Those of us who consume honey as a tastier alternative to refined sugar would have been disturbed to see headlines proclaiming Australian honey could be making us sick. Why, you might wonder, could this wholesome and all natural product be a threat to our health?
It is all because plants, famously, can’t run away from predators.
To stop animals eating them plants often use toxic chemicals. An example is the bitter alkaloid caffeine, which deters or can even kill insects trying to munch on the plants that contain it. A far less pleasant herbivore deterrent is the class of toxic chemicals are the pyrrolizidine alkaloids. These are a large group of related compounds that can cause severe liver and lung damage. Long term consumption of pyrrolizidine alkaloids may increase the risk of cancer.
Pyrrolizidine alkaloids are present in many plants ranging from comfrey to Patterson’s curse (Salvation Jane), and small amounts of pyrrolizidine alkaloids may be found in comfrey containing salads and herbal medicines. Importantly, one source is honey.
In many parts of Australia, especially southern Australia, the weed Patterson’s Curse/Salvation Jane is a significant source of nectar for foraging bees. Patterson’s Curse produces high levels of pyrrolizidine alkaloids. Indeed it is called Patterson’s Curse in part because of it poisoning stock. Honey produced from Patterson’s Curse nectar can have high levels of pyrrolizidine alkaloids, and must be diluted with honey from other sources to reduce the levels.
To date no adverse health effects, either acute or chronic, have been attributed to consumption of Australian honey.
The headlines were generated by a recent report that shows that Australian honey has on average four times more pyrrolizidine alkaloids than European honeys. Depending on how much you ate, consumption of some of the honeys would exceed recommended European intake guidelines.
Several European guidelines recommend that people be exposed to no more than 0.007 micrograms pyrrolizidine alkaloid per kilogram body weight per day, while Australian guidelines state that people should consume no more than 1 microgram pyrrolizidine alkaloid per kilogram body weight. Australian guidelines have set the intake limits as one hundred times lower than levels that show no evidence of toxicity or carcinogenicity in animal studies. Thus there is a substantial safety factor.
European guidelines are more stringent than Australian guidelines, due to a more conservative estimate of cancer risk. While pyrrolizidine alkaloids are able to produce cancer in rats, evidence for cancer in humans is indirect. As well, the human risk is likely lower as the mechanism of carcinogenicity are likely different to that in rats. However, the European guidelines take a “zero tolerance approach” and thus have lower intake limits.
Even with this more stringent approach for most Australian honeys the risk is low. Australian honey has on average 149 micrograms of pyrrolizidine alkaloids per kilogram honey (compared to 40 micrograms per kilogram for European honeys, to give you an idea of how small that is, a single grain of sugar weighs around 600 micrograms, now imagine a third of a single grain of sugar dissolved in a kilogram of honey).
For a 70 Kg person eating the average amount of honey (around three grams per day,
roughly three teaspoons, this is around double the average European consumption) consumption of most of the Australian honeys would be safe at both European and Australian guidelines.
There were a few exceptions where the levels were quite high, and would have exceeded the EU, but not Australian, limits substantially. Out of 59 honeys tested five had double the EU limit and one had nearly ten times the EU limit (ironically this was an “organic” honey) these honeys are of concern.
While for the average consumer the risk is low, people who are high consumers of honey are at much greater risk. The average Australian may consume only three grams
teaspoons of honey a day, but a small proportion of Australians consume much more.
Around 5% of Australians consume around 57 grams of honey a day. When consumed at these levels several honeys come close to the current Australian limits and substantially exceed the European guideline limits. The impact on children with lower body mass is likely to be greater as well.
On a brighter note the honey that was assayed in the headline-generating study was purchased in 2011 and 2012. Since then there has been a substantial campaign to reduce Patterson’s Curse infestation. While Patterson’s curse is not the only source of pyrrolizidine alkaloids (eg. weeds of the Heliotropium genus also contribute) this should reduce the amount of pyrrolizidine alkaloids entering into our honeys.
To reiterate, for the average consumer the risk from honey is low. However, further investigation and assays of more recent honey supplies will be needed to understand the risk to more vulnerable groups.
The bottom line is that there is no need to throw away your honey, but do eat sensible amounts of it, and make sure the kids are not eating too much.
 As pointed out in the comments, this may not be correct, I took this value from a FSANZ document, however, honey from different sources has different densities, according to what I can find, 2.74 grams of European honey will be two teaspoons, and 2.74 grams of Manuka honey will be around half a teaspoon.
 Updated 25/01/2016 Patterson’s Curse has a distinctive chemical fingerprint of the proportions of the different types of pyrrolizidine alkaloids it produces. The paper looked at a number of these pyrrolizidine alkaloids, enough to get a good idea of the range of sources. 10 of the 56 honeys tested clearly contain Patterson’s Curse Honey (for the technically minded, from the Echimidine/Lycopsamine ratio. This identification holds unless there are mega fields of Comfrey nearby), but six of the honeys with highest levels of pyrrolizidine alkaloids, including the honey with the highest level, have higher levels of Lycopsamine than anything else, often by a wide margin (the most problematic honey especially). This rules out Patterson’s Curse, Comfery, Borrage, the Heliotropium family (which were my best guess for sources of pyrrolizidine alkaloids, but virtually no honey had high levels of Heliotrine), it also rules out the orchid family, the legume family and whole range of flowering plants including sunflowers. I have no idea what this source could be. On the plus side Lycopsamine is less toxic than other pyrrolizidine alkaloids. On the minus side controlling Patterson’s Curse is unlikely to reduce pyrrolizidine alkaloids in all honeys.  The herb Borage is different from the plant called Blue Borage in New Zealand, and Vipers Bugloss in Australia. Blue Borage is a relative of Patterson’s Curse and has a similar ratio of pyrrolizidine alkaloids as Patterson’s Curse.