Levels of vitamin D in your body can be altered by changing your diet or exposing your skin to sunshine. Fish oil is commonly used as a supplement to supply omega-3 fatty acids to the human diet, but it's not likely to contain significant amounts of vitamin D. Cod-liver oil is the only type of oil from fish that is rich in vitamin D.
Most vitamin D is synthesized within your skin. The ultraviolet radiation from the sun converts a vitamin D precursor within your skin tissue to usable vitamin D. Very little vitamin D comes from your diet. If you have less than 20 nanograms of usable vitamin D per milliliter of blood, you are vitamin D deficient. A nanogram is one billionth of a gram. Decreased vitamin D levels can impair dietary calcium absorption and lead to bone loss. Also, vitamin D is important as an immune system modulator.
Manufacturers make fish oil supplements from a variety of fish. The type of fish used to make the oil determines the capsule's nutritional content. Fish such as salmon, sardines and halibut have omega-3 fatty acids and vitamin D, though amounts vary among types of fish. Salmon contains twice as much vitamin D as tuna. Also, processing techniques and manufacturing locations can affect the content of supplements.
Cod-liver oil is extracted from the liver of a cod fish. The cod is a large, cold-water fish found in the Atlantic and Pacific oceans. Collected oil is refined, deodorized and bleached for packaging. Cod-liver oil has both omega-3 fatty acids and significant vitamin D. It also contains vitamin A. It has been used since about 1900 to prevent bone deformity from vitamin D deficiency. Supplements can contain variable amounts, so read the labels carefully.
A 2011 review published in the journal "Medical Hypothesis" found important similarities between the anticancer effects of omega-3 fatty acids and vitamin D. The authors concluded that combining supplements is likely to prevent and treat cancer. Recently, researchers began a long-term study of cancer and cardiovascular disease prevention in 20,000 U.S. men and women with combined administration of supplemental vitamin D and fish oil. The rationale for this large study was discussed in "Contemporary Clinical Trials" in 2012, where researchers proposed the use of future data for risk assessment regarding cancer, diabetes, hypertension, depression, asthma and other diseases.
- The New England Journal of Medicine: Vitamin D Deficiency
- Linus Pauling Institute: Vitamin D
- Medical Hypotheses: The Anticancer Effects of Vitamin D and Omega-3 PUFAs in Combination Via Cod-Liver Oil: One Plus One May Equal More Than Two
- Contemporary Clinical Trials: The Vitamin D and Omega-3 Trial
- USDA National Nutrient Database for Standard Reference: Vitamin D Content of Selected Foods Per Common Measure , Sorted by Nutrient Content
Based in Michigan, Keri Gardner has been writing scientific journal articles since 1998. Her articles have appeared in such journals as "Disability and Rehabilitation" and "Journal of Orthopaedic Research." She holds a Master of Science in comparative medicine and integrative biology from Michigan State University.