Vitamin C is often taken to what ius called "bowel tolerance." That means one takes increasing doses until they start to have loose bowels. they then reduce their dose to proper bowel function.

An easy way to tell if you are peeing out vitamin C is by the color of your urine. Bright yellow urine contains vitamin C. You will notice that the same dose will not always show up ones urine. This is an example of how the body needs more vitamin C some days that others. Stress gobbles up vitamin C. Good health allows your body to expel some.

B-12 can be measured in blood as can folate. Many do not absorb B-12 well so they need the individually specific supplement.

As Dr Smith said, vitamins have been studied for years. My father was on a strong vitamin regimen since his early 30's. He passed away at 80 due to a neurological injury but with a very healthy body otherwise.

The Big Pharma's try to discount vitamins because they prefer to sell patented drugs to treat the same maladies. But lets not get into that controversy.