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Heinz’s new Funky Purple ketchup covers a couple boats of French Fries at an outdoor cafe in downtown Pittsburgh on Tuesday, July 31, 2001.KEITH SRAKOCIC/The Associated Press

In 2000, Heinz made culinary history by introducing the world to coloured ketchup. First came green, then purple, pink, orange and teal. The products were a massive hit with kids at first, selling millions of bottles. But within a few years, consumers seemed less keen to dip their fries into ketchup that resembled toxic goo. The EZ Squirt ketchup line was discontinued in 2006. Business Insider, a news website, called the initiative one of the biggest food flops of all time.

The colour of food greatly affects our perception of how good it will taste and whether we want to eat it. Food marketers have long since figured this out, which is why a surprising amount of the food Canadians eat contains dye.

The dyes added to food products are often made from synthetic ingredients. And in recent years, this has caused a sizable uprising from consumers who say they fear artificial food dyes can cause cancer, serious allergic reactions and attention deficit hyperactivity disorder.

In response, some large corporations have reformulated their products, which is why Smarties now come in paler, all-natural versions of their formally vibrant colours. Last year, Loblaw Cos. Ltd. announced it was removing all artificial colours and flavours from its President's Choice line of products. Industry watchers predict the trend toward natural colours will continue as consumer demand grows.

But some medical experts question whether artificial dyes truly are the nutritional bogeymen they are made out to be.

This isn't the first time the health profile of food dyes have come under scrutiny. Canada and other countries have banned or restricted the use of certain dyes, such as Orange No. 1, after research has proven they may be linked to cancer.

But critics, such as the Center for Science in the Public Interest, a Washington-based consumer advocacy organization, say that more need to be taken off the market. They single out a range of colours, like Allura Red (Red No. 40), Tartrazine (Yellow No. 5) and Sunset Yellow (Yellow No. 6) because they are the most widely used and are "contaminated with known carcinogens."

"These synthetic chemicals do absolutely nothing to improve the nutritional quality or safety of foods, but trigger behaviour problems in children and, possibly, cancer in anybody," CSPI executive director Michael Jacobson said in a press release. The organization is calling for restrictions and better labelling on artificial dyes. In Canada, companies don't have to disclose the names of what synthetic dyes they use.

No wonder so many consumers are looking for natural alternatives.

But Dr. Joe Schwarcz, director of McGill University's Office for Science and Society, isn't buying it. According to him, much of the commotion over the toxicity of food dyes is based on meagre facts and speculative data. He says the studies linking food dyes in today's food supply to cancer are seriously flawed and are unreliable. For instance, many are based on animals, not humans, which is a red flag.

Take the link between food dyes and petroleum. Schwarcz says synthetic food colourings are derived from compounds that are found in petroleum. But they have nothing in common with petroleum; the molecular structure of dyes is different, he notes.

When people base their argument against food colouring on the idea they must be bad because they come from petroleum, "You know they're missing a couple of scientific screws."

Scientists are more divided when it comes to the short- and long-term consequences of food dyes. While high-profile groups like CSPI argue otherwise, many experts say that dyes do not increase the risk for cancer. Schwarcz, who has read the research, says many of the studies that tie food colouring to cancer are flimsy and flawed.

But what about hyperactivity?

A now-infamous research paper conducted in Britain found that children who consumed beverages containing artificial colours were much more hyperactive than their peers who had naturally coloured drinks. That paper, now simply referred to as the "Southampton study" was proof enough for many that synthetic food dyes lead to bad behaviour.

In 2011, however, a U.S. Food and Drug Administration panel reviewed the available research and concluded there isn't enough evidence that dyes cause or exacerbate hyperactivity. However, they said it's possible some children with ADHD could be vulnerable to dyes. But that doesn't mean there is no evidence.

Eugene Arnold, professor emeritus of psychiatry at Ohio State University, said there's nothing wrong with being overcautious when the jury is still out.

"The thing is, if you see smoke, even though you can't see a fire, you want to evacuate the children from the building while you investigate," he said.

Schwarcz agrees there may be reason for caution. One difficulty with doing this behavioural research is that it's often subjective. In the Southampton study, for instance, parents noticed a change when kids had dyes, but teachers didn't.

Arnold and other experts want more robust labelling to help parents avoid food dyes if they wish.

Until then, if you are concerned about the possible health effects of food dyes, avoid them as much as possible by reading labels for mention of any artificial colours and opting for fresh, unprocessed foods. Schwarcz notes that in doing so, consumers will eat healthier fare and cut out many packaged goods – which is something most people should probably strive for anyway.