Hundreds of experiments have proved this. Weight gains, vigor, survival rates were markedly different. Synthetic vitamins were better than none, but the natural were more effective and less toxic. This is mainly because nature always gives us her vitamins in groups. If one is extracted it is bound to incorporate fragments of others, which both enhance it yet dilute it and give it balance.
Many people are allergic to the chemicals used as a base for synthetic vitamins.
If the product is properly prepared, much of the toxins will be refined out. However, no poisonous substance is used in preparing natural vitamins. The best source of any vitamin is, of course, the foods in, which they are in. Milk, eggs, butter and meats of every kind, but especially the organ meats such as liver are high in natural vitamins. Good too are, fresh fruits and vegetables, nuts and seeds and unprocessed, grains.