I have read quite a bit about vitamin D over the years. I've seen that mention also about vitamin D2 not being as helpful for animals. Vitamin D2 is the plant form of vitamin D, found naturally in small amounts in plants such as mushrooms. To be useful the body needs to convert D2 into the animal form of vitamin D which is D3. Some people have poor abilities at making the conversion from D2 to D3.
There is a study out there I remember showing that taking vitamin D2 led to shorter life when compared to vitamin D3. I do not remember the details of the study but imagine it to be a rodent animal study.
Personally I believe sensible sun exposure, where you avoid being burnt, is the best option for obtaining vitamin D. There are other substances that are created by the body when the rays of the sun strike our skin. For example there are other vitamin D compounds manufactured in our skin that we can not obtain from vitamin pills.
Additionally there are other substances created when exposed to the sun. Another item made by the sun is feel good endorphins. Many feel endorphins have health promoting abilities for those with IBD conditions such as Crohns and Colitis, along with autoimmune diseases such as MS, ALS, etc.
This benefit from endorphins might be seen with the use of the prescription drug Low Dose Naltrexone (LDN) One of the main theories on why LDN is helpful for a great many autoimmune diseases is that LDN raises endorphin levels significantly.
I was looking at an LDN testimonial sight recently and it is surprising at how many conditions LDN often appears to help improve. The sight can be seen here:
http://www.ldndatabase.com