There are 2 forms of vit D - D2 and D3. D3 is naturally occurring, in food typically in cod liver oil, eggs, and seafood. It is destroyed by heat, so the eggs and seafood would have to be consumed raw to get the D3 out of them. D3 is also manufactured in your epidermis when exposed to sunlight of sufficient intensity.
D2 is an easily synthesized form that was proven effective in preventing rickets and is the most commonly used form in clinical trials, despite having
been shown to be less efficacious in humans. D2 is added to pasteurized milk.
There is also an interaction effect with vit D and vit A, such that both are absorbed and used best by the body when they are taken in approximately a 1:10 ratio... but I need to go to bed now and I am not going to dig up that reference.
That article was poorly written and definitely on the scare-tactics side, BUT it's true that that baby might not have gotten rickets if his mother hadn't breastfed him. He would be more likely to have allergies, IBS, colic, stomach issues, immune system issues, require expensive dental work and have issues with obesity and god-knows-what-else, but not rickets. I don't think it's fair to say that breastfeeding was the cause - not sticking the poor kid outside for a few minutes with no clothes on is equally culpable, and a much cheaper fix than formula.
Anecdotally, my DD and I have been taking cod liver oil every day this winter, and we've had naught but a few days of sniffles, while everyone else we know has had 5-week nasty colds. We spend a lot of time at playgroups covered in other kids' germs. Make of that what you will, but I think it's worth getting over the gaggingly bad taste, and IME if you start kids on it early - like well before 1 year - they don't seem to mind the flavour.
ETA: Interestingly, the article I cited above has been referenced in a
more recent study, which shows that the above posters who blamed maternal levels of vit D were totally right - all the studies showing vit D not in breastmilk seem to have been done on vit D-deficient women (ie, most of North America & Europe) and if supplemented to an acceptable level, women's breastmilk has just fine levels of vit D. Someone at MSNBC didn't do their homework, if I was able to determine that from study abstracts on the internet in 15 minutes. Pffft.