[adrian_b]

Cod liver oil is probably the best choice, as it also includes DHA, EPA and vitamin A, providing most things that would not be provided by vegetable oil (the only essential fatty substance that is neither in vegetable oil nor in cod liver oil is vitamin K2).

Well made cod liver oil is tasty and you can add it to food together with whatever else kind of oil you prefer (after food is cooked, not before, as it is heat sensitive). No more than 10 mL/day is necessary.

At least at the analysis reports that I have seen in the EU, fish oil has never been found with high content of mercury, even if the fish from which it has been extracted are likely to have been contaminated with mercury. Moreover, cod liver oil is sold in the EU as recommended for children and pregnant women. I doubt that any company would have the guts to sell such products here without taking care to make frequent chemical analyses to ensure that the product is never contaminated.

Chicken liver is also rich in vitamin D, but it is not advisable to eat great quantities, because it may contain too much vitamin A (which is toxic in excessive amounts). The amount of vitamin A in chicken liver or turkey liver is pretty much unpredictable, because it may vary by more than an order of magnitude between various producers, depending on how they feed the birds.

Most vitamin D3 pills contain vitamin D3 that is produced from sheep wool (i.e. from lanolin).