A growing number of epidemiologic studies report a reduced lymphoma risk in relation to increased levels of a variety of personal UVR exposure indicators [4
]. Similarly, we found that people who reported a history (i.e., 5–10 years ago) of sunbathing with the intention to tan (>1 time per week compared to never) had a 72% lower risk of lymphoma. These findings are consistent with the overall findings reported from the InterLymph pooled analysis of sun exposure and NHL risk, the most comprehensive analysis of this association to date, which observed that increased recreational sun exposure was significantly associated with a decreased risk of NHL, yet the observed decreased risk with increasing overall sun exposure failed to reach statistical significance [10
]. In light of both the role of UVR in vitamin D production [11
] and published research to suggest a protective effect of vitamin D with regard to malignancy [13
], we subsequently evaluated the association between vitamin D levels and lymphoma risk.
Of the 9 previously published studies investigating the association between vitamin D and NHL, 6 have relied exclusively on recall of dietary intake on a food frequency questionnaire for exposure assessment [4
]. However, there are very limited dietary sources of vitamin D, and significant variability of vitamin D content in both the naturally occurring and fortified sources has been well established [29
]. Circulating 25(OH)D concentrations represent the combined contributions of both UVR and dietary (D2
) sources of vitamin D [16
], and the long half-life of this metabolite makes 25(OH)D the major circulating form of vitamin D [15
], and the preferred biomarker for determining vitamin D sufficiency. As such, we measured current serum 25(OH)D to evaluate the association between vitamin D and lymphoma risk. The OR estimate of the association between vitamin D insufficiency (serum 25(OH)D <30 ng/ml) and lymphoma risk indicates that vitamin D insufficiency was not an independent marker of lymphoma risk. Furthermore, secondary evaluation of the association using a three level ordinal vitamin D variable (tertiles defined by 25(OH)D distribution among the controls) did not demonstrate a dose-response, and the odds ratio for 25(OH)D as a continuous variable was near unity.
Our findings are consistent with the literature to date, which, overall, provides limited support for an association between vitamin D status and lymphoma [4
]. With the exception of the findings by Polesel et al. [23
], and Lim et al. [27
], the published estimates of association with dietary vitamin D intake or serum 25(OH)D and lymphoma risk are largely weak or null. Limitations in retrospective vitamin D insufficiency exposure assessment in epidemiologic research have been discussed in the literature [39
], and may be obscuring a true influence of vitamin D status on lymphoma risk in the current study and the previously published literature. In this study, we were limited to assessing current 25(OH)D with the assumption that this level was consistent with average adult vitamin D status. However, we were able to demonstrate an association between past (5–10 years prior) sunbathing frequency and lymphoma risk. Additionally, we confirmed that current sunbathing frequency (within the 4 weeks prior to study visit) was in fact independently predictive of serum 25(OH)D after adjustment for other dietary and demographic factors as well as season of blood draw, an observation consistent with published data indicating that active sunbathing was the strongest determinant of serum 25(OH)D in healthy Danish women [40
]. Together, these observations raise the possibility that vitamin D could have had a role in the association between past UVR exposure and lymphoma risk if there had been a change in sun exposure behaviors over the 5–10 prior to study participation. As current sun exposure variables were reported for only the 4 weeks prior to study participation, and are thus limited to a particular season, we are not able to assess this directly with our data. Although there is evidence of stability of serum 25(OH)D levels within individuals over several years [41
], our findings suggest the importance of more direct vitamin D status assessment at an etiologically relevant time period prior to diagnosis in order to determine the role of vitamin D in the reported relationship between UVR exposure and lymphoma risk.
A limitation of the case-control design is that we must evaluate whether the malignant disease process among the lymphoma cases may be affecting vitamin D status as opposed to vitamin D being related to lymphoma etiology. Lymphoma symptoms could possibly impact vitamin D levels by changing UVR exposure behaviors. However, strengths in our case selection and recruitment methods minimize this concern. Our study includes case patients who were recruited very soon after diagnosis (median time between diagnosis and study consent was 21 days), minimizing the likelihood that their diagnoses changed behaviors that would impact serum 25(OH)D. In addition, the case population was largely asymptomatic (79%), and limiting the analysis of the association between current measured 25(OH)D and lymphoma risk to patients without documented B symptoms at study participation did not change the results. Furthermore, patients with hypercalcemia, likely related to their hematologic condition if present [42
], were ineligible for study participation.
