One of the study's authors, Dr Hui Wang, said: "The results suggest vitamin D may influence the prognosis for people with breast cancer, colorectal cancer and lymphoma, in particular."
The meta-analysis, published in the Journal of Clinical Endocrinology & Metabolism, included 25 studies and 17,332 cancer patients. In the majority of the research included the patients were tested for vitamin D levels before undergoing any cancer treatment. The researchers from Shanghai’s Institute for Nutritional Sciences and other Chinese universities and institutions found a ten nanomole/liter (nmol/L) increase in vitamin D levels correlated with an increased survival rate of 4%.
Higher vitamin D (25(OH)D) levels were “significantly associated” with reduced cancer-specific mortality for patients with colorectal cancer and lymphoma, while improved disease-free survival for patients with breast cancer or lymphoma was observed.
Meanwhile the researchers found less evidence of a connection in people with lung cancer, gastric cancer, prostate cancer, leukemia, melanoma or Merkel cell carcinoma, but said the data available was positive.
The paper speculated that cancer patients with higher vitamin D levels had an improved overall survival because of better general health status. "Due to the limited data of randomized controlled trials, it is unclear whether vitamin D status is causally related to diseases or if circulating 25(OH)D merely acts as a biomarker for health status of patients," the researchers wrote.
No significant difference in levels was found for patients with early or later stages of the diseases.
"Considering that vitamin D deficiency is a widespread issue all over the world, it is important to ensure that everyone has sufficient levels of this important nutrient," Dr Wang said. "Physicians need to pay close attention to vitamin D levels in people who have been diagnosed with cancer."
The UK’s National Health Service (NHS) recommends the supplementation of groups at risk of deficiency including pregnant and breastfeeding women, children under the age of five not fed infant formula, people over 65 and those not exposed to much sun. It says supplementation should not exceed 25 micrograms (0.025mg) a day, as it “could be harmful”.
It said taking too many vitamin D supplements over a long period of time could cause more calcium to be absorbed than can be excreted, which could lead to kidney damage and softened and weakened bones.
Source: The Journal of Clinical Endocrinology & Metabolism
Published online ahead of print, doi: 10.1210/jc.2013-4320
"The Impacts of Circulating 25-Hydroxyvitamin D Levels on Cancer Patient Outcomes: A Systematic Review and Meta-Analysis"
Authors: M. Li, P. Chen, J. Li, R. Chu, D. Xie, H. Wang