The Vegetarian Society in the UK does not consider fish-eaters to be vegetarians, although dairy products that are free from animal derivatives, such as animal rennet, and free-range eggs are acceptable.
But with the explosion of omega-3-containing products onto the market, there is concern that some dairy and egg products contain fish oil that is not declared on the label.
Moreover the omega-3 in some dairy products and eggs are believed to be a result of feeding animals with fish - even though it does not form part of their natural diet.
"The whole area of food labelling is becoming more and more grey as these things are introduced," Collette Walsh, spokesperson for The Vegetarian Society told NutraIngredients.com.
She added that if a vegetarian were aware of a cow or chicken being fed fish they would avoid the product. Rather, they would be better advised to go for products whose provenance is known and that are labelled as vegetarian.
The society worked with the Food Standards Agency on a set of guidelines for vegetarian and vegan products, which were released in April.
However a spokesperson for the FSA told NutraIngredients.com that these guidelines are not legally enforceable, but that they are designed to improve labelling by providing criteria, help manufacturers avoid common mistakes, and help enforcement agencies identify misleading labelling that contravenes the 1990 Food Safety Act or the 1968 Trade Descriptions Act.
She confirmed that if a product claims to be vegetarian but is found to contain fish oil, it could be seen as misleading labelling. This interpretation excludes products that are generally presumed to be vegetarian, such as milk, but are not labelled as such.
For now the FSA is committing only to reviewing the guidelines once they have had time to take effect - although The Vegetarian Society's goal is the development of legal definitions of the terms.
For now, the society continues to operates an endorsement programme that allows products meeting its criteria to carry its recognisable 'Seedling Symbol'.
The rising interest and consumer awareness of omega-3 is attributed to a plethora of studies highlighting its role in cardiovascular health and cognitive function - as well as a number of other health benefits including joint health.
According to Mintel's Global New Products Database (GNPD) the launch of omega-3-containing product across Europe has increased from 153 in 2004 to 208 in 2005.
Fish oil is widely held to be the best source since it naturally contains both DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid), the long-chain fatty acids that are most bioavailable for humans.
Most vegetarian sources, on the other hand, such as flaxseed and linseed, contain ALA (alpha-linolenic acid), which must first be converted to the longer-chain form by the body before it can be used, with some of the benefit lost along the way.
In the past few years some omega-3 from microalgae has started to become available on the market, but most of this contains only DHA, not EPA as well.
Earlier this year, however, a Swiss company called Water4 announced that it has developed a process to source both DHA and EPA from microalgae, but so far this is available in a dietary supplement, called V-Pure, that is only available over the internet.
Walsh confirmed that V-Pure is endorsed by The Vegetarian Society.
An Oxford University study published in the August 2004 issue of the American Journal of Clinical Nutrition (Vol. 82, No. 2, 327-334) found evidence to suggest that when animal foods are wholly excluded from the diet, the endogenous production of EPA (eicosapentaeoic acid) and DHA (docosahexaenoic acid) results in low but constant plasma concentrations of these fatty acids.
However Walsh disputed that vegetarians are really missing out on omega-3, on the grounds that they tend to think more carefully about their diet than meat-eaters and ensure they are consuming all the nutrients they need.
"We are quite hot on omegas," she said. Of the current interest in omega-3, she said: "We welcome the awareness, but the vegetarian community has known about it for decades. It is non-vegetarians who have suddenly realised they should be getting these things because their diets are unbalanced."
She stressed the importance of adhering to a healthy balanced diet, and said that there is no point having omega oils if you are eating steak and chips, for instance, without any fibrous vegetables.
A 2002 Datamonitor report estimated that there are around 12 million vegetarians in Europe, while a UK-wide survey conducted by Food Standards Agency (FSA) in 2004 found that five per cent of households claimed to contain at least one vegetarian member.
In other parts of the world, however, vegetarianism is more prevalent, particularly in places where populations avoid meat for religious reasons (for example, Hinduism).
Mintel estimated the total UK meat free foods to be worth around £626m, with an increase of 12 per cent to £699 million at current prices by 2009.