Control selection is a critical factor in case-control study design. In clinic-based studies such as this, defining the base population from which the cases originated becomes difficult. The vast majority of the previously untreated lymphoma cases seen in the James P. Wilmot Cancer Center Lymphoma Clinic come from 17 Western New York counties. However, due to referral patterns to this academic clinic, not all cases from this geographic area had a chance of inclusion in the case set. As such, clinic-based controls are more appropriate, even over randomly selected healthy controls from the base population. The General Neurology Clinic has available an adequate patient base, and a similar referral pattern due to academic medical center affiliation, such that a sufficient number of controls with diseases unrelated to the exposure of interest could be identified. With the rapidly growing list of chronic diseases for which vitamin D is reported to possibly play an etiologic role, it is difficult to choose a clinic with a patient base having diagnoses unrelated to vitamin D. However, the enrolled control group was recruited in the outpatient setting, represents a heterogeneous group of neurological diagnoses and symptoms, and does not include patients with diseases (or on medications; e.g., seizure disorders and medications) known to impact serum 25(OH)D. Moreover, the prevalence of vitamin D insufficiency among the controls (73%) is consistent with the recently reported prevalence of vitamin D insufficiency (77%) among the healthy general adult population in the United States [12
], providing some confidence that the conditions among the controls are not associated with differential UVR or vitamin D exposure as compared to the general population. However, we do acknowledge that this comparison may be confounded by a wide variety of covariates. As such, we cannot fully rule out a decrease in 25(OH)D as a result of diagnoses, either directly or as a result of a change in behavior, among the controls.
Despite the considerable clinical heterogeneity of the lymphoma subtypes [3
], the studies to date that have evaluated the association between vitamin D status and lymphoma risk, current study included, have been designed to evaluate this association with all lymphoma subtypes combined as the primary hypothesis. As discussed recently by Evens and Chiu, evaluation of distinct etiologic processes within the NHL subtypes is one of the major and ongoing challenges in epidemiologic research [44
]. In exploratory analyses, we found no evidence of heterogeneity in effects by subtype, although we evaluated only the two most common lymphoma subtypes and statistical power was limited. It is possible that any potential association between vitamin D status and individual lymphoma subtypes could be masked when the subtypes are combined.
Furthermore, the true dose-response relationships between vitamin D (dietary or serum 25(OH)D) and cancer is unknown [45
]. We defined vitamin D insufficiency using a clinically relevant 25(OH)D threshold (30ng/ml), currently thought to be minimum level required for maximizing the vitamin D health benefit [15
]. However, there is evidence in the literature to suggest that the threshold levels for an effect of 25(OH)D may vary by cancer type, and preventive effects may be limited to higher levels of 25(OH)D than anticipated [45
]. As such, the range of serum 25(OH)D measurements in our study may be too low to observe a protective effect with regard to lymphoma risk.
Finally, the relevant etiologic period of exposure for lymphoma is not clear, as the complete natural history of lymphoma prior to onset of symptoms is still undefined. With particular regard to cancer, many steps are necessary for malignant transformation [47
]. Most recently, Lim et al. demonstrated a differential association between 25(OH)D and lymphoma risk by length of follow-up, with a statistically significant protective effective of higher serum 25(OH)D on lymphoma risk observed only in the subgroup of subjects with less than 7 years of follow-up [27
], a finding consistent with the evidence presented by Smedby [8
] and Hartge [4
], emphasizing the importance of exposure assessment timing in investigations of lymphoma etiology.
In conclusion, we observed a decrease in lymphoma risk with increased UVR exposure consistent with previous reports, thereby providing further evidence of a true association. With growing consistency in the literature to suggest a protective role for UVR exposure in lymphoma, the null association between measured serum vitamin D and lymphoma risk demonstrated in our study may suggest the need to explore possible alternative explanations of association between UVR and lymphoma that are independent of the role of vitamin D. For example, there is evidence from experimental studies of immune system modulation by UVR, resulting in decreased immune challenge responses to antigens applied to even non-UVR exposed areas [48
]. In light of ongoing investigation into antigenic stimulation with regard to lymphoma risk [2
], one could hypothesize that this is suggestive of a role for direct influence of UVR, through immune modulation, without the impact of an intermediary such as vitamin D. Additionally, another hypothesis is that folate and folate derivatives, on which cell replication is dependent, are degraded by UVR [53
]. Decreased folate availability via UVR degradation could limit DNA replication, and thus slow the cell cycle, particularly among cells that are rapidly dividing (such as pre-malignant and malignant cells) [53
]. These two hypotheses, and certainly countless others, indicate the need for further critical evaluation of the association, either direct or indirect, between UVR and lymphoma risk and potential mechanisms for pathogenesis.
Alternatively, the limited evidence of an association between vitamin D status and lymphoma risk to date may very well be due to methodological limitations, particularly in accurate assessment of serum 25(OH)D levels during etiologically relevant periods, and further investigation of this potential association while addressing these methodological difficulties is warranted.