Seventy five years ago, long chain omega 3 fatty acids were added to the list of essential nutrients. Later in the 20th century, the properties of marine polyunsaturated oils were linked with several health benefits, including protection from cardiovascular disease. However, a high quality systematic review published in this week's BMJ draws attention to uncertainties about some of the health benefits attributed to omega 3 fats.1

The review shows that the evidence for a reduction in cardiovascular events and mortality is less conclusive than we believed. A previous meta-analysis indicates that mortality is reduced owing to fewer fatal coronary events among people ingesting omega 3 fatty acids,2 but the current review found no strong evidence of a reduction in combined cardiovascular events. The claim that omega 3 fats reduce the risk of cancer is not supported here or by another recent systematic review.3 For each health outcome there are too few trials with adequate allocation concealment, and too few cohort …