Highlights
- An Ontario study found that intake of heme iron (found in meat) may be associated with a small increase in post-menopausal breast cancer risk, in particular for certain tumour sub-types.
- Although iron is an essential nutrient, it also plays a part in producing DNA damage, which may contribute to breast cancer development.
- More research is needed to understand the impact of dietary iron intake on breast cancer risk.
A large population-based Ontario study has found that heme iron may increase breast cancer risk in post-menopausal women and that this risk may differ by breast tumour sub-type. Heme iron is the more easily absorbed form of iron found only in animal-based foods (e.g., red meat) and is suggested to have stronger cancer-causing effects than non-heme iron found in plant-based sources (e.g., dark leafy green vegetables and fortified cereals).[1][2]
| BREAST CANCER SUB-TYPE | HEME IRON INTAKE (MILLIGRAMS PER DAY BY QUINTILE) | ODDS RATIO | 95% LOWER CONFIDENCE LIMIT | 95% UPPER CONFIDENCE LIMIT |
|---|---|---|---|---|
| All breast cancers | Quintile 1 (<0.45) (Reference) | 1.0 | — | — |
| All breast cancers | Quintile 2 (0.45–0.64) | 1.1 | 0.9 | 1.3 |
| All breast cancers | Quintile 3 (0.65–0.85) | 1.0 | 0.8 | 1.2 |
| All breast cancers | Quintile 4 (0.86–1.17) | 1.1 | 0.9 | 1.3 |
| All breast cancers | Quintile 5 (≥1.18) | 1.2 | 1.0 | 1.5 |
| ER+/PR+ breast cancer | Quintile 1 (<0.45) (Reference) | 1.0 | — | — |
| ER+/PR+ breast cancer | Quintile 2 (0.45–0.64) | 1.0 | 0.8 | 1.2 |
| ER+/PR+ breast cancer | Quintile 3 (0.65–0.85) | 0.8 | 0.6 | 1.0 |
| ER+/PR+ breast cancer | Quintile 4 (0.86–1.17) | 1.1 | 0.9 | 1.4 |
| ER+/PR+ breast cancer | Quintile 5 (≥1.18) | 1.1 | 0.9 | 1.5 |
| ER–/PR– breast cancer | Quintile 1 (<0.45) (Reference) | 1.0 | — | — |
| ER–/PR– breast cancer | Quintile 2 (0.45–0.64) | 1.3 | 0.9 | 1.9 |
| ER–/PR– breast cancer | Quintile 3 (0.65–0.85) | 1.3 | 0.9 | 1.9 |
| ER–/PR– breast cancer | Quintile 4 (0.86–1.17) | 1.1 | 0.7 | 1.7 |
| ER–/PR– breast cancer | Quintile 5 (≥1.18) | 1.7 | 1.2 | 2.5 |
Note: Estimates were adjusted for age, ethnicity, education, age at menarche, parity, age at first live birth, breastfeeding duration, oral contraceptive use, age at menopause, hormone replacement therapy, family history of breast cancer, personal history of benign breast disease, physical activity, body mass index, alcohol intake, dietary fat intake, and total energy intake. An odds ratio above 1.00 indicates a greater risk of breast cancer compared to the reference category.
Table 1 shows that compared with women in the bottom fifth of heme iron intake, those in the top fifth had a modest increased breast cancer risk (of borderline statistical significance). This association was stronger and statistically significant for estrogen/progesterone receptor-negative (ER–/PR–) breast cancers. Other types of iron intake (non-heme iron from foods, iron from supplements) were not associated with an increased risk of post-menopausal breast cancer (data not shown here, but can be found in published paper, Iron intake, oxidative stress-related genes and breast cancer risk).[3]
Although iron is an essential nutrient required for many important biological processes, including oxygen transport, it also participates in reactions that produce DNA-damaging free radicals.[4] This DNA damage is what may contribute to breast cancer development, especially in post-menopausal women who are not losing iron through menstruation.[5],[6] Given the widespread consumption of heme iron-rich foods, such as red meat, these findings may have implications for breast cancer prevention; however, further research is first warranted.
The causes and prevention of breast cancer are not fully understood, with known risk factors accounting for approximately 70% of cases.[7],[8] Established breast cancer risk factors include non-modifiable (e.g., family history of breast cancer, early onset of menstruation, having had no pregnancies or children, late menopause) and modifiable (e.g., obesity, physical inactivity, alcohol consumption) factors.[9],[10]
The study highlighted in this Cancer Fact, which examined more than just the effects of iron intake on breast cancer, originated from a need to better understand the causes of breast cancer and, in particular, modifiable risk factors. It was funded by research grants from the Canadian Breast Cancer Research Alliance and the Canadian Cancer Society as well as a Canadian Institutes of Health Research Doctoral Award.
The study of over 6,000 women in Ontario included those who were diagnosed with breast cancer and a control group who had never been diagnosed with breast cancer. These women did questionnaires asking about breast cancer risk factors, what they ate and supplements they took. The hormone receptor tumour status came from pathology reports for the women with breast cancer. Study findings were recently published by Chang et al. in the International Journal of Cancer.[3]
More information on risk factors and prevention can be found on My CancerIQ, an online tool developed so people in Ontario could assess their risk of 6 cancers, including breast cancer.