NOTE: This was from last October, but note that the UK's National Health Service (NHS) and Cancer Research UK (CRUK) are both unhappy with the coverage of this story. It's also worth noting that the dodgy claims re cancer were suggested by the John Innes Centre's press release, ie they were not made up by the press. They were just reported uncritically, presumably because they originated with what was perceived as an expert source. The John Innes Centre also appears to have made no effort to play down, or otherwise correct, the claims that were made as a result of its press release.
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Purple tomatoes 'beat cancer'
NHS Choices (2008), October 27th
2008 http://www.nhs.uk/news/2008/10October/Pages/Purpletomatoes.aspx

"Purple tomato can beat cancer" reports a front-page story in the Daily Express, which claims that British scientists have genetically modified tomatoes to create "the ultimate healthy superfood". The story features bold claims that the modified tomato could protect against cancer, keep you slim, ward off diabetes and help to safeguard eyesight.

These claims are not actually based on benefits seen in humans, but rather from a small-scale study of mice that were given an extract of genetically modified tomatoes. Researchers created the variety of tomato using genes from a snapdragon plant to produce fruit high in anthocyanins (pigments), which also give the fruit their purple appearance. They found purple tomato supplements increased life expectancy in a small group of mice by an average of 40 days. This study cannot determine what role these GM tomatoes might play in preventing disease in humans, as suggested by the Daily Express. Any potential benefits from these GM tomatoes can only be determined by further research.

Where did the story come from?

Dr Eugenio Butelli and colleagues from the John Innes Centre in Norwich and other research institutes across the UK, the Netherlands and Germany collaborated on this study. The work was funded by the European Union, the Centre for Biosystems Genomics (Netherlands) and by the Biological and Biotechnological Science Research Council (UK). The study was published in the peer-reviewed medical journal Nature Biotechnology.

What kind of scientific study was this?

This was a laboratory study in which researchers explored methods to increase levels of ‘health-promoting bioactive compounds’ such as anthocyanins in fruits and vegetables. The benefits of anthocyanins, a type of naturally occurring pigment (polyphenol), are thought to be a result of their antioxidant properties. Antioxidants can slow or prevent the oxidation of other molecules in the body.Anthocyanins are found in blackberries and blueberries.

The researchers considered tomatoes an 'ideal candidate' for this type of experiment as previous genetic modification experiments on tomatoes have succeeded in increasing the concentration of flavonoids (a type of antioxidant) in their flesh. Other experiments that increased the levels of a particular enzyme in the tomatoes increased the concentration of flavanoids in the skin by 78 times.

The researchers wanted to breed tomatoes with enhanced levels of flavanoids in their flesh. To do this they used bacteria to transport two genes into tomatoes both of which are responsible for the production of chemicals involved in turning on and off the expression of other genes in cells. These two genes are called Del and Ros1 and are found in the snapdragon, where they work together to turn on the production of anthocyanins.

The 'genetically-modified' tomatoes containing the snapdragon gene were then cultivated and researchers recorded the appearance of the tomato fruit as it grew. They also investigated the total anthocyanin content in the fruit and compared it to non-GM tomatoes. The extra anthrocyanin in the GM tomatoes gives them their purple colour. Researchers also extracted the anthocyanins from GM tomatoes and assessed the antioxidant properties.

In a second part of their experiment they tested whether the GM tomatoes offered an advantage over ordinary tomatoes to mice that were highly susceptible to cancer. Mice missing the Trp53 gene spontaneously develop a range of tumours at an early age and are often used to study the effects of potentially cancer-protecting substances. Using these mice, researchers compared the life expectancy of those fed a normal diet, those fed a diet supplemented with 10% red tomato powder and those fed a diet supplemented with 10% purple tomato powder.

What were the results of the study?

The genetically modified tomatoes contained higher concentrations of total anthocyanins than ordinary fruit in both the peel and in the flesh. They found that the water soluble extract of the GM tomatoes (which would contain the anthocyanins) had far greater quantity of antioxidants than the ordinary tomatoes.

In the animal experiments in the study, researchers found that the mutant, cancer-prone mice fed an ordinary diet lived an average of 142 days, those fed red tomato supplements lived 146 days, while those fed purple tomato supplements lived 182 days. They conclude that the difference in lifespan was significant between the the mice fed a normal diet and those given purple tomato supplement.

What interpretations did the researchers draw from these results?

The researchers conclude that they have engineered the highest levels of anthocyanins yet reported in tomato fruit and that these levels appear to be sufficient to give a substantial protective effect against tumour progression. They say their GM fruit could be used to evaluate the effects of high anthocyanin diets on other diseases. They also suggest that their results "support the arguments for inclusion of foods containing high levels of anthocyanins in all long-term dietary regimens".

What does the NHS Knowledge Service make of this study?

In this study, the researchers have developed a method to genetically modify ordinary tomatoes so that the resulting fruit contains much higher levels of anthocyanin pigments. These pigments can act as powerful antioxidants, and given that antioxidant properties may be implicated in reducing risk of a variety of diseases, the method will undoubtedly be used in further studies.

Anthocyanins can act as powerful antioxidants, which some claim can reduce the risk of various diseases. The evidence for this is limited. The researchers say that the beneficial activity of anthocyanins may not be linked to their antioxidant properties, which has been claimed in the past. Instead, they may activate defence systems and other reactions that indirectly delay 'oxidative damage and malignant progression' involved in the development of cancers.

As quoted by the Daily Express, Dr Lara Bennet of Cancer Research UK feels that "it is too early to say whether anthocyanins obtained through diet could help to reduce the risk of cancer".

The protective benefits of these tomatoes were tested in a group of 20 cancer-susceptible mice. While this part of the experiment found that the purple tomato supplement increased life expectancy, the small sample sizes used mean the results may have occurred by chance. Also until the tomato is tested in humans we cannot be sure that it will offer the same benefits, or that there will not be any unexpected harms.

The researchers' novel methods for creating genetically modified fruit and vegetables will pave the way for future studies. However, before the claims that these GM tomatoes 'can beat cancer' can be supported, there must be further research into the health benefits of this technology.

In the meantime current advice is to follow a diet containing five portions of fruit and vegetables a day.

Links to the headlines

Purple tomato can beat cancer. Daily Express, October 27 2008

Purple 'super tomato' that can fight against cancer. The Daily Mail, October 27 2008

Purple tomatoes help fight cancer. The Sun, October 27 2008

Purple tomato created to fight cancer. The Independent, October 27 2008

Purple GM tomatoes may ward off cancer. The Guardian, October 27 2008

Links to the science

Butelli E, Titta L, Giorgio M et al. Enrichment of tomato fruit with health-promoting anthocyanins by expression of select transcription factors. Nat Biotechnol 2008; Advanced online publication, Oct 26

Further reading

Brunner EJ, Rees K, Ward K, Burke M, Thorogood M. Dietary advice for reducing cardiovascular risk. Cochrane Database of Systematic Reviews 2007, Issue 4.

Bjelakovic G, Nikolova D, Simonetti RG, Gluud C. Antioxidant supplements for preventing gastrointestinal cancers. Cochrane Database of Systematic Reviews 2008, Issue 3

Evans JR. Antioxidant vitamin and mineral supplements for slowing the progression of age-related macular degeneration. Cochrane Database of Systematic Reviews 2006, Issue 2.

[For the John Innes Centre press release:
Purple tomatoes may keep cancer at bay
http://www.jic.ac.uk/corporate/media-and-public/current-releases/081026martin.htm ]