Iron and calcium are linked to a higher risk of lung cancer in new research from the Harvard School of Public Health.
High iron and "high calcium intakes from dietary or supplemental sources" are associated with higher risk of the disease, wrote the researchers in the journal Epidemiology (vol 16, no 6, pp772-779).
"The association between dietary micronutrients and lung cancer was stronger in current smokers," they added.
Lung cancer is the second most common form of cancer in the UK with smoking responsible for 90 per cent of the 37,000 annual new cases.
Beta-carotene supplements have been reported to increase the risk of lung cancer among smokers but the report from Harvard is the first to suggest a detrimental role for calcium and iron for this specific cancer.
High intake of iron, however, has previously been associated with higher risk of colorectal cancer, and although calcium seems to protect against some cancers, it has been linked to a higher risk of prostate cancer.
The role of zinc, on the other hand, appears to be beneficial, with several studies relating higher dietary intake of this nutrient to lower risks of breast, prostate and colon cancer.
Started in 1992, the new hospital-based study followed 923 patients with lung cancer and 1125 healthy controls. The association between dietary iron, calcium and zinc and the risk of lung cancer was established by using a 126-item food frequency questionnaire.
The researchers report that risk of lung cancer increased with increasing iron and calcium uptake (both dietary and supplementary). For volunteers with a daily iron intake of 22 mg or more, the risk of lung cancer was statistically doubled.
The risk of lung cancer increased by 50 per cent for calcium intake greater than 1270 mg per day. European recommended daily allowances (RDA) of iron and calcium are 14 mg and 550 mg respectively.
The study, led by Dr David Christiani, paid particular attention to iron, from the diet or as a supplement.
"We found that the increased lung cancer risk was the result of nonheme iron [iron from plant sources]… heme iron [from animal sources] was associated with decreased risk of lung cancer," said Christiani.
Unfortunately, "more than 90 per cent of iron [intake] is nonheme."
The researchers admitted there were limitations with the study, since a subset of control volunteers were recruited from friends and family of actual lung cancer cases. This could result in similar lifestyles for both cases and controls, particularly in terms of smoking and diet.
Professor Tim Key, epidemiologist for the British charity Cancer Research UK, told NutraIngredients.com that the new study would not change the well-established evidence showing that "smoking causes most cases of lung cancer".
"Previous research has not produced any compelling evidence that dietary factors have a significant impact on lung cancer risk, and this new study does not alter the position. The small differences in reported diet between people with lung cancer and people without lung cancer could be due to a combination of chance and changes in reporting diet among people suffering from lung cancer," he said.