. . they are often in a form convenient to manufacture but different from those found in vegetables and meats (like left handed vs. right handed molecules, as an example). They are sometimes much harder for the body to absorb or less effective and may have to be used in quantities that produce side effects.
Recent research on nutrition for flu resistance has shown natural sources to be better balanced and more effective. Suplements may be called for but should be examined with due caution.