Eat your reds and greens say researchers

By SIMON COLLINS science reporter

Greens are good for you, but research suggests that reds are valuable too - for plants and for people.

Auckland University biologist Kevin Gould has found that red in leaves acts as a powerful antioxidant, neutralising plants from harmful "free radicals".

And a team led by Professor Lynn Ferguson at the Auckland Medical School has found that reds, purples and blues in plants from blueberries to kumara appear to have the same health-giving effects in humans.

"Eat your purples," said a member of her team, Dr Martin Philpott. "The old saying, 'Eat your greens', is only part of the story."

Their discoveries are the outcome of an extraordinary story that began in the early 1990s when Dr Gould went walking with friends in the Waitakere Ranges.

As they admired the bright pinks and reds of the native parataniwha, a friend asked, "Why are the parataniwha leaves this colour?"

"As a botanist I felt I should have known the answer, but in fact I had never given the issue any thought," Dr Gould wrote last year in NZ Geographic.

The question was not new. It has fascinated people since Aristotle.

But Dr Gould decided to investigate, and won a three-year study grant from the Royal Society's Marsden Fund.



It was known that the red pigments in leaves are molecules known as anthocyanins. For many years scientific thinking, dominated by North America where leaves turn red only in autumn, held that anthocyanins were always there but only showed up when the sun weakened and leaves stopped producing as much green chlorophyll.

That did not explain the array of reds Dr Gould found year-round in New Zealand - red stripes in flax, at the tips of rimu leaves, in blotches on the horopito (pepper tree). He had even found red and green leaves on the same tree.

He noticed that the reds increased when the plants were under stress - in droughts, in extreme hot or cold, or when they were damaged.

Then he did some lateral thinking. He had been reading about the "French paradox". Despite a fatty diet and a high smoking rate, the French have low rates of heart disease. And they love red wine.

Research in the 1990s found that red wine was full of "antioxidants". These are molecules which combine with loose oxygen atoms and other "free radicals" - atoms which have empty spaces in their outer shells of electrons, which make them liable to pull in electrons from other molecules in the body.

Dr Ferguson said free radicals had changed scientists' thinking about cancer. They were known to be produced in cigarette smoke and other pollutants but scientists had now found that they were also a natural product of processes such as breathing and digestion.

"We have a very complex system to deal with them. We produce a lot of enzymes which neutralise almost all of them," Dr Philpott explained.

"But some leak out and can damage your DNA. If you are not able to repair it, that does cause a mutation in the DNA that may be a carcinogenic [cancer-inducing] event."

Observing that red wine seemed to help in humans, Dr Gould and a student, James McKelvie, used a dye which fluoresces under ultra-violet light only when free radicals are present, and injected it into red and green parts of horopito leaves.

They then stabbed leaf fragments with a needle, causing stress which showed up under the microscope as an explosion of free radicals.

"To our sheer amazement, the red-pigmented cells scavenged free radicals significantly faster than did green cells," Dr Gould said. The red cells had the free radicals back down to normal levels within five minutes; in green cells they stayed long enough to damage the leaf.

Between them, the two groups of scientists have found high concentrations of antioxidants in many native plants, including various berries, kumara and a so-far secret plant that Dr Gould calls a "wonder plant".

Two purple varieties of kumara, one bred by Crop and Food Research and one occurring naturally at Dargaville, have very high antioxidant levels - and are much cheaper than better-known sources.

Dr Philpott said the meat and grains that have come to dominate our diets meant "we no longer consume the number of antioxidants we once would have".

Dr Ferguson, the medical school's head nutritionist, says the research reinforces longstanding advice to eat lots of fruit and vegetables - red as well as green.

Why eat your reds?

* Antioxidants are molecules that neutralise dangerous "free radicals" such as loose oxygen atoms by combining with them chemically to form stable new molecules.

* Human bodies are geared to take antioxidants from fruit, vegetables and whole grains, which dominated our diets until we developed agriculture.

* Some of the most powerful sources of antioxidants are anthocyanins, molecules that make plants go red.

* A new purple kind of kumara may be the most economical source of anthocyanins.