How much you need will be dependent on numerous factors (what a 110lb sedentary female needs and what a 220lb male athlete needs are going to be quite different) and isn’t 100% clear (as Lyle has already mentioned in this thread–lots of debate on the subject). In general, if you aren’t having issues, then you probably are not deficient. That is quite dumbed down, but a lot of people will over do it or find things wrong with them that aren’t wrong at all. Look at your diet and see where things are going and if you might be deficient in something. Overall, if you are getting a variety of animal meats, fruits, and vegetables and perhaps take a multi-vitamin, chances that you are deficient in anything is low. If you are in an area with low sunlight, supplementing with vitamin D (or going to a tanning bed or one of numerous other options) is likely a good idea as well, but those things become high individualized. The main point I’m trying to make is to not go overboard with the supplementation for the sake of it if there isn’t anything wrong in the first place.
I agree. You really have to determine your individual needs, just like training. I think most people take supplements on faith without any really objective measure of what their true needs states are, which of course the supplement companies thrive on.
I think a simple multi is a cheap and harmless insurance policy, with some extra Vit D in the winter. But ultimately you have to test your levels to know what you need. Even if you do need something, changing whole food intake would be my first choice. Granted there are some nutrients that are difficult to obtain in optimal amounts even from a great whole food diet (e.g., Vit D, DHA, EPA).
The home Vit D test that the Vitamin D Council offers through ZRT looks promising. I just wish there were similar convenient home-based tests for other nutrients that did not require the hassle and cost of a doctor’s appointment. I think that would be an extremely profitable business venture.
Immune enhancing? Well, along with what fogelson said, make sure you get your sleep (deep sleep) and reduce stress. Many people forget about that!