Vitamin D Newsletter


Cod Liver Oil and Vitamin A Toxicity

Sue from Washington writes:

Dr. Cannell: Why don't you like cod liver oil? There are a lot of studies showing it helps.

Dr. Cannell replies:

There are lots of studies showing cod liver oil helps lots of things. Did you know there are at least seven studies from the '20s and '30s showing it reduces colds, flu, and absenteeism in both children and adults? Did you know there are two recent studies indicating it either helps prevent or ameliorates respiratory infections in children? Which one of the three nutrients in cod liver oil: vitamin A, vitamin D, or omega-3 (fish oil) fats is responsible, or is it two of them, or is it the three combined? We don't know—at least I don't. I am leery of the vitamin A. Cod liver oil has anywhere from 5,000 to 15,000 units of preformed vitamin A in each tablespoon, sometimes more, besides the vitamin D and omega-3 fats. The question is what is the ideal amount and ideal combination of all three nutrients? The answer for vitamin D is becoming clearer: enough to get levels up to about 50 ng/mL  year-round. Optimum omega-3 intakes are less clear and depend on the amount of omega-6 fats (most vegetable seed oils) in your diet. The answer for vitamin A is unclear, at least to me.

Vitamin A and vitamin D interact in unknown ways. Vitamin A protects against vitamin D toxicity and visa versa but we don't know why. There is increasing evidence that some Americans, perhaps quite a few, are suffering from sub-clinical vitamin A toxicity, mainly from the preformed vitamin A in their multivitamin supplements.

I am not convinced that because cod liver oil is "natural," it is therefore good. Tsunamis and strychnine are natural. In fact, cod liver oil is a processed food and Paleolithic man didn't drink it. There is evidence that at least one Paleolithic African women suffered severe vitamin A toxicity, probably from the liver she consumed. Chronic consumption of chicken liver caused vitamin A toxicity in twin girls.

A recent review of the literature found evidence that higher vitamin A levels are associated with bone problems. Women who consumed more than 15,000 units of vitamin A per day in food and supplements had a three and a half times higher risk for birth defects in their children than women who consumed less than 5,000 IU.

Vitamin A antagonizes the actions of vitamin D, probably at the receptor level, although that is not clear. The amount of vitamin A in one serving of liver antagonizes the rapid intestinal calcium response to physiological levels of vitamin D in man.

We just don't know if large amounts of vitamin A (more than 5,000 IU/day) causes problems when consumed with a large amount of vitamin D, as would be the case in people consuming large amounts of cod liver oil. A very well-written and entertaining rebuttal to my cod liver oil position, which I presented to the Weston A. Price Foundation, was recently published by that same foundation. However, Dr. Noel Solomon—a vitamin A researcher who spoke at the same conference—also cautioned against high vitamin A intakes.

In the end, my natural conservatism wins out. More is not better. What was the diet we evolved on? I know my Paleolithic ancestors didn't drink cod liver oil. Neither will I, unless my mother makes me, or until I have to choose between cod liver oil or nothing—and I don't.

Page last edited: 17 May 2011