If not deficient - no benefit
Treating Vitamin D deficiency will improve health, that is how I understand the definition of deficiency. It did therefore not surprise me in Zhang' s met-anaylsis (1) that a 16% survival benefit in cancer patients was found, but no reduced all cause mortality. This might be because more than half of all the participants of this met-analysis were not vitamin D deficient, their 25(OH)D levels were above 50 nmol/l. (last paragraph in “Strength and limitations”) Therefore any RCT in such groups will only show diluted results. Both, vitamin D and placebo group, have far too many replete individuals and giving vitamin D to those would not result in any health gain. Cancer patients, on the other hand, tend to have very low 25(OH)D levels (2) and will most likely all benefit (3). Zhang could therefore find a significant survival difference between the vitamin D and the placebo group in cancer trials.
Zhang Y, Fang F, Tang J, Jia L, Feng Y, Xu P, et al. Association between vitamin D supplementation and mortality: systematic review and meta-analysis. BMJ. 2019;366:l4673.
Hauser, K., Walsh, D., Shrotriya, S. et al. Low 25-hydroxyvitamin D levels in people with a solid tumor cancer diagnosis: the tip of the iceberg? Support Care Cancer (2014) 22: 1931
Personal experience as GP in 10 years. I visualise a ‘turn-over’ of vitamin D in the body. Vitamin D is involved in so many steps in the complicated cascade of events in our immune system and it appears to be ‘used up’. Therefore lower blood levels have always been found in diseases where inflammation is involved, rheumatoid arthritis, colitis, cancers etc. And correcting those deficient vitamin D levels brings usually great benefits.
Competing interests: No competing interests