Vitamin D is good for you
We’ll consider two categories of vitamin D supplementation: below and above the recommended levels.
1. What are the effects of vitamin D deficiency?
One theoretical meta-analysis paper shows that “several recent epidemiologic studies have demonstrated a strong association between vitamin D insufficiency and risk of cardiovascular disease, risk of diabetes and metabolic syndrome.”
This population study shows “low serum 25(OH)D levels are associated with a higher prevalence of PAD.”
Conclusion: If you are not getting the recommended dose of vitamin D, supplementation is advisable.
2. What are the effects of vitamin D at and above the recommended levels?
It looks like the effects on cancer prevention are dubious, although with more research we could find that it is effective on some types of cancer in particular. Likewise, its effect on cardiovascular disease seems nonexistent. Its effect on bone fracture is negligible, although with slight potential to be significant at very high doses.
Conclusion: supplementing vitamin D beyond recommended levels is unlikely to have significant effect. There is weak evidence that vitamin D can be harmful above recommended levels, so, overall, supplementation is around net neutral.
Parents:
If there aren’t side effects, it seems like the answer is probably yes, since vitamin D deficiency seems pretty real and not-vanishingly-rare. So it seems like the original question mostly comes down to (1) how common is vitamin D deficiency and (2) on average how bad are side effects for marginal vitamin D?