The results, from the same group of researchers from a variety of research institutions, are based on a pooled analysis and meta-analysis of studies, and may increase pressure on decision makers to increase the recommended daily intake of the vitamin.
Currently, the RDI is set at 400 IU, and the tolerable upper intake level (UL) in Europe and the US is set at 2000 International Units (IU), equivalent to 50 micrograms per day. However, recent research, particularly from clinical trials, suggests that this should be raised.
Moreover, a recent risk assessment by the US-based trade organisation, the Council for Responsible Nutrition (CRN) concluded that the UL could be raised to 10,000 IU (250 micrograms per day).
The first of the two studies, focusing on breast cancer and published in the Journal of Steroid Biochemistry and Molecular Biology, pooled dose-response data from two earlier studies totalling 1760 individuals, and found that individuals with the highest blood levels of 25-hydroxyvitamin D, or 25(OH)D (50 nanograms per millilitre), had a fifty per cent lower risk of breast cancer, compared to those with the lowest blood levels (10 nanograms per millilitre or less).
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive. The latter is derived from plants and only enters the body via the diet.
Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
"The data were very clear, showing that individuals in the group with the lowest blood levels had the highest rates of breast cancer, and the breast cancer rates dropped as the blood levels of 25-hydroxyvitamin D increased," said lead author Cedric Garland from the University of California in a statement.
"If the oral dose must be kept at or below 2000 IU/day, a 50 ng/ml concentration of 25(OH)D could be achieved by oral intake of 2000 IU/day, and, if appropriate and climate allowing, about 12 min/day in the noontime sun on a clear day with 50 per cent of the skin exposed to the sun," wrote the researchers.
The colorectal cancer meta-analysis, published in the American Journal of Preventive Medicine, pulled together five studies, totalling 1,448 individuals (all Caucasian) that explored the association of blood levels of 25(OH)D with risk of colon cancer.
As with the breast cancer study, the dose-response data on a total of 1,448 individuals were put into order by serum 25(OH)D level and then divided into five equal groups, from the lowest blood levels to the highest. "Through this meta-analysis we found that raising the serum level of 25-hydroxyvitamin D to 34 ng/ml would reduce the incidence rates of colorectal cancer by half," said lead author Edward Gorham.
"We project a two-thirds reduction in incidence with serum levels of 46 ng/ml, which corresponds to a daily intake of 2,000 IU of vitamin D3. This would be best achieved with a combination of diet, supplements and 10 to 15 minutes per day in the sun."
The meta-analysis on colorectal cancer included data from the well-publicised Women's Health Initiative, which reported a null relationship between vitamin D intake and colorectal cancer risk. However, the researchers wrote, the meta-analysis indicates that a higher dose may reduce its incidence.
"An intake of 2000 IU/day would raise the population median to 46 ng/ml. This is well below an intake level that would induce even mild hypervitaminosis," they wrote.
The link between vitamin D intake and protection from cancer dates from the 1940s when Frank Apperly demonstrated a link between latitude and deaths from cancer, and suggested that sunlight gave "a relative cancer immunity."
Vitamin D levels have been linked to skin colour - darker skinned people produce less vitamin D on exposure to the sun, relative to fair-skinned people.
Calls for raising the recommended daily allowance of the vitamin have been growing after reports that higher intakes could protect against osteoporosis and certain cancers. Consumer awareness of these health links is also increasing with some outlets reporting massive boosts in sales.
Sources: Journal of Steroid Biochemistry and Molecular Biology Published on-line ahead of print; doi: 10.1016/j.jsbmb.2006.12.007 "Vitamin D and prevention of breast cancer: Pooled analysis" Authors: C.F. Garland, E.D. Gorham, S.B. Mohr, W.B. Grant, E.L. Giovannucci, M. Lipkin, H. Newmark, M.F. Holick, F.C Garland
American Journal of Preventive Medicine Volume 32, Number 3, Pages 210-216 "Optimal vitamin D status for colorectal cancer prevention - A quantitative meta-analysis" Authors: E.D. Gorham, C.F. Garland, F.C Garland, W.B. Grant, S.B. Mohr, M. Lipkin, H. Newmark, E.L. Giovannucci, M. Wei, M.F. Holick,