>Now, please correct me and tell me that this analogy of mine is >incorrect
or partly incorrect, and then convince me that it's >harmless to take
supplements, or at least some of them.

Fredrik, it is a very good analogy. so supplements can work to the good or
ill. but mostly people are deficient in many, such as zinc, copper, vitamins
b12, folate, biotin, chromium, selenium.
this is because these are found strongly in liver, shellfish and sea
mammals.

but if you take too much b3, it will speed up carbohydrate metabolism, not
useful in a low-carb diet.

andrew