I don't think the doctor is correct. 40+ yrs ago the only D available was D2 on RX ergocalciferol, and was made by Lilly and called Deltalin. This was the go to treatment for rickets. Many of the therapeutic manuals still have this in them, and that is why doctors continue to prescribe it. Lilly sold its patent long ago and the D2 is now brand Drisdol and many generics.

However I will look again. I am reminded of a fellow I ran into at an HMO clinic who took FOUR 50,000 IU D2s a day, for his medical problems. I have to say I suspected him of mailing them abroad, as many patients get doctors to do prescribe high for them as a favor.
Either that or his D2 was not really working well for him?

Doctors sometimes make stuff up, to impress patients, when they are caught without a good answer based in fact.

I'll come back here later today...

edit:
http://press.endocrine.org/doi/full/...0/jc.2004-0360
This article is from 2004. It is pretty clear.
But as it states, few studies get done on this subject.

This is a 2011 article... very long and very complex:
http://www.ncbi.nlm.nih.gov/books/NBK56061/

The way I look at this issue is this:
Claims are D2 is "less toxic"... well if that is true and
no studies are done on the claim, it suggests that D2 is not acting in the tissues hence is eliminated from the body.
Also D3 is the form made in the skin from sunlight. So it is natural to assume that it is more biologically compatible with the body.

The claims about "rickets" suggests only that the lower end of serum values really need to be met for success.
The ricket level usually is given as 30. The therapeutic levels today from research suggest 50-100, the 100 being mostly targeting MS patients. We see now that most foods in the US are fortified now with D3, and the D2 has been discarded.

If people insist on using D2 today, that is their choice. But studies are showing D3 to be superior to it.

And the bottom line? The comment by en bloc's doctor about the "cellular level" ? Both forms D2 and D3 have to undergo conversion to the active form called calcitriol.
This is considered a drug also on RX for people with kidney disease who do not convert the other two forms properly by the kidneys.
Its brand name is Rocaltrol (by Roche).
So D2 and D3 are not working on the cellular level until converted by the kidneys to the active calcitriol. The links I put up here, seem to say that D2 is slower in this regard.