High intake of vitamin D and calcium from the diet and supplements may cut the risk of colorectal cancer by over 30 per cent, suggests a new study.
A growing body of studies is reporting protective effects of calcium-rich dairy foods for colorectal cancer, a condition that accounts for nine per cent of all new cancer cases and kills half a million people every year worldwide.
"Our findings support the hypothesis of a protective role for calcium in colorectal cancer risk in both men and women, as a protective role for intakes of vitamin D, milk, and dairy products in men and in women who did not use supplemental calcium," wrote lead author Song-yi Park from the University of Hawaii.
Last year, researchers from Sweden reported that a diet rich in dairy products could cut the risk of colorectal cancer by half, but the authors did not attempt to measure the impact of vitamin D status, and called for future studies to look into the effects of this vitamin.
The new study, published in the current issue of the American Journal of Epidemiology addresses this need and not only supports the observation that calcium and dairy protect against colorectal cancer, but reports that vitamin D may have a protective effect, particularly in men.
The Multiethnic Cohort Study based in Hawaii and Los Angeles recruited 85,903 men and 105,108 women (average age 60, average BMI 26 kg per sq. m) and calculated nutritional intake from the diet and supplements using a food frequency questionnaire with over 180 food items.
During the follow-up period of 7.3 years 2,110 incident cases of colorectal cancer (1,138 in men and 972 in women) were documented by the researchers. After adjusting the results for potential confounding factors such as age, smoking habits, BMI, fibre intake, Park and co-workers divided calcium, vitamin D and calcium intake into five groups (quintiles).
The researchers report that the highest intake (at least 611 mg per 1,000 kcal per day) of total calcium intake (from foods and supplements) was associated with a 30 and 36 per cent reduction in colorectal cancer risk in men and women, respectively. Compared to the lowest intake (less than 288 mg per 1,000 kcal per day).
A protective effect was also observed for total vitamin D intake, but only for men. An average daily intake of at least 276 International Units (IU) per 1,000 kcal per day was associated with a 28 per cent reduction in risk, compared to a daily intake of less than 39 IU per 1,000 kcal per day.
Dairy products, a rich source of both calcium and vitamin D, were also observed to have a protective effect, with associated risk reduction of 23 and 34 per cent for men and women with intakes of more than 161 grams per 1,000 kcal per day who did not use calcium supplements.
"The findings support the hypothesis of protective roles for calcium, vitamin D, and dairy products in the risk of colorectal cancer," said Park.
The study does have several limitations, particularly related to vitamin D measures. The vitamin is made in the body on exposure to sunlight and no information on sunshine exposure was taken at the start of this study. Also, a higher proportion of women than men admitted to taking calcium supplements, many of which contain vitamin D, but this was not taken into account, said the researchers.
"Although previous cohort studies suggested that moderate calcium intake may reduce colorectal cancer risk, with no appreciable further effect from higher intakes, our results suggest a continued dose-response relation," concluded the researchers.
The research was supported by the US National Cancer Institute.
Source: American Journal of Epidemiology
April 2007, Volume 165, Number 7, Pages 784-793; doi:10.1093/aje/kwk069
"Calcium and Vitamin D Intake and Risk of Colorectal Cancer: The Multiethnic Cohort Study"
Authors: S.-Y. Park, S.P. Murphy, L.R. Wilkens, A.M.Y. Nomura, B.E. Henderson and L.N. Kolonel