Cod liver oil
The richest and most cited UK food source, cod liver oil, is actually very poor when compared to recent daily intake recommendations ranging from 1,000 internationl units (iu) to 2,000iu a day. The reason is that much of it is highly polluted, and it needs to be purified.

Various companies have tried to find ways to de-pollute and make cod liver oil palatable but, in all cases, these processes appear to have reduced the oil’s natural vitamin D content considerably. All but one of Nordic Naturals’ cod liver oil products, for instance, famous for their purity and an excellent source of essentially fatty acids, contain no more than 40iu of vitamin D per teaspoonful. Their capsules never claim more than 20iu. The one exception is their vitamin D-fortified lemon-flavoured Arctic-D Cod Liver Oil. One teaspoon (5ml) contains 400iu but, to date, we have not been able to find a source in the UK.

View Vitamin Research Products' vitamin D3 1,000iu supplement

Medicinal mushrooms
Certain sun-dried medicinal mushrooms are probably the richest food source, but try eating 20 grams every day.

These special mushrooms make their own vitamin D (specifically D2) when alive, and carry on making it after being harvested as long as they are exposed to UVB by being dried in the sun! Drying indoors does not increase their D2 content. The most astonishing by far are maitake and shiitake mushrooms, whose respective 136iu and 460iu per 100 grams when harvested soar to 21,400iu and 31,900iu per 100 grams after six to eight hours of drying in the sun. [1]

The human body converts D2 to D3 in a 3:1 ratio so would need 6,000iu of D2 to make 2,000iu of D3. That means you would need to consume 20gm of sun-dried maitake or 28 grams of sun-dried shiitake mushrooms every day to give the body sufficient vitamin D2 from which to make 2,000iu of D3 a day. Researchers have confirmed that the body was as good at generating D3 from mushroom D2 as from D2 supplements. [2]

Ed.- (i) According to Danish and Icelandic food statistics, haddock liver oil contains twice as much D3 as cod liver oil, but the only study we have found on this was in Polish and we have not found any UK sources of haddock liver oil.

(ii) Clearspring’s Dried Shiitake Mushrooms are the only sun-dried shiitake mushrooms easily available in the UK that we have found to date. If your local health food shop does not stock them, ask them to get some for you from Suma Wholefoods, who distribute for Clearspring. We have not been able to identify a source of sun-dried maitake mushrooms.

(iii) Paul Stamets, who led the research on sun-dried shiitake mushrooms, also noted that if the mushrooms were dried gills-up the level of D2 rose even higher - to 46,000iu/100g. Paul went on to write a book on the nutritional contents of mushrooms, entitled Running Mycelium.

View Vitamin Research Products' vitamin D3 1,000iu supplement