Formula feeding increases cancer risk

Breastfeeding protects against breast cancer
IBFAN Photo by Gonda Heijman-Bruyn

Unravelling the protective capacity of breastfeeding never fails to reveal remarkable facts. And its ability to safeguard against one of world's most alarming diseases is no exception. Breast cancer is now the most common cancer in women globally. The disease is expected to strike one in every nine Canadian women. At the World Congress on Breast Cancer, hosted by Canada in July of this year, the relationship between not breastfeeding and increased risk was a substantive topic of discussion and media attention. The large body of accumulative evidence can no longer be dismissed as inconclusive.

In 1993 INFACT Canada wrote in it's Fall issue, "public health implications for reducing cancer incidence through breast feeding promotion can be substantial". In 1999 the evidence for improving the odds is even greater as a review of the data confirms.

United States

Over the past several years researchers have uncovered significant data on the relationship between not breastfeeding and increased breast cancer rates. Most recently, data from the Carolina Breast Cancer Study1, a population-based, case controlled study of 751 case and 742 control North Carolina women showed a reduction in the risk of breast cancer among both younger and older women. When women who had breastfed were compared to those who had never breast fed, the odds ratio of 0.8 for younger; 0.7 for older and 0.7 for all women were found. To detect a dose-response relationships this study was limited as acknowledged by the researchers, because the median lifetime duration of lactation was only 8 months Previous research2,3 from the US has shown an even stronger capacity of breastfeeding to reduce the incidence.


Romieu et al.4, examined not only the relationship between breast cancer and lactation history, but the dose response relationship as well. Between 1990 and 1992, 506 subjects were selected from newly reported cases in Mexico City.

Duration of breastfeeding, even short term, was associated with reduced risk of breast cancer. Women who had breastfed up to three months had an age-adjusted odds ratio of 0.48 compared to women who had never breastfed. Women who had breastfed for 37 to 60 months in total had an age-adjusted odds ratio of 0.27, and women who breastfed for more than 60 months had an odds ratio of 0.23. After adjustment for social, economic, family and reproductive history, the results remained the same; a significant decreasing trend in breast cancer associated with the total duration of breastfeeding. They concluded that breastfeeding is "a cost-effective intervention".

New Zealand

Researchers5 from New Zealand compared 891 women aged 25 to 54 with newly diagnosed cases of breast cancer with 1864 controls in order to determine the relationship between breast cancer and reproductive risk factors. A major finding from this study was that prolonged breastfeeding substantially reduced the risk of breast cancer especially among young women.


  • Breastfeeding protects against breast cancer in young women
  • The risk decreases with increasing duration of breastfeeding, with breastfeeding each baby for three months or longer giving greatest protection
  • Risk of breast cancer was not linked to the number of births
  • Breastfeeding is good for the mother as well as for the baby

--The United Kingdom National Case-Control Study Group8    


Japanese researchers, Keun-Young Yoo6 et al. made an important contribution to clarify the link between breastfeeding and reduced risk of breast cancer. Using statistical adjustments to control confounding effects of other closely related factors, they used a hospital based case control study of 521 women with breast cancer and 521 controls. They determined that women who had never breastfed compared with women who had breastfed had an adds ratio of breast cancer of 0.62. They also found that a decreasing risk with increasing months of total breastfeeding was statistically significant. For premenopausal women who had ever breastfed for 7 to 9 months the risk was lowest, an odds ratio of 0.39. They concluded that breastfeeding has an "independent protective effect against breast cancer in Japanese women".


An early report from Shanghai7, determined that women who had breastfed for a total of 9 years had a 63 per cent reduction in breast cancer. Women who had breastfed for a total of 3 to 6 years showed a reduced cancer rate of 50 per cent. The study looked at 534 women with breast cancer matched with controls to determine the risk factors contributing to breast cancer.

A number of hypothesis1 have been put forward to explain the relationship between breastfeeding and breast cancer risk reduction.

  • Lactational amenorrhoea reduces a woman's exposure to ovarian hormones, estrogen and progesterone. Studies showing a dose-response relationship between breastfeeding and reduced risk support this.
  • Breastfeeding reduces potential carcinogens from a woman's body and thus reduces risk. This hypothesis would suggest a greater risk reduction for older women and those with a long duration of breastfeeding.
  • The ability of breastmilk to destroy cancer cells by apoptosis. When a mother stops breastfeeding her extra breast tissue is resorbed . This hypothesis also suggests a greater protective effect for those whose lifetime breastfeeding experience is extensive.
  • The protective effect may be brought about by structural changes in the breast tissue due to lactation. Breastfeeding induces epithelial cells to differentiate in order to decrease their susceptibility to cancerous substances.

United Kingdom

The United Kingdom National Case-Control Study Group8 also used a population based case-control study to investigate the determinants related to the risk of breast cancer. The study group consisted of women diagnosed with breast cancer under the age of 36, case-control pairs, 755, matched for age, were interviewed.

The UK results confirmed what the Japanese, Chinese and earlier US studies had shown that the risk of breast cancer declines with increased duration of breastfeeding and with the number of babies breastfed. The authors also acknowledge that inconsistent results in some previous studies could be due to the lack of sufficient detail on the lactation.

Public health implications were also highlighted by the UK Group.

1.Furberg, H. et al. Lactation and breast cancer risk. Internat J of Epidemiol. 28:396-402,1999 BACK

2. McTiernan, A. et al. Evidence for a protective effect of lactation on risk of breast cancer in younger women: results from a case-control study. Am J Epidemiol 124:353-358,1986 BACK

3. Newcombe, P.A. et al. Cancer of the breast in relation to lactation history. N Engl J Med. 330:81-87,1994 BACK

4. Romieu, I. et al. Breast cancer and lactation history in Mexican women. Am J Epidemiol 143:543-552,1996 BACK

5. McCredie, M. et al. Reproductive factors and breast cancer in New Zealand. Internat J of Cancer 76:182-188,1998 BACK

6. Keun-Young Yoo, et al. Independent protective effect of lactation against breast cancer : a case-control study in Japan. Am J Epidemiol 135:726-733,1992 BACK

7. Yuan, J-M. et al. Risk factors for breast cancer in Chinese women in Shanghai. Cancer Res. 48:1949-1953,1988 BACK

8. United Kingdom National Case-Control Study Group, Breast feeding and risk of breast cancer in young women. Br Med J 307:17-20,1993 BACK

INFACT Canada Top | Spring 99 Contents |