Summary of findings
This study, of 1,447 prostate cancer cases and 1,449 healthy controls from the ProtecT study, investigated associations of circulating plasma total 25(OH)D with prostate cancer risk and, in cases, with stage and Gleason grade. There was evidence of a 2-fold increased risk of advanced versus localized prostate cancer and high-grade versus low-grade cancer in men deficient in vitamin D. There was no evidence of an association between total 25(OH)D and overall prostate cancer incidence (p = 0.50).
Our findings agree with some previous epidemiological studies. Our published systematic review and meta-analysis found no overall association between total 25(OH)D concentrations and prostate cancer (14 studies, 4353 prostate cancer cases; random-effects OR per 10 ng/mL increase in total 25(OH)D = 1.04, 95% CI: 0.99,1.10; p = 0.12), or aggressive prostate cancer (6 studies, 871 aggressive prostate cancer cases; random-effects OR per 10 ng/mL increase in total 25(OH)D = 0.98, 95% CI:0.84,1.15; p = 0.78).12 There was, however, weak evidence of a possible small decreased risk of aggressive prostate cancer with increasing 1,25(OH)2D (2 studies, 696 aggressive prostate cancer cases; random-effects OR per 10 pg/mL increase in 1,25(OH)2D = 0.86, 95% 0.72,1.02; p = 0.09). This result was based on only two studies, so CIs were wide, but a potentially important protective effect of 1,25(OH)2D on advanced prostate cancer could not be excluded. The “Aggressive” prostate cancer was defined as advanced stage and/or high grade (as defined in individual studies) and the analysis compared cases with controls, rather than case-only analyses. Our current analysis supports others in indicating no association between total 25(OH)D and overall prostate cancer risk, but does support previous epidemiological and in vivo studies, suggesting that higher levels of vitamin D protect against more aggressive forms of prostate cancer amongst cases. A recent study by Fang et al.25 also found evidence that prostate cancer patients with lower 25(OH)D had a higher risk of developing metastatic or fatal prostate cancer.
The association between total 25(OH)D and more aggressive prostate cancers amongst cases is consistent with experimental data showing that low total 25(OH)D concentrations reduce differentiation, and enhance cancer invasion and angiogenesis.3, 4, 6 Prostate cancer cells have reduced 1-alpha-hydroxylase activity as compared to normal prostate epithelial cells, and thus have a reduced ability to locally convert 25(OH)D to 1,25(OH)2D.26 Genes related to the vitamin D pathway suggest a link with advanced disease in particular, with a recent genetic association study and meta-analysis of 13 studies finding an association between three vitamin D receptor gene (VDR) polymorphisms (BsmI, ApaI and TaqI) and prostate cancer grade27: ApaI-a and BsmIb increased the risk of a high grade, TaqI-t was protective. These polymorphisms modulate the activity of the VDR and therefore may modulate a protective effect of 1,25(OH)2D on aggressive prostate cancers.
Some studies show positive associations of both low and high total 25(OH)D14, 28–30 with prostate cancer, but a review by Yin et al.16 concluded that the overall literature is inconsistent. A recent study by Albanes et al.15 found that men with higher 25(OH)D concentrations were at increased risk of developing prostate cancer over up to 20 years follow-up (40–60% increased risk in men with the highest vs. lowest levels). They found no evidence of greater risk in men with low vitamin D status. Our analysis found no evidence to replicate these previous findings of an increased risk of prostate cancer with both high and low 25(OH)D levels. The range of total 25(OH)D was comparable between these studies and our data [mean total 25(OH)D ranging from 18.5 to 24.6 mg/mL]. Differences in total 25(OH)D across studies may relate to different methodology used to measure total 25(OH)D, or to other factors such as differing latitude or fortification practices between countries in which the studies were conducted.12 Our data add to those of Yin et al in suggesting no evidence of a U-shaped relationship.
