Health Notes

Exposure to toxins can be good for you, say scientists

The dogma that there is no safe level of exposure to toxins, radiation and carcinogens is wrong and may have unnecessarily terrified the public and led to billions of pounds being wasted on environmental clean-ups.

A little poison or radiation can even be good for you, according to a study being published in Nature.

For 30 years, toxicologists have assumed a simple relationship between the dose of a toxin and effect: double the dose and the effect is doubled. But this simple relationship is wrong, conclude Prof Edward Calabrese and Dr Linda Baldwin of the University of Massachusetts at Amherst. "The economic implications of this conclusion are substantial,'' they conclude, saying it will allow less costly environmental standards and "revolutionise public attitudes towards risk''.

They report that the dose-effect relationships are usually represented by a U-shaped curve where tiny quantities of a toxin or radiation can have an effect that first declines - and can even be beneficial - before increasing in harmful impact.

The scientists trace the problem back to the formative years of toxiciology, the 1930s and 1940s, when the "threshold model'' of toxicity testing was adopted. Now it is enshrined in all textbooks.

The method that should be used, they say, is one that eschews the concept of "safe'' and "dangerous'' levels but instead assumes that a toxic substance acts like a stimulant in small doses, but as an inhibitor in large ones.

A systematic study by Prof Calabrese and Dr Baldwin found 5,000 examples of this effect, known as hormesis.

"The hormetic model is not an exception to the rule - it is the rule,'' they say. Yet, even today, most toxicological experiments are not set up to investigate hormesis: they focus on doses that are beyond the dip in the U-shaped curve and then assuming a linear relationship between dose and effect.


'Delhi belly' bacteria may protect against cancer

Travellers suffering from "Montezuma's revenge'' or "Delhi belly'' can take comfort from the discovery that the bacteria responsible may protect against cancer, say American researchers.

They were struck by how colorectal cancer strikes disproportionately more often in industrial nations than developing countries. Conversely, traveller's diarrhoea, caused by intestinal E.coli infections, is more prevalent in developing countries.

Dr Giovanni Pitari and colleagues at Thomas Jefferson University, Philadelphia, investigated the effects of a toxin produced by E.coli on human colon cancer cells.

After adding the toxin, the scientists found that the growth rate of a population of actively dividing cancer cells soon slowed to a crawl. Further tests revealed that the toxin mimics substances that control cell division, they report in the Proceedings of the National Academy of Sciences.

These results suggest a new way to treat colorectal cancer, by exploiting the same pathway targeted by E.coli toxins to stem the rapid cell proliferation that takes place in cancer.


© The Telegraph Group Limited, London 2003