In the present study, 400 IU/day of vitamin D3 tended to increase total 25(OH)D levels more when compared with the same dosage of vitamin D2 (p = 0.08). The underlying basis for this finding appears to be a concurrent decrease in 25(OH)D3 after supplementation with vitamin D2. This finding is in keeping with a number of studies which demonstrated a decrease in 25(OH)D3 after vitamin D2 supplementation either daily or weekly [8–10]. However, there was also one other study which could not demonstrate a concurrent decrease in 25(OH)D3, and found that vitamin D2 and D3 were equipotent in improving vitamin D status . It is noteworthy that nearly all of the studies that demonstrated less efficacy of vitamin D2 in increasing 25(OH)D levels used relatively higher doses of vitamin D, indicating that the less efficacy of vitamin D2 may be dose-dependent [9, 12]. The recent meta-analysis indicated that vitamin D3 is more efficacious at raising serum 25(OH)D concentration when given as a bolus dose [50,000 IU single dose (oral), 300,000 IU single dose (oral and intramuscular) and 50,000 IU/month (oral); p = 0.0002] compared with administration of vitamin D2, but the effect was lost with daily supplement [1,000-4,000 IU/d; p = 0.10] . However, our findings suggest that this is not the case, since we were able to demonstrate the trend of the difference between D3 and D2 supplementation even at a lower dose, i.e. 400 IU daily. All of the previous studies were performed in Caucasians, and it is unclear if ethnic differences in vitamin D metabolism could be the basis of our findings. The decrease in 25(OH)D3 after vitamin D2 supplementation, same as reported in the study of Demetriou ET, et al. , likely due to competition for the 25-hydroxylase enzyme by vitamin D3 and vitamin D2. However, it is probable that enzymatic catalyzation by other enzymes with relatively minor roles, such as CYP24A1  and CYP3A4, may be different for vitamin D3 and D2, and thus be partially accountable for the observation.
The ability of either vitamin D2 or vitamin D3 to increase circulating 25(OH)D varies considerably among individuals. The explanations for the large between-individual difference include differences in adiposity , enzymatic degradation of vitamin D metabolites , and dietary composition , as well as fat malabsorption . In the present study, vitamin D2 or vitamin D3 supplementation resulted in varying increases in 25(OH)D. No association with adiposity as assessed by BMI, however, was demonstrated. Since the number of study subjects was small, the lack of power to detect association could possibly be responsible. Dietary composition and enzymatic degradation of vitamin D metabolites were not assessed in the present study.
The present study demonstrated that DBP genetic variation is another factor which can influence the responsiveness to vitamin D supplementation. The major function of DBP is the binding, solubilization and transport of vitamin D and its metabolites . A previous study found that both serum 25(OH)D3 and 1, 25(OH)2D3 concentrations were significantly lower in mice lacking DBP, compared to wild-type mice . In another recent study, Lauridsen et al. showed that DBP phenotype determines the median plasma concentration of 25(OH)D3 and 1, 25(OH)2D3. With regard to genetic variation, single nucleotide polymorphisms in the DBP gene have been demonstrated to be related to 25(OH)D levels in Caucasians and Africans [4, 6]. A difference in the response of serum 25(OH)D after vitamin D3 supplementation according to DBP genetic variants has also been reported . It is of note that the DBP genetic variants affected the change in vitamin D status only for vitamin D3 but not vitamin D2 supplementation in our study. One study reported that vitamin D3 had greater affinity for the DBP than vitamin D2 and 25(OH)D3 also had greater affinity for the DBP than 25(OH)D2. However, the binding capacity of DBP cannot totally explain the difference in responsiveness to vitamin D3 supplementation as demonstrated in Armas, LA.’s study . After receiving single dose of 50,000 vitamin D3 or vitamin D2 orally, a similar rising of 25(OH)D levels in the first 3 days were observed in both group. However, much more rapid decline of serum 25(OH)D in the vitamin D2-treated subjects after 3 days seem to reflect substantially more rapid metabolism or clearance of the vitamin D2 metabolite . We speculated that the greater affinity of 25(OH)D3 for the DBP and less clearance of 25(OH)D3 would maintain its serum levels and accountable for the influence of DBP genetic variants in the case of vitamin D3,not vitamin D2. Further studies such as the differences in metabolism between vitamin D2 and vitamin D3 after binding to DBP are warranted.
There are a number of limitations in this study. The sample size is relatively small and may not have enough power to detect small changes, particularly those related to vitamin D2 supplementation. A 50 nmol/L difference in baseline 25(OH)D2 and 25(OH)D3 was present. This may influence the increment of 25(OH)D2 or 25(OH)D3 levels after receiving vitamin D2 or vitamin D3 supplementation, and thus possibly affect the total 25(OH)D levels at the end of the study. The change in 25(OH)D at the end of the study may also be influenced by lifestyle factors influencing the degree of sun exposure during the course of the study. This potential confounding effect cannot be assessed since there was not a non-supplemented group in our study. It would be important to know if both the vitamin D2 and vitamin D3 supplements had the same amount of vitamin D. However, vitamin D contents of both preparations were not measured. However, total 25(OH)D levels of subjects in both groups were similar at 3 months, suggesting that the variability in vitamin D content between vitamin D2 and vitamin D3 preparations, if any existed, was likely to be small. The other limitation is that multivitamin tablets containing vitamin D were used (in the vitamin D2 group), and it is unknown if other constituents of the multivitamin would affect vitamin D absorption or metabolism. It has been demonstrated that statins, atorvastatin and rosuvastatin in particular, can influence the circulating levels of 25(OH)D [21–23]. To our knowledge, no interference from common vitamins and minerals with vitamin D metabolism has been reported.