Key Points:

Anxieties about the health effects of aspartame appear to have turned many consumers against the artificial sweetener, sparking new public health worries this could boost obesity.

Supermarket sales of low- or no-calorie diet soft drinks such as Coke Zero have slumped this year while their sugar-charged alternatives have experienced a boom, according to figures from research agency Nielsen. This reverses an earlier trend.

"I think it's a concern," Auckland University population health expert Professor Rod Jackson said yesterday.

In the face of New Zealand's growing obesity rate, he supports replacing sugary drinks with those containing aspartame - except for those with a rare disease that prevents them from processing the sweetener - to reduce energy intake.

"There's no such thing as a safe anything. Anything you eat is going to have some harms and possibly some benefits.

"But what's clear is that having too many calories increases your weight, your blood pressure, your risk of diabetes, your risk of heart disease."

New Zealand's acceptable daily aspartame intake is 40mg/kg of bodyweight - 17.5 cans for a 70kg adult if measured by aspartame drinks. In 2003, individuals were typically consuming 6-15 per cent of the recommended limit, according to the Health Ministry.

The aspartame controversy erupted after Abby Cormack's case became the subject last year of a petition to Parliament's health select committee calling for restrictions on the chemical.

Ms Cormack, who chewed four packets of sugar-free gum a day and drank large quantities of diet soft drink, reported dizziness, tingling, insomnia, paranoia and other problems which she said ended when she quit aspartame.

The committee did not agree to the petitioners' request, but in a minority report, the Green Party called for warning labels to highlight aspartame's "potential adverse effects".

Coca-Cola Oceania has financed the trip to New Zealand of Canadian aspartame expert Dr Bernadene Magnuson, who will speak at Nutrition Foundation seminars this week on the "myths" surrounding the chemical.

Dr Magnuson, an independent consultant toxicologist and Toronto University adjunct professor in nutritional sciences, contributed to a review of aspartame last year. It was funded by a leading producer of the sweetener, but the researchers were not told this until after they had reached their conclusions.

"At current levels of consumption of aspartame, the evidence is that it's completely safe," Dr Magnuson told the Herald yesterday. "There is no credible evidence of an effect of aspartame in cancer, neurological problems or reproductive problems."

She discounted studies which purported to show adverse health effects from the chemical.