity) and total duration of breast-feeding. Note that the risk increases with age at first pregnancy but declines with parity and total duration of breast-feeding.
Source: Adapted from a figure in Layde et al. (1989).

More recently, the Collaborative Group on Hormonal Factors in Breast Cancer conducted a large-scale, worldwide review of available information on the relationship between breast-feeding and cancer. The report, published in 2002, examined information from 47 epidemiological studies in 30 countries, covering a total of 50,000 women with breast cancer and twice as many who were cancer-free. The Collaborative Group review confirmed the protective effects of pregnancy by showing that women with breast cancer had 15% fewer births on average. The review also revealed that, among those who had given birth, just over 70% of women who developed cancer had ever breast-fed. By contrast, almost 80% percent of women who remained cancer-free had breast-fed to some extent, even if only for a few months. A further difference was detected regarding average lifetime duration of breastfeeding. Women who developed cancer breast-fed for only 10 months altogether, compared with 151/2 months for cancer-free women. The most important finding reported by the Collaborative Group was that relative risk of breast cancer decreased by 7% for every birth and by more than 4% for every year of breast-feeding. Combining all findings to estimate the cumulative incidence of breast cancer up to age 70 for developed countries yielded the following conclusion: If all women had the average number of births and lifetime duration of breast-feeding that characterized third world countries until recently, deaths from breast cancer could be more than halved  —  from one in 16 to one in 37. Almost two-thirds of this projected reduction would be due to the influence of breast-feeding.

Breast-feeding and ovarian cancer

Starting in the 1970s, various studies also linked breast-feeding to a reduced risk of cancer of the ovaries. In a 2001 review devoted to effects of breast-feeding on the mother, Miriam Labbok confirmed on the basis of 13 studies that breast-feeding is significantly associated with a reduction in the risk of breast cancer. She also summarized results from 15 studies that, taken overall, identified a protective effect against ovarian cancer. It emerged that breast-feeding during the first 2-7 months after birth is associated with a significant decrease in the risk of ovarian cancer by an average of about 20%. However, there was no clear evidence that greater duration of breast-feeding provided increased protection. Labbok specifically mentioned the “incessant ovulation hypothesis” proposed by John Casagrande and colleagues in 1979, according to which “excess” ovulation due to fewer pregnancies and lack of breast-feeding leads to the reported increase in ovarian cancer. In support of this interpretation, they reported that using oral contraceptives reduced the risk of ovarian cancer. But more data are needed to test the validity of that hypothesis.

Adapted from a figure in Labbok (1989)
Relative risk of breast cancer (above) and ovarian cancer (below) for women who have breast-fed with those who have not. There is some variation in results, but most studies have reported a reduction in risk associated with breast-feeding for both types of cancer.
Source: Adapted from a figure in Labbok (1989)

A more recent paper published by Dada Su and colleagues in 2013 reported results from a 2-year case-control study in Guangdong Province in southern China. Almost 500 women undergoing treatment for ovarian cancer were compared with a similar number of control patients with other conditions. After taking potential confounding factors into account, it was found that both the number of children breast-fed and the total duration of breast-feeding were significantly associated with a decreased risk of ovarian cancer. Moreover, a dose-response relationship was clearly evident. Compared to women who had only a single child, the risk of ovarian cancer was decreased by over 15% in mothers with two children and by over 60% in mothers with three or more. Compared to women with a total breast-feeding duration of 10 months or less, the risk of ovarian cancer was reduced by over 35% in mothers who breast-fed for 11-20 months, by almost 70% in mothers who breast-fed for 21-30 months and by a staggering 90% in mothers with an overall breast-feeding duration of 31 months or more.

The take-home message

Let it be emphasized that the take-home message from the undeniable advantages of breast-feeding for both mothers and children is not that women should be pressurized to breast-feed at all costs. It is that we should use the biological evidence to find practical ways of enabling women to cope with their reproductive history in ways that will maximize health benefits for mothers and children alike. For instance, in cases where mothers are unable to breast-feed for whatever reason, a key step is to ensure that the composition of milk formula is matched as closely as possible to the baby’s biological needs. As regards the health benefits of breast-feeding for mothers, the solution is by no means as obvious. But we should surely be conducting research to identify the mechanisms underlying those benefits and developing ways to replicate them in women who do not breast-feed. As a starting point, it is encouraging to know that long-term use of oral contraceptives certainly reduces the risk of ovarian cancer and may also lessen the risk of breast cancer.

References

Casagrande, J.T., Pike, M.C., Ross, R.K., Louie, E.W., Roy, S. & Henderson, B.E. (1979) “Incessant ovulation” and ovarian cancer. Lancet 314:170-173.

Chua, S., Arulkumaran, S., Lim, I., Selamat, N. & Ratnam, S.S. (1994) Influence of breastfeeding and nipple stimulation on postpartum uterine activity. British Journal of Obstetrics & Gynaecology 101:804-805.

Collaborative Group on Hormonal Factors in Breast Cancer (2002) Breast cancer and breastfeeding: collaborative reanalysis of individual data from 47 epidemiological studies in 30 countries, including 50 302 women with breast cancer and 96 973 women without the disease. Lancet 360:187-195.

Gunderson, E.P., Quesenberry, C.P., Ning, X., Jacobs, D.R., Gross, M., Goff, D.C., Pletcher, M.J. & Lewis, C.E. (2015) Lactation duration and midlife atherosclerosis. Obstetrics & Gynecology 126:381-390.

Ing, R., Ho, J.H.C. & Petrakis, N.L. (1977) Unilateral breast-feeding and breast cancer. Lancet 310:124-127.

Labbok, M.H. (2001) Effects of breastfeeding on the mother [review]. Pediatric Clinics of North America 48:143-158.

Layde, P.M., Webster, L.A., Baughman, A.L., Wingo, P.A., Rubin, G.L. & Ory, H.W. (1989) The independent associations of parity, age at first full term pregnancy, and duration of breastfeeding with the risk of breast cancer. Journal of Clinical Epidemiology 42:963-973.

Lee, S.Y., Kim, M.T., Kim, S.W., Song, M.S. & Yoon, S.J. (2003) Effect of lifetime lactation on breast cancer risk: a Korean women's cohort study. International Journal of Cancer 105:390‐393.

Rosenblatt, K.A. & Thomas, D.B. (1993) WHO Collaborative study of neoplasia and steroid contraceptives: Lactation and the risk of epithelial ovarian cancer. International Journal of Epidemiology 22:192-197.

Schwarz, E.B., Ray, R.M., Stuebe, A.M., Allison, M.A., Ness, R.B., Freiberg, M.S. & Cauley, J.A. (2009) Duration of lactation and risk factors for maternal cardiovascular disease. Obstetrics & Gynecology 113:974-982.

Shema, L., Ore, L., Ben‐Shachar, M., Haj, M. & Linn, S. (2007) The association between breastfeeding and breast cancer occurrence among Israeli Jewish women: a case control study. Journal of Cancer Research & Clinical Oncology 133:539‐546.

Su, D., Pasalich, M., Lee, A.H. & Binns, C.W. (2013) Ovarian cancer risk is reduced by prolonged lactation: a case-control study in southern China. American Journal of Clinical Nutrition 97:354-359.

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