I have just bought a flowerpot of sage - Salvia officinalis. I know that the leaves can be used for tea. But can they also be eaten fresh, as a snack or as part of the salad? Is there any limit on the amount of fresh Salvia leaves that is safe to eat per day?
Sage (Salvia officialis) is a staple herb in various cuisines. It pairs with veal in an Italian Saltimbocca or pork in the British sage and onions stuffing and is eaten even on its own, e.g. battered and fried. So yes, it’s clearly edible. However, personally I would not serve it as a salad leaf, it’s probably too pungent to be truly enjoyable, but taste is of course personal preference. There are recipes that use sage in vinaigrette, though.
As the flavor is quite intense and a little goes a long way, most consumers will never nibble on enough to get in the range where the thujone content matters (similar to the amygdalin in apple pips). But let’s do a rough estimation to get a ballpark number. The amount of oil that can be extracted from S. officinalis leaves is between 0.5 and 1 %. That oil can contain up to 50% thujone, so we can just use the 0.5% as thujone content of fresh leaves. Considering that the LD50(mice) of thujone is 45mg/kg and that 30mg/kg gives a 0% mortality, a healthy 75kg “average person” could probably eat 2g of thujone or 400g sage leaves and be fine - but it wouldn’t be a good idea nevertheless, taste-wise. Excessive, especially habitual/long term use is sometimes discouraged.
The EU has limited the amount of thujone from sage a food product may contain to 25mg/kg. That would equal 50g sage leaves in 1 kg prepared food.
Conclusion: Enjoy cooking with your sage, forget about the salad leaves idea.
Further reading: https://pubmed.ncbi.nlm.nih.gov/20727933/
If you were a human-sized rat, it seems that in the worst case, 2 kg of leaves might be enough. That's a rather handwavy amount, of course.
To go into more depth: the presence of papers like "Toxicity of Salvia officinalis in a newborn and a child: an alarming report" means that dying from eating sage isn't common - apparently, two very young children having seizures after ingesting sage essential oil is news enough to get published as a cautionary case report. So it is unlikely that anybody has ever measured the exact amount needed for an adult human to die from sage poisoning.
What people have measured is the composition of the essential oil extracted from sage. A good example is "Composition of the essential oil of Salvia officinalis L. from various European countries", which makes a good comparison of samples - because of course, there is huge variation between plants of the same species.
If we make several assumptions, we can make a back-of-the-envelope calculation:
- the only toxic compound we are interested in is thujone
- the conversion ratio for fresh to dry sage is 3:1 by weight
- the sage you eat produces as much essential oil as the highest-yield sage in the paper (the range was 2.2 to 24.8 ml per kg dried leaves)
- the sage you eat has as much thujone as the highest-thujone sage in the sample (50% alpha and 13% beta thujone)
- both alpha and beta thujone count equally
- the LD50 dose for rats (192 mg/kg) is relevant for humans
- your weight is 70 kg
then my calculation is that you would have to consume 13.44 g of thujone to reach the LD50, while a kilogram of fresh sage will have a bit over 6 grams of thujone.
This is of course a very inaccurate calculation, please do not rely on the exact numbers. But if should give you an idea of the rough range of dangerous amounts.