This is an old misconception stemming from the time when accurate
chemical analysis was not available. In the early days, nutrients were
detected by their biological activity in culture, and not by their
chemical composition. Thus, different sources of a nutrient were
compared, even though they were different substances.
"Natural" vitamin A was actually beta-carotene, which is virtually
nontoxic. "Synthetic" vitamin A was actually retinol which is toxic in
high amounts. The "natural" version was obviously superior to the
"synthetic" version, but only because it was a different chemical
substance.
"Natural" water contains healthful minerals, whereas "synthetic" water
(distilled water) lacks these. Obviously, "synthetic" water did not
include the mineral nutrients. Obviously, differring ingredients are
being compared.
"Natural" vitamin D3 is nontoxic and occurs in small amounts in foods.
"Synthetic" vitamin D2 is made by irradiating milk. This form of
vitamin D can be toxic in high amounts. Again, different chemicals are
being compared.
"Natural" vitamin C contains bioflavinoids and many cofactors that help
the nutrient, and also is often acid-buffered in foods. "Synthetic"
vitamin C is pure ascorbic acid, which lacks the other nutrients, and
often its acidity can cause gastric distress. Of course, different
chemical forms act differently.
Many times when people refer to "natural" versus "synthetic", they are
not comparing the same chemical substances. Often, they are comparing
different forms. The "natural" forms often have various foods,
cofactors, pH buffering, that provide different affects than is found
with a single, pure nutrient. All chemical function is based on
chemical content. The same substance will always act the same,
regardless of its source. Differing substances or mixtures will act
differently, each according to its ingredient list.
-- Harvey Newstrom <mailto:harv@gate.net> <http://www.gate.net/~harv> ...author of the "ultimate nutrient reference book" --American Libraries