Vitamin D3 increases the total 25(OH)D concentration more than vitamin D2. Vitamin D2 supplementation was associated with a decrease in 25(OH)D3, which can explain the different effect on total 25(OH)D.

Results: Seven articles and 2 unpublished study data sets with 640 subjects and 14 study groups met the inclusion criteria and were included in this meta-analysis. Compared with controls, the consumption of fish increased 25(OH)D concentrations, on average, by 4.4 nmol/L (95% CI: 1.7, 7.1 nmol/L; P , 0.0001, I 2 = 25%; 9 studies).The type of the fish also played a key role: the consumption of fatty fish resulted in a mean difference of 6.8 nmol/L (95% CI: 3.7, 9.9 nmol/L; P , 0.0001, I 2 = 0%; 7 study groups), whereas for lean fish the mean difference was 1.9 nmol/L (95% CI: 22.3, 6.0 nmol/L; P , 0.38, I 2 = 37%; 7 study groups). Short-term studies (4-8 wk) showed a mean difference of 3.8 nmol/L (95% CI: 0.6, 6.9 nmol/L; P , 0.02, I 2 = 38%; 10 study groups), whereas in long-term studies (w6 mo) the mean difference was 8.3 nmol/L (95% CI: 2.1, 14.5 nmol/L; P , 0.009, I 2 = 0%; 4 study groups). Conclusion: As the major food source of vitamin D, fish consumption increases concentrations of 25(OH)D, although recommended fish intakes cannot optimize vitamin D status.Am J Clin Nutr 2015;102:837-47.

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Daily supplementation of 20 µg vitamin D during winter is unlikely to change cardiovascular risk profile.