Great thread. There's so much benefit to be gained from 'natural' methods that is kept from us and it's a crime, if you ask me.
(Watch out for Codex... I can only hope and pray that it never comes to fruition.)
I take vit D3 1000mg/day (conservative I know!), but also take several grams of vit. C daily, fish oil, high-dose vitamins, etc. I've had a lot of major health issues and between supplementing and a whole foods, cooked-from-scratch diet has brought me up to a higher level of health than I ever knew. I won't tell others how to eat, but if there's any advice to be receptive to, it's to stop consuming sugar fullstop. It depresses the immune system.
All the chemicals in processed foods etc. should really be avoided too - none of them do anything very beneficial for you, and lots have been shown to cause harm.
But back to vit. D3, been taking this for just under 2 years (along with other supps.) and would say it's helped heaps
Not had a cold/flu in 18 months, which is much improved from 2-3 per year.