Researchers from the School of Public Health at the University of Minnesota, found that diets rich in magnesium reduced the occurrence of colon cancer. Rectal cancer, so often grouped together with colon cancer, did not seem to be affected by magnesium intake.
A previous study from Sweden (Journal of the American Medical Association, Vol. 293, pp. 86-89) reported that women with the highest magnesium intake had a 40 per cent lower risk of developing the cancer than those with the lowest intake of the mineral.
"These findings offer further evidence that a diet high in magnesium may reduce the occurrence of colon cancer amongst women," said lead researcher Aaron Folsom.
The research is important because dietary surveys show that a large portion of adults do not meet the RDA for the mineral, found naturally in green, leafy vegetables, meats, starches, grains and nuts, and milk.
The new research, published in the February issue of the American Journal of Epidemiology (Vol. 163, pp. 232-235), used a food frequency questionnaire to assess dietary nutrient intake of 35 196 women with an average age of 61.
During the 17 years of follow-up, three per cent of the women developed colorectal cancer.
The hazard ratio, a measure of the risk, was statistically 25 per cent lower for the volunteers with the highest intake of magnesium (more than 356 mg per day). This is still less than than the RDA for magnesium: 320 mg per day for women and 420 mg per day for men.
Because the intake of magnesium from supplements was less than five per cent of the daily intake the researchers could only base their conclusions on dietary magnesium.
"Foods high in magnesium, such as vegetables, grains, and fruit, are already considered useful for reducing colorectal cancer," said Folsom.
The protective mechanism of magnesium is not clear but the researchers suggest that reductions in insulin resistance, oxidative stress, and cell proliferation could be possible.
The errors involved with using food frequency questionnaires suggest the results should be treated with caution; the research does support however the much larger Swedish cohort study.
Folsom said: "If the association is further replicated by observational studies, a clinical trial would be needed to determine whether it is magnesium, specifically, that may offer benefit."
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.