One Web site that I think is good for vitamin information is www.drweil.com
. I think the vitamin potency question is a bit complicated because the marketing hype doesn't always match the bio-availability/bio-potency. As with everything in the consumer shell-game, it's easier to do labels and ad copy than it is to produce a product that is effective when and how it is used.
One vitamin example that has real potential for a disconnect between a synthetic product and a naturally derived product is Vitamin E. As I understand the current belief, there are about eight forms of tocopherols in the "natural" Vitamin E family. It is fairly common for synthetic Vitamin E products to contain three or fewer forms and some contain only one form (alpha). (The alpha form probably makes up the largest single component of a native tocopherol mix, but I don't believe that justifies ignoring the other forms.) A natural source "should" contain all forms, but the inquiry really doesn't end there. There still is the question of whether there is enough of each form in the mix and whether the components are pure and stable so that they remain available when they are ingested. One would think that a plant extract would retain its original proportions, but that can depend on how well the extraction is done and the quality of the original material. (At least one of the multi-level marketed supplement lines that claims some "natural" connection appears to fail fairly badly, despite their slick marketing.) The state of the art probably is such that we don't know everything about how the Vitamin E family works in vivo, so who knows what we will think about the optimum tocopherol mixture in 20 years?
I still believe that the best place to get vitamins is from fresh organic food, but there's an additional question about vitamin potency/availability from food. Particularly for the water soluble vitamins (C & B complex for the most part), the issue arises as to how much cooking and shelf time degrades the vitamin content (I wonder if what cafeterias call "cooked broccoli" has any Vitamin C left in it at all). It is one thing to say that an organic orange fresh off the vine has a certain amount of Vitamin C, but it is quite another to say that juice made from the same orange and consumed 3 weeks/months later has the same Vitamin C content. The same thing would seem to apply for processed foods where vitamins have been added/sprayed on before packaging, like Total cereal. For those that pore over the USDA's nutrient content lists, the difference between raw food content and cooked/processed food content can be striking. Their vitamin content values for cooked/processed food is the maximum that a digestive tract would see, and they can be reduced quite a bit from that. (I have a related problem with how the USDA's calorie values and protein content are derived, but that's another windy day topic.) One thing to note about recommended daily values for vitamins is that they lean a lot more on the "vitamin functions" that the vitamins perform and lean a lot less on the more general antioxidant functions that we believe some of them perform.
Credible evidence is mounting to show that organic fruits and vegetables can and often do start with higher nutrient values, particularly when antioxidants like polyphenols are considered within the nutrient definition. Whether that advantage can be neutralized by detrimental cooking or storage techniques is another issue. There seems to be general confusion in the public about the entire antioxidant concept, but that's a topic for another day. One nagging question is whether and to what extent synthetic pesticide residues adversely affect the overall vitamin and antioxidant process in vivo. Because we don't know everything about how things interreact in the body, we don't know much about how pesticides and combinations of toxic contaminants affect body functions. I hope that the "Body Burden" study being done by Mount Sinai School of Medicine in New York, in collaboration with the Environmental Working Group and Commonweal, will generate some work that could help answer that (a description of the Body Burden study is at http://www.ewg.org/reports/bodyburden/es.php