Are all of these Vitamin D-s inactive ? I have heard that Vitamin D in high doses cures some.
Commercial vitamin D supplements are the inactive form. Last I checked active vitamin D3 was selling for about $600/kg, so it is just too expensive for manufacturers to use in supplements. Especially when the public is not aware of the differences. This is the same reason that inactive vitamin D2 is added to milk. It is not going to do any good unless the body can convert it in to active D3. You will notice though that the milk containers only list "vitamin D" not "vitamin D2" or "vitamin D3". The average public has no idea what the difference is. So if they can get away with adding small amounts of D2 at lets say $3/kg as opposed to active D3 at $600/kg they are going to do it since it saves them money and the public does not know the difference. Many supplement manufacturers are no different. They are going to go with the cheapest stuff they can get by with based on the knowledge of that substance by the general public.
As a really good example of what I am referring to look at magnesium or calcium magnesium supplements as well as the magnesium found in most multivitamins. I would say that at least 90% of the time the magnesium being used will be magnesium oxide. I have even seen bottles labeled as "magnesium citrate" on the front and the first ingredient (indicating the predominant ingredient) is magnesium oxide mixed with a little magnesium citrate. Magnesium oxide is the cheapest garbage they can use in these supplements. It is also the least absorbed forms of magnesium, and it interferes with the absorption of other nutrients and digestion of proteins. To make matters worse magnesium oxide reacts with water forming the caustic magnesium hydroxide. Hydroxides (OH) are some of the strongest free radicals known, and are severely damaging to the tissues. Magnesium hydroxide is frequently used as a laxative such as with Milk of Magnesia. The reason it acts as a laxative is because the magnesium hydroxide chemically burns the intestinal wall leading to an influx of water and increased peristalsis. This also increases intestinal inflammation and the risk of cancer. But again the general public does not understand the difference between magnesium oxide, citrate, malate, asparatate, etc. Most people are going to go with cheap instead of researching effectiveness and safety. And the manufacturers know that given the choice between a $3 bottle of magnesium oxide and an $11 bottle of magnesium malate most people will go with the magnesium oxide because they don't know the difference.
Getting back to vitamin D high doses are great for a number of conditions including cancer and autoimmune conditions. But again D2 and inactive D3 are not going to do much of anything unless they can be converted in to active D3, which requires healthy liver and kidney function.