An interesting question came up on the Track Your Plaque Member Forum about vitamin D2 vs. vitamin D3. This often comes up among our patients, as well.
Vitamin D is measured in the blood as 25-OH-vitamin D and is distinct from 1,25-diOH-vitamin D, a kidney measure, a test you do not need unless you have kidney failure.
The human form of vitamin D is cholecalciferol and is usually obtained via activation of a precursor molecule in the skin on activation by the sun. You can also take cholecalciferol and it increases blood levels of 25-hydroxy vitamin D reliably.
However, there is a cheap, plant-sourced, alternative to vitamin D3, called vitamin D2, or ergocalciferol. D2 has far less effect in the body. Taking D2 or ergocalciferol orally is an extremely inefficient way to get D. Unfortunately, it's the form often used in milk and many supplements, even the prescription form of D. About half the multivitamins and calcium supplements I've looked at contain ergocalciferol rather than cholecalciferol.
Taking vitamin D2 yields very little conversion to the effective D3. This particular issues is maddening, as the USDA requires dairy farmers to add 100 units of vitamin D to milk, and D2 is often used. In other words, the D in many dairy products barely works at all. There are many children who rely on D from dairy products who are at risk for rickets and are not getting the D they need from dairy products because of this cost-saving switch. Do not rely on milk for vitamin D for your children.
D2 or ergocalciferol is often included in the blood measures of vitamin D along with vitamin D3. The only reason it's checked with blood work is to ensure "compliance,", i.e., see whether or not you're taking a prescribed ergocalciferol. Beyond this, it has no usefulness.
25-OH-vitamin D3 , or cholecalciferol , is both the blood measure and the supplement you need. This is the one that packs all the punch. Keep in mind also that it is the oil-based gelcap you want, with more consistent and efficient absorption. Tablets usually barely work at all, even if it contains cholecalciferol. Most people who take calcium tablets with D, or multivitamin with D, not only are getting a powdered form of D, but also in trivial doses. It's the pure vitamin D3, cholecalciferol, in gelcap form you want if you desire all the spectacular benefits of vitamin D.