The researchers analysed data from almost 1,600 patients treated for non-metastatic bowel cancer, i.e. bowel cancer that had not spread to other distant parts of the body.
One in five, or 20% of the group with the lowest levels of vitamin D had died five years after diagnosis. However for those patients with the highest levels of vitamin D in their blood, 20% died ten years after diagnosis.
This research, funded by Cancer Research UK, was recently published in the Journal of Clinical Oncology, the leading peer-reviewed international oncology journal.
Dr Lina Zgaga, first author of the study and an Associate Professor of Epidemiology in the School of Medicine at Trinity College Dublin, said: "This study was the first to investigate the relationship between vitamin D levels post-diagnosis in bowel cancer patients and their survival of the cancer."
"We found significantly improved prognosis in patients with higher levels of vitamin D. However, the outstanding concern was to disentangle whether vitamin D is directly affecting survival, or if it is just a marker of a healthier lifestyle. For example, more active people tend to spend more time outside and also have higher vitamin D levels, and because the two come together, it is difficult to determine if it is physical activity or vitamin D that drives the higher rate of survival."
"By using genetic markers in our analysis, we were able to get further evidence that suggests vitamin D is causally related to improved survival in bowel cancer patients. This is very important, as it strongly suggests that vitamin D supplementation could be beneficial for bowel cancer patients."
Professor Malcolm Dunlop of the Edinburgh Cancer Research UK Centre and lead author of the study said: "Our findings are promising but it is important to note that this is an observational study. We need carefully designed randomised clinical trials before we can definitely confirm whether taking vitamin D supplements offers any survival benefit for bowel cancer patients."
The current study investigated the effects of vitamin D concentration in blood, but it was not designed to assess whether vitamin D supplementation is beneficial. While it is reasonable to assume that taking vitamin D supplements would increase levels of vitamin D in the circulation, and that this could improve survival as this study suggests, further studies explicitly designed to explore the effects of vitamin D supplements on survival are needed before supplementation can be recommended for bowel cancer patients.
"These initial findings are promising and particularly relevant for Ireland, because the number of people living with cancer is increasing and vitamin D deficiency is very common here." said Dr Zgaga.
While similar beneficial effect of vitamin D has been reported for breast cancer patients, little evidence is available for other cancer types. Therefore, further studies are needed before scientists can make recommendations for treating other cancers, as every cancer is a different disease.
Some of this research is already under way. A team of researchers from Trinity College Dublin and St James's Hospital led by Dr Zgaga has secured funding from the European Commission to explore the role of vitamin D in patients diagnosed with cancers of stomach and oesophagus. This study is to take place over the next 4 years in Ireland.
"The result from the bowel cancer study has prompted us to explore the role of vitamin D in cancers of the stomach and the oesophagus. Prognosis in patients diagnosed with oesophageal or gastric cancers is very poor, even when the cancer is caught in an early stage. We must explore any therapeutic options that have the potential to improve prognosis in these patients." Dr Zgaga said.