However researchers at the departments of nutrition, and endocrinology at Soochow University, also note that there is some possibility of publication bias, and that other dietary co-variables, such as vitamin D, may also have an effect on the observations.
Breast cancer is the most common cancer diagnosis in women, and ranks fifth for cancer mortality. The World Cancer Research Fund calculates that nearly 40 % of breast cancer cases could be prevented with changes to lifestyle. Established modifiable factors that increase the risk of breast cancer are greater intakes of alcohol and increased obesity. And decreases in risk are linked to breast-feeding and possibly to more exercise and being active.
Calcium intakes from both foods and supplements was collected from more than 870,000 women from 11 surveys. The studies were all prospective, so none of the women had a diagnosis of breast cancer during recruitment.
Intake of calcium was calculated either by a food frequency questionnaire, or by one or more 24-hour food diaries.
Of the 11 studies selected for meta-analysis, five were based in the US, five were based in Europe, and one was based in Singapore. One of the European-based studies was part of the European Prospective Investigation into Cancer (EPIC) research programme (Abbas, 2013).
The women recruited were monitored for a period of at least seven years, and during this time, more than 26,000 were given some diagnosis of breast cancer.
The overall relative risk of breast cancer in women with high intakes of calcium (compared to low intakes) was 0.92. So high intakes of calcium from all sources suggested a reduced risk of diagnosis of breast cancer of about 8%.
Further sub-group analysis showed strongest protective effects of calcium intakes in diagnosis of breast cancer made prior to menopause; associations between calcium and breast cancer diagnosed in postmenopausal women were weaker.
More is (a little) better
There was also a dose-effect. The researchers revealed that each 300 mg* increase in calcium intake was associated with a 2% reduction in the total risk of getting the diagnosis of breast cancer.
There was a slight trend of more benefit from additional calcium observed in women in the lower range of intakes (below 800 mg per day), whereas this was less strong in women in the higher range of calcium intakes. The population reference intake (PRI) figure for calcium in adults defined by EFSA is 950 mg per day.
Uncertainty
Fiona Osgun, senior health information officer at Cancer Research UK, said: “This study, which combined the results of other research, suggests that there may be a slightly lower risk of breast cancer in women having the most calcium, compared to those having the least.
"But many of the studies included asked women to remember what they had eaten, which isn’t always reliable, and it was hard to separate out the effect of calcium from other elements of people's lives, so we can’t be sure that there’s a real link between calcium and breast cancer."
*300 mg calcium would be obtained in
- 250 ml milk or
- 40 g semi-hard cheese, such as cheddar or edam or
- 200g of yogurt or
- 120g almonds
Source: British Journal of Nutrition
2016 vol 116, pp 158-166 (Published online, doi:10.1017/S0007114516001768)
“Calcium intake and breast cancer risk: meta-analysis of prospective cohort studies”
Authors: Hidayat Khemayanto, Chen Guo-Chong, Zhang Ru, Du Xuan, Zou Sheng-Yi, Shi Bi-Min and Qin Li-Qiang