Now that we have learned that each vitamin has its own letter designation, we can discuss number designations. We learned above that ergocalciferol is known as vitamin D. There are a few different types of vitamin D. To keep things simple, we use a subscript (small number to the right and a little below the vitamin letter) to designate the different types. It's really quite simple. Vitamin D₂ is the designation for ergocalciferol. Vitamin D₃ is the designation for cholecalciferol. Now that you have learned all the designations used for vitamins, let us now construct a more complete list.

We can even further divide some vitamins into a smaller category by placing a letter next to the number.

Don't let this confuse you. By adding the letter "A" to the B₁₂ it simply means that this vitamin (B_12a) is similar in structure to B₁₂ ,but has a minor modification, making it slightly different. This is just like owning a VW Golf, E or EF. They are both VW Golfs, but one of them has a slight modification. It's that simple.

Just in case your extra enthusiastic and want to know the difference between these two forms of vitamin B₁₂, they only differ slightly in structure. Vitamin B_12a is similar to B₁₂, with the exception of an added hydroxyl group. A hydroxyl group is merely an oxygen and hydrogen. However, this difference in chemical structure enables them to perform slightly different functions in different areas of the body.

Sources, Absorption and Utilization