RT Journal
A1 Glaser JH
T1 VItamin d supplements vs plasma level
JF Archives of Internal Medicine
JO Archives of Internal Medicine
YR 2009
FD September 12
VO 169
IS 16
SP 1536
OP 1536
DO 10.1001/archinternmed.2009.280
UL http://dx.doi.org/10.1001/archinternmed.2009.280
AB 
An alternative explanation is that vitamin D supplements have a relatively small contribution to vitamin D plasma levels. Chiebowski et al3 found that many women who, before the WHI study, were ingesting the lowest level of vitamin D (diet plus supplements) had the highest plasma levels of vitamin D. The low correlation coefficient (r = 0.19) for supplements vs plasma levels shows that other factors such as sunlight exposure, body mass index, or genetic factors have a more significant effect on plasma levels. In fact, Chlebowski et al3 showed a similar contradiction as noted by Giovannucci et al2: plasma levels of vitamin D had no statistically significant effect on breast cancer, but the women who, before the WHI study, were ingesting the highest intake of vitamin D had an increased risk of breast cancer.