Cod liver oil is renowned for containing Vitamin D3, vitamin A and the omega-3 fatty acids EPA and DHA. Levels of these nutrients are what conventional cod liver oil manufacturers focus on, and so they strip away as much of the oil as possible and add these nutrients into the oil (and they are almost always synthetic versions). This is why conventional cod liver oils often have very rounded figures for these nutrients. However, there is a lot more in true and authentic cod liver oil than these 4 nutrients, important though they are.
Beyond the label
Fats: Cod liver oil contains a range of other omega-3 fatty acids, including DPA and ALA. It also provides various omega-6,7 and 9 fatty acids too. In addition, ~30% of the oil is healthy poly-unsaturated fatty acids and around ~20% is saturated fat. This provides a very diverse and extremely nutritional range of fatty acids. There is also ~5% of cod liver oil that is ‘unidentified’. Although this offers no insight into health benefits, it does demonstrate how limited food testing is, and how much there is still to learn.
Vitamin D: You then have the various forms of vitamin D which are naturally present. Aside from vitamin D3, there is obviously D2 (which is common in supplements, and often considered the ‘plant’ form of the vitamin). However, there is a vast array of various forms of vitamin D. Some estimates are over 100 forms, and all interact with the body in slightly different ways. Measuring all these forms is near impossible. Vitamin D3 is commonly tested for, for the sake of giving a vitamin D result, but the actual vitamin D ‘activity’ can be quite different.
Polyphenols: Vitamin A is less complex that vitamin D, however, there are nutrients closely related to vitamin A called polyphenols. These are often plant based chemicals that are usually responsible for pigments, such as the colour of blueberries or carrots. Cod liver oil has a rich yellow colour, which is in part due to vitamin A, but also due to a range of polyphenols that are in the oil. These polyphenols are difficult to test for, but their presence can be noted from the colour of the oil. Incidentally, a reason some oils are paler than others may be due to the vitamin A and polyphenol content.
Although vitamin D3 and specific omega-3 fatty acids are important for health, nothing in the body developed to work in isolation. The body benefits more from a variety of vitamin D forms rather than one isolated form. A range of fats beyond just EPA and DHA and even omega-3’s is important for a healthy cardiovascular system, brain etc. The body needs more than what you can test for or create in a lab.