Vitamin D3 seems like the better choice for supplementation.

Despite considerable research, the health benefits of vitamin D 
supplementation in the general population remain controversial. In 
this systematic review and meta-analysis, investigators determined 
whether blood vitamin D levels and vitamin D supplementation were 
associated with risk for death.

One analysis involved 73 observational studies (mean follow-up, 0.3–
29 years) that involved 850,000 participants (median age, 63; median 
baseline blood 25-hydroxyvitamin D [25(OH)D] level, 20.7 ng/mL). 
Overall, compared with participants whose blood 25(OH)D levels were in 
the top third, those whose levels were in the bottom third had 
significantly greater risks for cardiovascular (CV)-related death 
(adjusted relative risk, 1.4), cancer-related death (ARR, 1.1), and 
all-cause death (ARR, 1.4). For each 10 ng/mL lower increment of 25(OH)
D, risk for all-cause death increased by 16%.

Another analysis involved 22 randomized, placebo-controlled trials 
(31,000 older participants; mean follow-up, 0.4–6.8 years) with data 
on the effect of vitamin D supplementation on all-cause mortality; 8 
trials provided vitamin D2 (dose range, 208–4500 IU/day), and 14 
trials provided vitamin D3 (dose range, 10–6000 IU/day). Vitamin D3 
supplementation significantly lowered mortality risk (relative risk, 
0.9), but vitamin D2 supplementation did not.

Comment

In this analysis, the observational data showed an inverse association 
between blood 25-hydroxyvitamin D levels and death, but reverse 
causality is possible (i.e., ill people having low vitamin D levels 
rather than low vitamin D levels causing illness). In randomized 
trials, vitamin D3 supplementation modestly lowered all-cause 
mortality risk; however, the optimal dose and duration of vitamin D3 
supplementation are unknown. Thus, widespread vitamin D 
supplementation should not be recommended.

Chowdhury R et al., BMJ 2014 
Apr 1; 348:g1903