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鈥淚 wouldn鈥檛 drink that juice if you paid me!鈥�

And so began my conversation with a fellow traveller sitting next to me at the airport, as we waited for our flight. She had glanced at my laptop and saw an item I was perusing with the headline 鈥淧urple tomato juice from genetically modified fruit engineered for health benefits.鈥�

Courtesy required some sort of response, which I made with trepidation, having a feeling about the direction the discussion was about to take. I thought about mentioning that I wasn鈥檛 in the business of paying strangers to try unusual beverages, but I finally I came up with the more benign 鈥淎nd why not?鈥� As I suspected, that unleashed a torrent of rhetoric about evil multinational companies foisting untested genetically modified foods on us.

鈥淲hat health benefits?鈥� she said. 鈥淭hose foods are making us sick.鈥�

I wasn鈥檛 really surprised by the onslaught; after all, this is not a unique view. No, genetically modified foods are not making us sick. Studies attesting to their safety are overwhelming. But can we say with certainty that no untoward effects will ever arise? Of course not. That is a naive expectation that science can never meet. All we can do is come to some conclusion about risks and benefits, based on the current state of knowledge.

My new acquaintance was wrong about genetically modified foods making us sick, but she was correct in questioning their health benefits. Up to now, it is only farmers who have benefited from crops resistant to herbicides and insects grown from genetically modified seeds. Indeed, one of the reasons for public skepticism about genetic modification is the lack of any obvious benefit to the consumer. The purple tomato is a step, admittedly a small one, toward demonstrating that health benefits are possible, and that pursuing the technology along these lines is worthwhile.

The purple colour is due to an accumulation of anthocyanins, compounds that occur widely in nature, although not in conventional tomatoes. They are responsible for the stunning hues of autumn leaves and the various colours of flowers, fruits and berries. But plants do not produce anthocyanins as entertainment for our eyes; it is reproduction they have in mind. Colourful flowers and fruits attract insects and animals to spread pollen and seeds. The tomato, however, produces very little of these compounds, relying mostly on lycopene from the carotenoid family to attract attention. But use a clever bit of genetic engineering to introduce a couple of genes from the ornamental snapdragon, and presto, you have a purple tomato, and an obvious question. Why would anyone want such a thing? It all comes down to paying attention to the growing body of evidence that the optimal diet is mostly plant-based.

Numerous studies have linked a lower rate of heart disease, cancer, obesity and diabetes with a largely plant-based diet. Is that because plants contain special disease-preventing compounds, or because a meaty diet contains disease promoters? Or is it a combination of these factors? Based on laboratory studies, animal experiments and human epidemiological data, the anthocyanins have stirred interest as potential disease-preventing compounds.

In cell cultures they can be shown to scavenge free radicals, stimulate the production of detoxicating enzymes, reduce the proliferation of cancer cells and interfere with the formation of blood vessels that supply nutrients to tumours. Animals with chemically induced cancers fare better when given an anthocyanin-rich diet. While human studies have not shown a cancer-preventive effect for anthocyanins, they have for heart disease. The Nurses Health Study monitored the medical status of more than 93,000 nurses who periodically filled out food-frequency questionnaires over a period of 18 years. Based on the foods consumed, researchers calculated the amount of anthocyanins in their diet and found that nurses with a high intake had a reduced risk of having a heart attack. Most of the anthocyanin intake was accounted for by berries, with the data showing a roughly 30-per-cent decrease in risk for those who consumed more than three portions a week, compared with those who ate the berries less than once a month. A number of other chemicals found in plants were also assessed, but only anthocyanin intake was associated with a reduction of heart attack risk. Of course, plants contain thousands of compounds and it is impossible to disentangle their effects. Anthocyanins may just be markers for the presence of other active compounds. A further complication is that more than 600 anthocyanins are known to be present in plants, possibly with different health effects. But if anthocyanins as a class do indeed have a protective effect, the pertinent question is whether the amounts found in the genetically modified tomato are significant. It turns out that they may well be. The samples with the highest content weighed in at 450 mg of anthocyanins per tomato. That鈥檚 almost as much as a cup of blueberries (500 mg) and far more than strawberries. But if you really want to load up on these chemicals, go for black raspberries at 850 mg per serving, or seek out chokeberries or elderberries at a whopping 2,000 mg per cup. Alternatively, you鈥檒l be able to drink a glass of purple tomato juice, if and when it becomes commercially available. It is noteworthy that in the Nurses Health Study, health benefits were noted with as little as 35 mg of anthocyanins consumed every day. The newfangled tomatoes were developed in England by Professor Cathie Martin at the John Innes Centre in Norwich. But because of an anti-GMO atmosphere in Europe, they are being grown experimentally by New Energy Farms in Ontario. Juice from the tomatoes will be sent to Professor Martin for testing of health effects with seeds removed to ensure that there are no genetically modified components in the finished product. As research zeros in on the specific 鈥減hytochemicals鈥� responsible for the healthy nature of a mostly plant-based diet, we are likely see further developments in genetic engineering geared toward improving the nutritional content of our food supply.

I tried to explain all this to my seatmate, and yet she still maintained that she would steer clear of any genetically modified foods since she had 鈥渘o need of the genes they put into purple tomatoes.鈥�

Having made her point, she proceeded to take a sip from the soft drink she had been nursing and ripped open a bag of potato chips.

Not an anthocyanin in sight.

Sigh.