The overall level of circulating total 25(OH)D (mean = 22.9 ng/mL) was slightly higher than that reported by the National Diet and Nutrition Survey, designed to be representative of the UK population, which found that the median concentration of plasma 25(OH)D in men aged 50 to 64 years was 20.3 ng/mL (SD = 8.5; n = 190).31 This may be method-related or because included men were healthier than the general population in order for them to be eligible for the ProtecT study (as men were excluded due to pre-existing comorbidities that would rule out surgery or radical radiotherapy). Also, ProtecT includes few men who identified themselves as black or South Asian, who are likely to have lower total 25(OH)D levels.32 An age-related decline in total 25(OH)D levels is expected by, on average, between 0.3 and 0.6 ng/mL per year.33 We found that vitamin D levels increased with age, which may be because retired men who are healthy have more opportunity for sun exposure.34
Variation of total 25(OH)D by season of blood draw, with the highest levels of total 25(OH)D occurring in the summer months and the lowest in winter, has been consistently demonstrated.33, 35 We accounted for seasonal variation in our analysis by matching cases and controls on strata that included date of blood draw. In the case-only analysis, where strata-matching was not accounted for, we adjusted for season of blood draw. Increasing adiposity has consistently been associated with decreasing circulating total 25(OH)D concentrations,33–37 but BMI was not strongly associated with prostate cancer in ProtecT38 and it did not confound our observed associations.
Strength and limitations of our study
All of our men were resident in the UK and 99% of our subjects self-reported their ethnicity as white so we cannot assess differences by large changes in latitude or ethnicity. It is also possible that we are studying a relatively healthy population, within which there is not enough variation in total 25(OH)D to be able to ascertain correlations with very high or very low levels. It is widely accepted that total 25(OH)D is the best indicator of short term vitamin D status39 but measuring circulating total 25(OH)D may not reflect biologically active levels in tissues, since the prostate can locally produce 1,25(OH)2D.40 However, levels of 1,25(OH)2D are tightly regulated and only fluctuate during insufficiency or severe deficiency of total 25(OH)D. 25(OH)D3 has a half-life of 2 to 3 weeks,33 so a single measurement of total 25(OH)D at study baseline may not reflect vitamin D status over the life-course, which may be most relevant to cancer etiology.41 Indeed, prostate cancer risk in UK South Asians is consistent with high sun exposure during young adulthood conferring life-long protection.42 However, studies assessing the reliability of measurements have shown a low within-person variability over 5 years (14.9%, 95% CI 12.4, 18.1)43 and a high agreement across time points with intraclass correlation coefficients of 0.7 between samples collected three28 and 5 years apart.43 Serum stored for over 40 years was found to have the same predictive power as serum stored for up to 2 years,44 indicating that even though our samples were stored for 4.4 years on average, this should not affect our results.
There is potential for residual confounding, as vitamin D status is likely to be a marker of overall health but we have adjusted for the main risk factors for prostate cancer. Since the decision to biopsy was based on PSA level, some of the controls with PSA <3 ng/mL will have unidentified prostate cancer45 (misclassification bias) but this would not affect our analysis of advanced versus localized cancers (as all cancers were biopsy confirmed). Studies have suggested an inverse association between total 25(OH)D and PSA level,46, 47 but there was no evidence of an association between PSA level and total 25(OH)D in our data. Therefore any misclassification of cancer status is likely to be nondifferential with respect to vitamin D status, at most moderately attenuating any effect-estimates. More affluent men tend to have PSA tests and thus be diagnosed with prostate cancer, and such men may have higher vitamin D exposure48 but this would not affect our study where all men had PSA tests. Since the men had prostate cancer at the time of 25(OH)D measurement, it is possible that our results are due to reverse causality (more advanced cases causing lower circulating vitamin D levels). Our case-only comparison uses 25(OH)D levels measured in men diagnosed at the time of their advanced/high-grade cancer versus men diagnosed at the time of their localized/low-grade cancer. We do not have information on 25(OH)D levels before disease progression, and therefore cannot rule out reverse causality.
The strengths of our study are the large sample, about which we have extensive information recorded. Circulating total 25(OH)D concentrations were measured at one laboratory, in as few batches and in as short a time frame as possible (thus attenuating any potential technical errors of measurement). The likelihood of falling into the various vitamin D exposure categories is highly dependent on the season of blood draw. However, we know the date of blood draw, and can therefore adjust analyses for season. Recall bias is unlikely since the questionnaire data were predominantly collected before the results of the PSA test being known (in 85% of men). The study is population-based and thus subject to little selection bias. Using total 25(OH)D as a biomarker captures vitamin D exposure from sunlight, diet and supplements.