there are 4 general chemical compounds to be aware of when eating weeds
1 Oxalic Acid:
This occurs in quite a few weeds -however in context, spinach and silverbeet that we eat without regard also contain oxalic acid in quite high amounts. The highest amount of oxalate in a weed is in Fathen - Chenopodium album, then Purslane and then Spinach - the other weeds are less than Spinach. Oxalic acid gives plants an acid taste and it can be removed by boiling for a few minutes and then replacing the water. This also helps remove any bitterness of alkaloids or other chemicals as well.
There are no recorded deaths in Australia from oxalate poisoning - there was one in Canada a long time ago (1913) from someone who ate rhubarb leaves. The LD50 for oxalic acid seems to be 25gm for a 65 kg human - to eat this much of Fathen would require eating 2.5kg in one sitting. To eat such an amount would be unlikely.
Theoretically - too much soluble oxalate in your blood stream would combine with Calcium with the possible consequence of muscle weakness, cardiac rhythm disturbance and when excreted, kidney stones. Interestingly in my practice, Calcium oxalate stones are the most common type but the patients with kidney stones are not the ones eating lots of green leafy vegetables! Also, theoretically, too much oxalate chronically, might contribute to thinning of the bones (osteoporosis) but it may also bind to toxic metals such as mercury and lead allowing them to be excreted and thus be beneficial.
So, on balance, if you eat spinach and silverbeet you are already getting a fair dose of oxalic acid. As a precaution, we tend to blanch the wild greens with elevated levels of this chemical.
2 Alkaloids - especially pyrrolizidine alkaloids.
Alkaloids are ubiqitous -caffeine is an alkaloid, as is nicotine , morphine etc and they are not all bad in small amounts. One class of alkaloids is however harmful and they are called pyrrolizidine alkaloids. These can cause liver damage and cancer when eaten over for a prolonged period. The banning of comfrey was due to it containing certain pyrrolizidine alkaloids and indeed some of weeds have also been reported to have contain similar compounds. The only one that we have tried that contains pyrrolizidine alkaloids is Thickhead - Crassocephalum crepidiodes. I am also suspicious of Cobbler's pegs (Bidens pilosa) having pyrrolizidine alkaloids but have not found any evidence for this on the internet.
Cyanide :
Surprisingly, cyanide occurs in some of our foods -lima beans, chick peas, almonds, bamboo shoots, cassava and apple pips - and also in the leaves of 2 weed species that I am aware of , the leaves of Black Nightshade and Stinking Passionfruit. It is readily destroyed by cooking so is not really an issue . Humans can cope with very small doses of cyanide with it being excreted through the lungs anyway.
Nitrates and Nitrites:
A large number of weeds are nitrate concentrators - fathen, amaranth, sonchus - as is spinach, silverbeet, lettuce and broccoli. Nitrates are ok anyway - lethal dose is 8-15gm for an adult male, but nitrate can convert to nitrite in certain conditions and this is lethal at about 1gm for an adult male. To eat such an amount would require eating prodigous quantities of overfertilised, wilted, insect chewed leaves, and this would apply to any other leafy vegetable anyway
So risks - oxalic acid and pyrrolizidine alkaloids worth remembering -but from my viewpoint the amounts we might eat in a salad or as a boiled vegetable would be way to little for this to be a real concern.
Obviously avoid eating weeds that may have been sprayed - eg roadsides/reserves and weeds that are growing on contaminated sites as many are heavy metal accumulators.
As a general rule - pick and eat the very youngest leaves as these will have less of any bad chemicals in them and taste better!
Tom
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