Fish Oil versus Cod Liver Oil: Is Vitamin D a Reason to Go Back to the Future

To the Editor: As Dr. Oh1 points out in his article, fish oil supplementation has gained popularity in recent years because of the many health benefits of omega-3 long-chain polyunsaturated fatty acids (LCPUFAs). Dr. Oh’s article provides a thorough review of the literature on this topic and recommendations for fish oil supplementation. However, although fish oil is an excellent source of omega-3 LCPUFAs, it doesn’t provide the significant levels of vitamin D that cod liver oil does.2 Vitamin D is concentrated in the liver of the fish and thus plentiful in cod liver oil, a time-honored source of vitamin D. Fish oil is made from the whole body of the fish and has an insignificant amount of vitamin D.

Hypovitaminosis D is a significant public health concern.3–5 In a study of healthy adolescents, Gordon et al5 found that 24.1% were vitamin D deficient and 42.0% were vitamin D insufficient. LeBoff et al4 found an association between hip fractures in older women and lower levels of 25-hydroxyvitamin D. Vitamin D deficiency may contribute to metabolic syndrome.6,7 Vitamin D seems to play a protective role against breast, prostate, and colon cancer.8 Animal experiments link vitamin D deficiency to abnormal brain development. The broad spectrum of effects from suboptimal levels of vitamin D reflect its varied functions. Vitamin D plays important roles in bone health and mineral homeostasis, immune modulation, muscle function, nervous system function, control of the renin-angiotensin system, control of insulin secretion, skin function, regulation of apoptosis, and regulation of cell growth.

Cod liver oil contains both vitamin D and omega-3 LCPUFAs.2 Individuals at higher risk for vitamin D deficiency may wish to consider substituting cod liver oil for fish oil supplementation.