A survey of Which? online panel members found that 45% are more likely to buy a product that claims to be high in omega 3 than the same food without the nutrient.
Although the best source of omega 3 is oily fish, some food companies are cashing in by adding it to a number of everyday foods such as bread, fruit juice, yoghurt and baked beans.
Which? examined a range of foods that are promoted as containing or being high in omega 3 and found that labelling on Asda's healthy wholegrain bread made statements about the nutrient that were "just plain wrong".
The label claims that four slices of the bread provide 31.3g of omega 3 but Which? tests revealed that it only contained 0.009g per 100g, meaning you would have to eat just over 11 loaves a day to benefit from the omega 3 fatty acids.
Asda admitted a typing error with the claimed amount of omega 3 in four slices.
Which? also found that even products with added fish oil often fail to make it clear how much you'd have to consume to get a useful amount of omega 3.
People would need to drink one and a half litres every day of Tesco's Healthy Living pomegranate juice to get a beneficial measure of omega 3.
Hellman's mayonnaise states on the label that it is a "good natural source of omega 3 from plant sources" but Which? said omega 3 from plant sources does not have the same health benefits as omega 3 from fish.
Neil Fowler, editor of Which? magazine, said: "A good many food manufacturers are riding the money-making omega 3 wave by adding it to all sorts of food and failing to mention that it may not be the right sort of omega 3, or enough of it, to be as beneficial as simply eating oily fish."