Sometimes I read older papers and learn new things. One such paper was written by Professor Emeritus Lars Olof Bjorn of Lund University in Sweden 13 years ago.
The topic was on vitamin D in an ecological context, as the title suggests, meaning how vitamin D presents itself in the environment/ecology.
I learned that some plankton need to ingest fungi in order to contain vitamin D and those fungi usually contain vitamin D2, which is common in plants. However, most phytoplankton and zooplankton have been producing vitamin D for more than 500 million years. It is my understanding that zooplankton produce D3 by themselves.
Tiny shrimp eat the zooplankton, small fish eat the shrimp and cod fish eat the small fish, explaining why cod liver oil used to contain so much vitamin D. I say used to contain as virtually all manufacturers of cod liver oil now add vitamin D to their product, although that was not always the case.
For unclear reasons, the vitamin D content of cod liver oil has fallen dramatically in recent decades. This may be because deodorization of cod liver oil removes vitamin D that is then replaced later in the process. Even still, the vitamin D content of naturally occurring cod liver oil seems to have fallen with time. For example, one manufacturer sells natural cod liver oil (no vitamin D or vitamin A added or withdrawn). A tablespoon of it contains only 3 to 60 IU of vitamin D.
As sunlight exposed zooplankton is the ultimate source of vitamin D for fatty fish, this reduction in the vitamin D content of cod liver oil over time may be due to the current commercial cod fisheries being in the artic, while they were in more temperate latitudes last century. In the artic, the sun is not up high enough on the horizon for the plankton to make much vitamin D.
I also learned from Professor Bjorn that the leaves of tomato plants contain vitamin D3. Not the tomatoes, only the leaves. Why tomato plants have D3 and not D2 is anyone’s guess.
I also learned that some grasses have vitamin D, but that may be due to fungi living inside the grasses.
Finally, I learned from Professor Bjorn that reindeer lichen, the moss on the tundra in northern climes, contains mostly D3. How it is manufactured in lichen when the sun never gets very high in the sky is a mystery to me. However, it explains why reindeer meat is a one of the few meat sources of substantial amounts of vitamin D3. It is also why the 25(OH)D levels of reindeer have little seasonal variation.
Vierimaa H, Timisjärvi J, Eloranta E, Saarela S, Ruokonen A, Leppäluoto J. Effects of seasonal photoperiod on serum 25-hydroxycholecalciferol and calcium in reindeer, Rangifer tarandus tarandus. Int J Circumpolar Health. 2000 Jan;59(1):33-7.
In the artic, reindeer get their vitamin D from eating lichen, not from the sun. While vitamin D is found in a variety of plants, it’s in such insignificant amounts that getting enough from diet is something man simply cannot do to fulfill their vitamin D requirements.