Vitamin D supplements do help your immune system, reducing respiratory tract infections, and seem to help with some autoimmune diseases. There have also been some positive findings regarding clinical depression, and improved bone health.
Currently there is no evidence that they help with cancer or cardiovascular disease, but that doesn't mean that someone who is deficient should not try to solve that deficiency with supplements.
Dosing the supplements is the difficult part, ingested Vitamin D absorption is different from person to person, and it's further complicated by variable sun exposure and skin production. 4000 IU per day should be a safe dose, but it's probably not enough with zero sun exposure during winter...
Currently there is no evidence that they help with cancer or cardiovascular disease, but that doesn't mean that someone who is deficient should not try to solve that deficiency with supplements.
Dosing the supplements is the difficult part, ingested Vitamin D absorption is different from person to person, and it's further complicated by variable sun exposure and skin production. 4000 IU per day should be a safe dose, but it's probably not enough with zero sun exposure during winter...