Cod liver oil as a suitable source of vitamin D

Townsend Letter for Doctors and Patients, May, 2007 by Alan Gaby

In a recent statement in the Townsend Letter, Dr. John Cannell wrote that cod liver oil is not advisable as a source of vitamin D because of the potential toxicity of the vitamin A in fish liver oils. He cites a review article that states that relatively small increases in vitamin A intake are associated with an increased risk of osteoporosis. However, that assertion is based solely on observational studies, the results of which are conflicting. Even if there is a true association between vitamin A intake and osteoporosis risk, it is likely attributable to confounding factors.

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The main food sources of vitamin A in the American diet, aside from liver, are fortified milk (5,000 IU per liter), fortified breakfast cereals (up to 1,250 IU per serving), and fortified margarine (approximately 500 IU per tablespoon). A daily diet that includes two cups of fortified milk plus either one serving of fortified breakfast cereal or two tablespoons of margarine would put a person in the top quintile of vitamin A intake.

There is circumstantial evidence that some or all of the foods that are high in vitamin A can promote the development of osteoporosis, for reasons unrelated to their vitamin A content. For example, fortified breakfast cereals often contain large amounts of added sugar. A high intake of refined sugar has been shown to increase urinary calcium excretion in humans (1) and to cause bone loss in experimental animals. (2) Most brands of margarine contain substantial quantities of trans fatty acids. Ingestion of trans fatty acids can promote a deficiency of essential fatty acids, (3) which are essential for bone health. (4) Fortified milk, another significant dietary source of vitamin A, may also be a less-than-perfect food for the bones, despite its high calcium content. Milk is one of the major sources of dietary phosphorus, a mineral that is often present in excessive amounts in the American diet. Consumption of too much phosphorus can impair bone health. (5) In addition, dairy products are the only significant dietary source of naturally occurring trans fatty acids. (6) Studies examining the relationship between milk consumption and bone health have yielded conflicting results, (7) and one study showed an increased risk of fractures in women who consumed two or more glasses of milk per day, as compared with women who consume one glass or less per week. (8)

Liver, the other major dietary source of vitamin A, accumulates various environmental toxins, including lead and cadmium, (9) both of which can cause osteoporosis. In addition, some environmental chemicals that might concentrate in the liver could promote bone loss by inhibiting androgen activity. (10)

Supplementation with massive quantities of vitamin A has caused bone abnormalities in experimental animals, but the doses required to produce these effects are equivalent to nearly one million IU per day for humans. The assertion that as little as 5,000 IU per day could damage the bones of adult humans has no biological plausibility.

It seems unlikely that the possible association between vitamin A intake and hip fractures represents a cause-effect relationship. Perhaps the real culprits are refined sugar, trans fatty acids, milk, and environmental toxins.

Alan Gaby, MD

Notes

1. Lemann J Jr, et al. Possible role of carboydrate-induced calciuria in calcium oxalate kidney-stone formation. N Engl J Med. 1969;280:232-237.

2. Saffar JL, et al. Osteoporotic effect of a high-carbohydrate diet (Keyes 2000) in golden hamsters. Arch Oral Biol. 1981;26:393-397.

3. Holman RT, et al. Effects of trans fatty acid isomers upon essential fatty acid deficiency in rats. Proc Soc Exp Biol Med. 1956;93:175-179.

4. Odutuga AA. Effects of low-zinc status and essential fatty acid deficiency on bone development and mineralization. Comp Biochem Physiol. 1982;71A:383-388.

5. Calvo MS, et al. Changing phosphorus content of the U.S. diet: Potential for adverse effects on bone. J Nutr. 1996;126:1168S-1180S.

6. Oomen CM, et al. Association between trans fatty acid intake and 10-year risk of coronary heart disease in the Zutphen Elderly Study: A prospective population-based study. Lancet. 2001;357:746-751.

7. Weinsier RL, et al. Dairy foods and bone health: Examination of the evidence. Am J Clin Nutr. 2000;72:681-689.

8. Feskanich D, et al. Milk, dietary calcium, and bone fractures in women: A 12-year prospective study. Am J Public Health. 1997;87:992-997.

9. Boyer KW, et al. Trace element levels in tissues from cattle fed a sewage sludge-amended diet. J Toxicol Environ Health. 1981;8:281-295.

10. Sohoni P, et al. Several environmental oestrogens are also anti-androgens. J Endocrinol. 1998;158:327-339.