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Norwegian researchers found that among more than 3,000 middle-aged women, those who took cod liver oil as children generally had lower bone mass than women who had not used the fish oil.
Because sunlight is needed to trigger the synthesis of vitamin D in the skin, people in Nordic countries are at particular risk of vitamin D deficiency. Cod liver oil is a traditional source of supplemental vitamin D, and is still widely used in Norway, where few foods are fortified with the vitamin.
Many people also take cod liver oil as a source of heart-healthy omega-3 fatty acids, or to ease arthritis symptoms.
Given the role of vitamin D in maintaining healthy bones, the new findings are "unexpected" and "paradoxical," the researchers note in the American Journal of Epidemiology.
They speculate, however, that the high vitamin A content in cod liver oil could be to blame.
Vitamin A accumulates in body fat, and excessive levels may have a negative effect on bone metabolism and actually raise fracture risk, explained Dr Siri Forsmo, the lead researcher on the study and an associate professor at the Norwegian University of Science and Technology in Trondheim.
Too much vit A to blame
It's possible that cod liver oil, on top of the traditionally vitamin A-rich Norwegian diet, provided some of these women with too much of the nutrient, Forsmo told Reuters Health.
Since 2002, Norway has required that cod liver oil producers cut the supplement's vitamin A content by 75 percent - from 3,300 International Units per dose to 825 IU. Forsmo said she is unaware of any other countries that have made similar moves.
In the US, the recommended daily intake for vitamin A is 3,000 IU for men and 2,310 IU for women; for children, the recommendation is between 1,000 and 2,000 IU per day.
In contrast to the US and certain other countries, where milk and many breakfast cereals are fortified with vitamin D, relatively few foods in Norway have added vitamin D, Forsmo noted. Butter, margarine and one type of low-fat milk are the exceptions.
So cod liver oil remains a major source of vitamin D there, Forsmo said, adding that she still takes it during the winter.
Importantly, the researcher noted, the current study looked at bone mass, and not whether women who used cod liver oil as children actually had a higher rate of bone fractures. That is a question for future studies. - (Amy Norton/Reuters Health)
SOURCE: American Journal of Epidemiology, February 15, 2008.
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