Vitamin D Newsletter


Cod Liver Oil Possible Reason for Conflicting Studies

Kirstin from New Mexico writes:

Dr. Cannell: I'm a nutritionist and I'm confused. Two studies came out in the last several months about vitamin D and pancreatic cancer. One showed it helped and the other showed it made it worse. What's up?

Dr. Cannell replies:

 We are all confused. Now two studies, both from Scandinavia, have found higher risks of cancer with the highest vitamin D blood levels. One study was in prostate cancer and it conflicts with numerous other prostate cancer studies that show the opposite. Now we have a pancreatic cancer study that found Finnish male smokers with the highest vitamin D blood levels at baseline were at a three-fold risk for pancreatic cancer a decade later.  Stolzenberg-Solomon RZ, Vieth R, Azad A, Pietinen P, Taylor PR, Virtamo J, Albanes D. A prospective nested case-control study of vitamin D status and pancreatic cancer risk in male smokers. Cancer Res. 2006 Oct 15;66(20):10213–9.

A month earlier, a group headed by Dr. Skinner at Northwestern, found higher vitamin D intakes reduced the risk of pancreatic cancer. Skinner HG, Michaud DS, Giovannucci E, Willett WC, Colditz GA, Fuchs CS. Vitamin D intake and the risk for pancreatic cancer in two cohort studies. Cancer Epidemiol Biomarkers Prev. 2006 Sep;15(9):1688–95.

Several months ago, a prospective study by Dr. Giovannucci at Harvard showed vitamin D exposure reduced the risk of pancreatic cancer. Giovannucci E, Liu Y, Rimm EB, Hollis BW, Fuchs CS, Stampfer MJ, Willett WC. Prospective study of predictors of vitamin D status and cancer incidence and mortality in men. J Natl Cancer Inst. 2006 Apr 5;98(7):451–9.

An earlier Japanese study indicated sunlight reduced the incidence of pancreatic cancer. Mizoue T. Ecological study of solar radiation and cancer mortality in Japan. Health Phys. 2004 Nov;87(5):532–8.

A possible explanation for the inconsistent results from Scandinavian countries is the high consumption of cod liver oil in Scandinavia. I say possible because it is too early to indict cod liver oil. However, cod liver oil has noticeable concentrations of known carcinogens. Storelli MM, Storelli A, Marcotrigiano GO. Polychlorinated biphenyls, hexachlorobenzene, hexachlorocyclohexane isomers, and pesticide organochlorine residues in cod-liver oil dietary supplements. J Food Prot. 2004 Aug;67(8):1787–91.

Furthermore, the concentration of these carcinogens was much higher in the 1980s (when the Scandinavian pancreatic cancer study began) than it is now. Falandysz J, Tanabe S, Tatsukawa R. Most toxic and highly bioaccumulative PCB congeners in cod-liver oil of Baltic origin processed in Poland during the 1970s and 1980s, their TEQ-values and possible intake. Sci Total Environ. 1994 May 16;145(3):207–12.

The male Finnish smokers whose high vitamin D levels preceded their pancreatic cancer had those blood levels measured in the late 1980s, perhaps after a lifetime of ingesting cod liver oil with high concentrations of these known carcinogens. Also, cod liver oil has high amounts of vitamin A and, as I've written before, even the amount of vitamin A in most multivitamins—not to mention the huge amounts of vitamin A in cod liver oil—may be causing subclinical vitamin A toxicity.

Three studies suggest vitamin D reduces the risk of pancreatic cancer and one study suggests it increases the risk. Overall, there are several hundred studies indicating vitamin D, or sunlight, or both, dramatically reduces your risk from dying of cancer, while two studies in cod liver oil Scandinavian countries show high vitamin D levels increase risk. Take your pick.

Page last edited: 07 November 2010