Sascha Abbas and colleagues from a working group headed by Dr. Jenny Chang-Claude at the German Cancer Research Center who collaborated with researchers at the University Hospitals in Hamburg-Eppendorf said previous studies concentrated chiefly on nutritional vitamin D.

The researchers studied 25-hydroxyvitamin D, referred to as 25(OH)D, which is a marker for both endogenous -- from sunlight -- vitamin D and vitamin D from food intake.

The researchers said the study involving 1,394 breast cancer patients and an equal number of healthy women after menopause was surprisingly clear -- women with a very low blood level of 25(OH)D have a considerably higher breast cancer risk.

The effect was found to be strongest in women not taking hormones for relief of menopausal symptoms. However, the authors note that, in this retrospective study, diagnosis-related factors such as chemotherapy or lack of sunlight after prolonged hospital stays might have contributed to low vitamin levels of breast cancer patients.

Besides its cancer-preventing influence with effects on cell growth, cell differentiation and programmed cell death, vitamin D regulates, above all, the calcium metabolism in the body, the researchers said.