It's true that shark liver oil supplements are being promoted as a means of boosting the immune system, treating cancer and reducing the side effects of conventional cancer treatments, but none of those claims is based on what I regard as compelling scientific evidence. The marketing around these products is based on a limited number of studies that have suggested that compounds found in shark liver oil have cancer-fighting properties. These include alkylglycerols (also found in human bone marrow and breast milk), as well as squalamine and squalene. Although some laboratory investigations have hinted that these substances have anti-tumor effects in animals, I've seen no evidence to show that they can help combat cancer in humans. The only studies to document the usefulness of alkylglycerols were done by Scandinavian researchers in the 1970s and 1980s. To the best of my knowledge, their findings haven't been confirmed by scientific investigators elsewhere.
Among the claims for the substances found in shark liver oil are that they fight disease by stimulating immune system cells to consume germs and damaged cells and by inhibiting a regulator of cell growth called protein kinase C. According to the American Cancer Society (ACS), the claim that alkylglycerols can reduce the side effects of chemotherapy and radiation is based on the ability of these substances to protect cell membranes. Promoters of shark liver oil supplements also assert that alkylglycerols can combat colds, flu, chronic infections, asthma, psoriasis, arthritis and AIDS, but the ACS notes that none of these claims has been studied in controlled clinical trials.
Squalamine and squalene are also promoted for their supposed cancer-fighting effects. Some studies have shown that squalamine can slow the growth of blood vessels that nourish tumors, leading promoters to claim that shark liver oil may be useful in cancer treatment with or without chemotherapy. You should be aware that when squalamine was studied for treatment of age-related macular degeneration and cancer, it was given by injection; researchers said it was unclear whether it would have the same effects if taken orally.
Squalene (also found in olive oil) was shown in a single lab study to appear to protect normal bone marrow cells from the toxicity of chemotherapy without compromising the drugs' effects on cancer cells. Here, too, the ACS says that it is unclear whether this protection will apply to animals and humans with cancer.
Supplements containing shark cartilage are promoted for treatment of arthritis and inflammation as well as for enhancement of the immune system and as an anti-cancer agent. I've seen no scientific evidence supporting any of these claims.
While shark liver oil is a good source of omega-3 fatty acids, it is not one that I recommend. Shark populations are endangered all over the world. According to the advocacy group Shark Savers, tens of millions of sharks are being killed every year for their fins, used largely in shark fin soup. As a result, one third of shark species are threatened with extinction. There are many equally good sources of omega-3 fatty acids. Here's where you can find my omega-3 recommendations.
Andrew Weil, M.D.