Like most vegetable oils, linseed oil contains linoleic acid, an essential fatty acid needed for survival. But unlike most oils, it also contains significant amounts of another essential fatty acid, alpha linolenic acid (ALA).
ALA is an omega-3 fatty acid. To a limited extent, the body turns ALA into eicosapentaenoic acid (EPA) - an omega-3 fatty acid found in fish oil, which in turn converts to beneficial prostaglandins. These are hormone-like substances made in many parts of the body rather than coming from one organ, as most hormones do. While fish oil has been shown to have anti-inflammatory activity, an anti-inflammatory effect of linseed oil has not been demonstrated conclusively.
Some studies have argued that, because ALA can be converted to EPA and DHA (the fatty acids found in fish oil), linseed oil should be useful for the same conditions as fish oil. However, the conversion of ALA to EPA and DHA is limited, so that argument may turn out be incorrect. For example, while numerous studies have shown that fish oils are beneficial for rheumatoid arthritis, linseed oil failed to work for this condition. However, linseed oil may help lower cholesterol as research specific to linseed oil indicates.