Is the world mad when Irish scientists focus their attention on Australian honey and find high levels of pyrrolizidine alkaloids? And the results are sensationalised by the Australian press a year later, talk about a slow response! Headlines in January 2016 proclaimed that “Australian honey could make us sick” and the article stated that “Australian honeys are the most contaminated in the world with natural poisons linked to chronic disease including cancer”. If that didn’t frighten you, what would?
And truely, pyrrolizidine alkaloids are natural toxins linked to chronic disease including cancer. Typically the compounds affect the liver and in some cases the lungs causing serious illness. Animal experiments have also shown that certain pyrrolizidine alkaloids are genotoxic carcinogens, the worst of the worst of toxins.
So what are they?
Pyrrolizidine alkaloids are produced as a protection against herbivores by about 6,000 plant species, representing 3% of all flowering plants, most of which are weeds. There is a great variety of compounds with more than 500 different pyrrolizidine alkaloids known to date. Besides in honey, pyrrolizidine alkaloids in food have been detected in products of plant origin, for example, in herbal teas and supplements, cereals, and salads. Cases of elevated contamination in wheat are known to have occurred in Afghanistan associated with illness and similarly contaminated salad in Germany.
To be fair to the Irish, the study was all about developing better analytical methods for detecting multiple pyrrolizidine alkaloids and the scientists probably selected Australian honey to be certain of having positive samples. They could as well have selected South American samples also known for containing high levels of pyrrolizidine alkaloids.
Nevertheless, their results showed that 41 of the 59 honey samples were contaminated by pyrrolizidine alkaloids with a mean total sum of 153 µg/kg. This is on average four times more pyrrolizidine alkaloids than in European honeys and is quite high as an average level. Echimidine and lycopsamine were most common and found in 76% and 88%, respectively, of the positive samples. The scientists also attempted to calculate possible average daily exposure based on the results and found that adults could have an exposure of 0.051 µg/kg bodyweight per day and children 0.204 µg/kg bodyweight per day of pyrrolizidine alkaloids.
What does it mean?
It is debatable if all pyrrolizidine alkaloids should be treated equally when considering their toxicity due to their expected cumulative effects or if some of the compounds could be considered to be less toxic.
Conveniently the Australian authority, Food Standards Australia New Zealand, considers that echimidine is less toxic and used a Tolerable Daily Intake approach in establishing a safe level of exposure of 1 µg/kg bodyweight per day. This was calculated by applying an uncertainty factor of 10 to what was considered to be a human no-observed-effect level of 10 µg/kg bodyweight per day for liver failure due to veno-occlusive disease. But carcinogenic effects were not considered. Using this approach the Irish exposure estimates are well within safe limits.
Not so says a number of national and international organisations like the World Health Organization International Programme on Chemical Safety, the Dutch Rijksinstituut voor Volksgezondheid en Milieu, the UK Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment, the German Bundesinstitut für Risikobewertung, and the CONTAM Panel of the European Food Safety Authority. They have all concluded that 1,2-unsaturated pyrrolizidine alkaloids may act as genotoxic carcinogens in humans (that is they may cause cancer and damage DNA, the genetic material of cells).
The safety of genotoxic carcinogens should be evaluated using the Margin of Exposure approach and not the Tolerable Daily Intake approach. A benchmark dose lower confidence limit for a 10% excess cancer risk (BMDL10) of 70 μg/kg bodyweight per day for induction of liver haemangiosarcomas by lasiocarpine in male rats was calculated as the reference point for comparison with the estimated dietary exposure. As a Margin of Exposure of 10,000 or higher, based on a BMDL10 from an animal study, is considered to be of low concern from a public health point of view, exposure to 0.007 µg/kg bodyweight per day or less of pyrrolizidine alkaloids would not be a worry. But the Irish presented much higher exposure estimates.
What margin is safe?
The different interpretations of what is a safe exposure to pyrrolizidine alkaloids is confusing to scientists and the public alike. Honey consumption has a long and varied history as a remedy for several health afflictions. Although, due partly to low numbers and questionable quality of human studies, some of the suggested health benefits of honey have been difficult to prove scientifically. Nevertheless, the public perception is that honey is a wholesome and natural product beneficial to health and a tastier alternative to refined sugar. There is a small committed group of consumers that regularly consume relatively large amounts of honey. So the findings of pyrrolizidine alkaloid contamination is disturbing.
However, there are som alleviating factors to reassure honey consumers. The presence in honey of lasiocarpine used to calculate the BMDL10 is rare and most other pyrrolizidine alkaloids are at least a magnitude less toxic. This could raise the level of exposure of no concern to 0.07 µg/kg bodyweight per day. Also retail honeys are often mixed from several sources to reduce the overall level of pyrrolizidine alkaloids in the consumer-ready product. And finally the Australian honey industry is claiming that they have reduced access of bees to Paterson’s curse, a main source of pyrrolizidine alkaloids in Australian honey. But the future will tell if that is right.
So some caution is justified for regular honey consumers. Vary your source of honey to limit exposure and hopefully you will be fine. For now.