“Cod fish only contains vitamin D3. There is no vitamin D2 in either cod fish or cod liver oil.” -Dr. Jacob Exler, Nutritionist, USDA Nutrient Data Laboratory
Rosita EVCLO contains exactly what you would find present in the liver oil of a “living” wild codfish – nothing more and nothing less. Though vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) are available in a fish’s diet vitamin D2 is almost absent in fish and fish liver oils. A possible explanation regarding why this is the case is provided below:
What Forms of Vitamin D are Available in a Fish’s Diet?
- Phytoplankton Microalgae live at the surface of the water and vitamin D2 is probably formed via sun exposure of provitamin D (ergosterol). This is supported by the fact microalgae tested in August have a higher vitamin D content than in October and December.
- Phytoplankton Microalgae have been reported by some to also contain (in addition to ergosterol and vitamin D2) both vitamin D3 and provitamin D3. There is evidence for microalgae (phytoplankton) as the basis of the food chain is the origin of vitamin D3 in fish. The occurrence of vitamin D3 in microalgae suggests that vitamin D3 may exist in the plant kingdom and vitamin D3 has also been identified in several plant species.
- Zooplankton Zooplanktons principally contain 7-dehydrocholesterol as their provitamin D content. Codfish also consume zooplankton (i.e. Copepoda). Levels of vitamin D3 may vary depending on the time of year and depth in the sea that the zooplankton are found.
- Fish Skin Although there is some evidence for the synthesis of vitamin D3 in the skin of fish, vitamin D3 synthesis is likely to be negligible or minimal because of the limited UVB-light in their natural habitats combined with low amounts of 7-dehydrocholesterol in fish skin.
What forms of vitamin D are present in fish and fish livers?
It is obvious from what has been written above that both vitamin D2 and vitamin D3 are available for fish in their diet, but vitamin D2 is almost absent in fish. The only form of vitamin D that has been identified at significant levels in fish oils (both body and liver) is vitamin D3. This is also supported by the work of world famous vitamin D researchers including Dr Michael Holick who has never found vitamin D2 (ergocalciferol) in cod liver oil or other livers oils including flounder, red fish, mackerel, eel, and mullet liver oils. In addition, similar results have been obtained by the Institute of Aquaculture, Stirling University who have sampled fish livers and a great deal of unrefined fish liver oils over many years.
The Mystery Why Fish Liver Oils Only Contain Vitamin D3
What remains a mystery to scientists and marine experts is why the only form of vitamin D that has been identified in fish oils is vitamin D3. You would expect vitamin D2 would be present especially if vitamin D2 produced in phytoplankton was concentrated in the food chain. The main explanations for this observation include:
- Fish have the ability to discriminate between vitamin D2 and vitamin D3 and therefore only concentrate vitamin D3 in their visceral oils.
- Alternatively, fish or zooplanktons that consume phytoplankton have a similar capacity as Paramecium to convert the side chain of vitamin D2 into the side chain of vitamin D3.
- There is also a possibility the bioavailability of vitamin D2 is lower than for D3 in fish.
More about vitamin D
Humans obtain vitamin D from diet and sunlight. There are two main forms of vitamin D: cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2). The main source of vitamin D3 is from exposure to sunlight, which accounts for more than 90% of the body’s vitamin D requirement. Vitamin D3 is synthesized in the human skin by a photochemical conversion via ultraviolet B (UVB) exposure of provitamin D3 (7-dehydrocholesterol). Vitamin D2 is sourced from the UV irradiation of ergosterol, which is a steroid found in some plants but largely fungi. Humans have a combination of vitamin D3 and vitamin D2 available to them from sunlight UV exposure (vitamin D3), egg yolks and oily fish (vitamin D3), fortified margarine, milk and breakfast cereals (fortified vitamin D2) and dietary supplements (vitamin D2 and vitamin D3).
Both vitamin D2 and vitamin D3 undergo similar enzymatic hydroxylation processes which converts them to the active metabolite 1,25-dihydroxyvitamin D2 or D3 (calcitriol). The liver and kidneys participate to make this active form of the vitamin that the body can use. However, there may be differences in their respective efficacies in raising serum 25(OH)D, which is a marker of vitamin D status. Differences may be due to their differing affinities for the vitamin D receptor. It is also thought that vitamin D3 is the preferred substrate for hepatic 25-hydroxylase.
Some studies indicate that vitamin D3 is more efficacious at raising serum 25(OH)D concentrations than is vitamin D2; and that vitamin D2 is 30% to 50% less effective than vitamin D3 in maintaining serum 25(OH)D levels. However, other studies have suggested that both forms of vitamin D are similarly effective.