A lack of the sunshine vitamin causes breast cancer tumours to grow faster and spread, a new study found.
Vitamin D is mainly made from sunlight on our skin but can be found in a small number of foods such as oily fish like salmon, sardines and mackerel, eggs and fortified fat spreads and breakfast cereals.
The Stanford University School of Medicine study highlights a direct link between circulating vitamin D levels and the expression of a gene called ID1, known to be associated with tumour growth and breast cancer metastasis.
It adds to previous findings that a Vitamin D deficiency not only increase a person's risk of developing breast cancer, but are also correlated with more aggressive tumours and worse prognoses.
The research was carried out in mice but researchers found in a study of 34 breast cancer patients that levels of circulating vitamin D were inversely correlated with the expression levels of ID1 protein in their tumours.
Women with lower levels of vitamin D expressed more ID1 in their tumour tissues than did women with higher levels of vitamin D.
It confirmed a vitamin D metabolite directly controls the expression of the ID1 gene in a human breast cancer cell line.
Assistant professor of paediatrics Dr Brian Feldman said: "Our study shows that a deficiency in vitamin D levels, or an inability of tumour cells to respond appropriately to the presence of vitamin D, is sufficient to trigger non-metastatic cancer cells to become metastatic.
"It's enough to significantly accelerate tumour progression in our mouse model.
"Further studies are warranted, but this direct association between vitamin D levels and ID1 expression is very interesting to us.
"Although much more research needs to be done, research from our lab and others suggests that people at risk for breast cancer should know their vitamin D levels and take steps to correct any deficiencies."
The study was published in Endocrinology did not set out the optimal amount of vitamin D needed as scientists are divided.
But the Institute of Medicine recommends 600 international units per day for people age 70 and younger, and 800 IU for older adults.
The NHS recommends pregnant women, those over 65 and those housebound should take a daily supplement containing 10 micrograms (0.01mg) of vitamin D.
Too much of about 4,000 to 10,000 IU per day has been linked to damage to the kidneys, cardiovascular system and other organs.
The new study involved mice fed a Vitamin D deficient diet over 10 weeks and a control group
The mice on a poor diet developed palpable tumours an average of seven days sooner than their peers and after six weeks of growth those tumours were significantly larger.
The results were compared to the 34 breast cancer patients undergoing treatment.