A comfrey contention

JONATHAN CAMERON/Dominion Post

FRIEND OR FOE: Comfrey has long been used medicinally and as a garden fertiliser, but concerns have been raised about the potentially damaging alkaloids it contains.

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Debate is continuing over the use of the common garden plant comfrey, with a New Zealand toxicologist cautioning against it.

Meanwhile the country's largest organic gardening organisation believes the plant is safe when used sensibly.

Comfrey has traditionally been used both topically, for inflammation and wound healing, and orally for such things as gastrointestinal and respiratory concerns. It has also long been used in gardening practices.

However, Nelson man Tony Gibbons, a keen gardener and an immediate past treasurer of the Soil and Health Association in Nelson, wrote to The Nelson Mail in mid-December, concerned about advice given in the newspaper's garden pages advocating lining potato trenches with comfrey leaves and making liquid comfrey fertiliser.

Mr Gibbons said comfrey was dangerous and "a well established and potent human carcinogen and liver toxin".

"While the most obvious results are found in those animals grazing on it, there is also a considerable body of opinion that carcinogens are cumulative. That is, each of the carcinogens you ingest adds to the likelihood of your other carcinogenic intake beginning its deadly effect," he wrote.

"Oral intake is simply dangerous while the use of liquid fertiliser may not only lead to subsequent oral intake when eating the fertilised produce but the toxic alkaloids can anyway be ingested through the skin, making application more than problematical."

National Poisons Centre toxicologist Leo Schep said comfrey contained a variety of alkaloids known as pyrrolizidine alkaloids, which are produced by about 5 per cent of flowering plants, including ragwort.

In people who ate a lot of comfrey, the alkaloid could be concentrated by the liver into a more toxic form, Dr Schep said.

Sometimes the liver gets rid of the toxin, and sometimes it doesn't, in which case it can slowly cause liver damage. It could change DNA and some research models had shown cancer of the liver, he said.

The United States Food and Drug Administration asked for comfrey products to be withdrawn voluntarily from the market in 2001 because of concerns about it.

Dr Schep said research had shown mutations in the liver and a couple of cases of cancer in rats that were fed comfrey.

The Australia New Zealand Food Authority had said people should have no more than one microgram of comfrey per kilogram of body weight in their daily diet, he said.

One research paper in Britain said ragwort should be composted for a year before use on the garden, and he would treat comfrey the same.

He had no knowledge of any studies demonstrating whether the toxin is present in produce when comfrey is used in the gardening process or as a liquid fertiliser. However, he would err on the side of caution. If people used it for gardening, they should "dot their i's" and compost it for at least a year to allow the alkaloids to break down.

"I wouldn't encourage it [using it in the garden] because of the uncertainty. There's plenty of other composts you can use."

He said the problem with eating comfrey was that people could not see the underlying damage it might be doing.

"It's a long-term cumulative effect," he said. "You could have low-grade liver damage going on and you wouldn't know about it."

However, Soil and Health Association of NZ spokesman Steffan Browning said as with any food, too much comfrey could cause problems, and "toxicity has occurred with over-the-top consumption of carrots".

Mr Browning said the bad press for comfrey came from a case of over-consumption that caused toxicity in a person. No registered alternative practitioner would recommend the level of consumption that caused the problem, he said, adding that small amounts of comfrey were used in salads without harm by many people in Nelson.

Mr Browning said using comfrey as a fertiliser was highly unlikely to cause toxicity.

Through soil biology, comfrey was beneficial to plant growth. The plant being fertilised benefited from the improved mineral levels available but that did not mean it then carried a toxic level of the minerals or alkaloids that might be present in the plant-based fertiliser.

Mr Browning said that while some synthetic pesticides can be picked up by plants in either their original form or a dangerous metabolite, comfrey had been used successfully as a fertiliser for considerable time without apparent harm.

"The claim of comfrey as a potential cumulative carcinogen again will depend if the particular carcinogen is active in the same way as another, and at what part of the body/function.

"The reality is that the research has not been conclusive and those that I know that include small amounts of comfrey appear to be very well and often long-lived."

Mr Browning said comfrey was fantastic as part of good gardening, but care needed to be taken against eating vast amounts.

- © Fairfax NZ News