The study does not only add to the debate on the link between calcium and the cancer, it also reports that fibre intake, so often associated with a decreased risk of colorectal cancer, may not offer protective effects.
Previous randomised clinical trials have reported an inverse link between calcium supplements and the risk of colorectal cancer, but the situation remains controversial. Indeed, studies looking at the effects of calcium and vitamin D on the risk and incidence of the cancer, most notably the WHI (New England Journal of Medicine, Vol. 354, pp. 684-696) and the Nurses' Health Study (Cancer Epidemiology Biomarkers Prev. 2004 Vol. 13, pp. 1502-1508) did not report any potential benefits.
The results of the Shanghai Women's Health Study, published in the International Journal of Cancer (Vol. 119, pp. 2938-2942), indicate that the among the 73.314 women (average age 55.5), those who had the highest daily calcium intake had a significantly reduced risk of cancer of the colon or rectum than women with the lowest daily intake.
Dietary intakes of 77 food items, covering 90 per cent of commonly consumed foods, was evaluated using a validated food frequency questionnaire. Intakes of the nutrients calcium, vitamins A, B1, B2, B3, C, and E, carotene and fibre were calculated accordingly.
After 5.74 years of follow-up, 129 cases of colon cancer and 91 cases of rectal cancer were recorded. The researchers, led by Wei Zheng from Vanderbilt Uiversity Medical School, report that the relative risk of colorectal cancer for the highest calcium intake group was 40 per cent lower compared to the lowest intake group.
No statistically significant relationship between the other nutrients and colorectal cancer was calculated by the researchers.
The potential mechanism was not evaluated in this study, but the researchers note that previous research has linked calcium to a range of cancer inhibiting effects, including binding to carcinogenic bile acids, and promoting the differentiation and stopping the growth of cells in the colon.
"High calcium intake is associated with reduced risk of colorectal cancer in a cohort of Chinese women whose dietary consumption of calcium is lower than Western populations," concluded the researchers.
The study's main strength is following a large sample in a free-living population.
The study has several limitations, namely the use of FFQs which rely on the recall of the participants and may affect links between nutrient intake and cancer risk.
Another limitation is the long latency period of colorectal. None of the participants had colorectal cancer at baseline, and it should be noted that the latency period of the cancer is between 10 and 20 years.
Colorectal cancer accounts for nine per cent of new cancer cases every year worldwide. The highest incidence rates are in the developed world, while Asia and Africa have the lowest incidence rates.
It remains one of the most curable cancers if diagnosis is made early.