In a study of patients with colorectal cancer, higher levels of vitamin D were associated with better survival than lower levels, and the "magnitude of effect was substantial and clinically important," the researchers report.
However, because all the data so far have come from observational studies, it is too soon to recommend vitamin D supplementation for colorectal cancer patients, according to an expert not involved in the study.
The findings were published online July 7 in the Journal of Clinical Oncology.
When the study participants were divided into tertiles by plasma 25-hydroxyvitamin D (25[OH]D) level, overall mortality was approximately a third lower in the highest tertile than in the lowest.
In addition, disease-specific mortality was 20% at 5 years in the lowest tertile, but "10 years elapsed before mortality reached 20%" in the highest tertile, report Evropi Theodoratou, PhD, Chancellor's Fellow at the Centre for Population Health Sciences, University of Edinburgh, United Kingdom, and colleagues.
For all-cause mortality, the trend was similar; it was 20% at 3.5 years in the lowest tertile and at 6.0 years in the highest tertile.
There were also interactions between 25(OH)D level and vitamin D receptor (VDR) genotypes, which suggest a causal relation between vitamin D and survival.
"We believe that we should more seriously consider the role of vitamin D in colorectal cancer," Dr. Theodoratou told Medscape Medical News. "However, the nature of the relationship between vitamin D and survival after a colorectal cancer diagnosis is not clear, as this was an observational study."
"We can therefore not exclude the possibility that low vitamin D levels were associated with other predictors of poor survival," she cautioned.
Link Already Known, But Gaps in Knowledge
Previous studies have shown a link between vitamin D levels and cancer risk and survival. Some studies have suggested that vitamin D levels are linked to colon cancer risk and an overall improvement in survival for a number of cancer types, including colorectal.
An "umbrella review" Dr. Theodoratou was involved in found that using vitamin D for colorectal cancer prevention, and possibly treatment, is "biologically plausible" (BMJ. 2014;348:g2035), according to David J. Kerr, CBE, MD, DSc, professor of cancer medicine at the Oxford Cancer Centre in the United Kingdom.
"Vitamin D has important effects on cell growth, cell morphology, and cell cytokinetics — a whole range of biological factors that would be relevant and germane to the control of cancer," he explained in a recent Kerr on Oncology Medscape video.
In their umbrella review, Dr. Theodoratou and colleagues note that "despite biological plausibility and a positive trend toward improvements in outcomes, in terms of prevention and in terms of reducing elements of disease recurrence, there were insufficient prospective randomized trials," Dr. Kerr pointed out.
"There is a key gap in the knowledge — the defining evidence, the 'home run' if you like — that we would require to be utterly convinced of the role that vitamin D plays in the prevention and treatment of colorectal cancer," he said.
Lower Mortality with Higher Levels
In the current study, Dr. Theodoratou's team examined the association between plasma 25(OH)D and stage-specific survival in a prospective cohort of 1598 patients with stage I to III colorectal cancer. They also evaluated the interaction of 25(OH)D level with VDR gene locus and VDR polymorphisms (rs1544410, rs10735810, rs7975232, rs11568820), and calculated a protective score for alleles that suppress disease in otherwise susceptible individuals.