The long-held theory, that low exposure to sunshine makes tumour development more likely, was developed in 1980 by Dr Frank C Garland, writing in the International Journal of Epidemiology.
The researchers, led by Professor Joan Lappe, examined the effects of vitamin D3 and calcium supplements in 1,156 healthy women over the age of 55 with sufficient levels of vital D absorption.
Study participants were given 2,000 international units of vitamin D3 (or 25mcg) and 1,500mg of calcium daily. A further 1,147 were given a placebo. The participants were asked to limit their consumption of both supplements outside the intervention.
Forty-five new cases of cancer were diagnosed in the group of participants taking the supplements, and 64 cases from the placebo group. The difference is statistically insignificant.
Lappe said: ‘Considerable interest exists in the potential role of vitamin D for prevention of cancer. Supplementation with vitamin D3 and calcium compared with placebo did not result in a significantly lower risk of all-type cancer at four years.
‘Further research is necessary to assess the possible role of vitamin D in cancer prevention.’
This was a randomised controlled trial (RCT) with a four-year follow-up period. 2300 women were randomised to receive either placebo or vitamin D (2,000 international units) and calcium (1,500mg) daily supplements. Women of 55 years old and over were recruited. The number of patients in the trial was sufficient to detect a significant difference in outcomes. The average starting level of vitamin D was 32.8ng/ml rising to 45.1 ng/ml in the treated arm of the study.
Outcomes of interest included new cases of cancer as well as diabetes, cardiovascular disease, hypertension and other outcomes.
At the conclusion of the study no differences in incidence of cancer was seen between the two groups.
This could be explained in several ways. The starting average serum vitamin D level was higher in study participants than in the population on average and this could have contributed a protective effect regardless of further supplementation. Another explanation could be that significantly higher doses of vitamin D may be needed to affect cancer rates. The follow-up period was also relatively short and perhaps a longer follow-up would have shown more significant results.