That means some supplement products may not contain what the label says.
One study showed that cod liver oil supplementation during pregnancy and infancy decreased asthma development risk among school-age children.
This decrease was hypothesized to be an effect of the high vitamin D levels available through cod liver oil.

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This may be due to omega-3 fatty acids' effects on heart rate and arrhythmia.
However, how fatty acids act as antiviral agents is unclear.
What Are the Side Effects of Cod Liver Oil?
There are no known side effects from cod liver oil consumption itself.
Supplementing any child under the age of 1 with cod liver oil is no longer recommended.
What Happens If I Take Too Much Cod Liver Oil?
Toxicity or overdose of cod liver oil has not been researched and, thus, cannot be determined.
However, cod liver oil contains high vitamin A levels.
The symptoms of vitamin A toxicity are listed in the Side Effects section above.
Discuss with your healthcare provider whether to avoid taking a vitamin D supplement while taking cod liver oil.
There is evidence that cod liver oil has particular benefits in decreasing inflammation compared to omega-3 fatty acid supplements.
Whenever possible, make a run at obtain high-quality cod liver oil from Atlantic cod.
This can help them prevent possible interactions and side effects.
They can also ensure you take cod liver oil at appropriate doses.
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