I absolutely love stinging nettle, it is one of my favourite spring time greens. Why? Because it is absolutely delicious. Nettle is also a plant that doesn’t give up its leaves without a fight – it has stinging hairs that cause discomfort when touched. I appreciate any relationship that demonstrates clear communication of boundaries – something that keeps nettle close to my heart.

This year my first nettle experiment has been a take on a wonderfully simple and delicious tomato soup recipe I found online this past winter.

Ingredients

  • 3 tablespoons butter
  • 1 large onion
  • 3 generous handfuls of nettle
  • 156mL tomato paste
  • 2 tablespoons blue cheese (optional)
  • 2½ cups water (or chicken/ vegetable broth)
  • 1/2 teaspoon nutmeg
  • 1 teaspoon black pepper
  • 1 teaspoon salt

Directions

  1. Sautée onion in butter with salt.
  2. Add nettle leaves, allow to wilt in pan.
  3. Add tomato paste, cheese and water (slowly).
  4. Allow to simmer until nettle is cooked and cheese is melted.
  5. Purée soup.
  6. Add black pepper and nutmeg and simmer soup for 5 minutes.
  7. Serve hot with grilled cheese.

Below you will find a plant info card for stinging nettle. I have taken special care to debunk the numerous myths surrounding the so-called dangers of stinging nettle. Some of these myths are applied to other wild edibles, so it is well worth taking note and reading on. Unless properly referenced, most news articles and even books on the subject of wild food should be taken with a grain of salt, or should I say pan fried in butter with a pinch of salt. 😉

Stinging Nettle – Urtica dioica
Family: Urticaceae
Lifecycle: perennial, rhizomatous, dioecious
Key features: Opposite triangular-shaped leaves, stinging hairs. Flowers are not showy, looking like strings of green beads.
Parts used: Leaves, seeds, and roots.

Harvesting considerations:
* For plants growing in disturbed areas be aware of pollution and other environmental contaminants
* Harvest young tops before plant is in flower.
* Use gloves when harvesting to avoid getting stung.

How to prepare:
* Dry or cook leaves to avoid getting stung.
* Seeds can be dried and added to food as a wild source of fatty acids and vitamin E (1)
* Roots and rhizomes are used as a medicine to support prostate health

Safety considerations:
* Stinging hairs:
The stinging hairs of stinging nettle contain the compounds acetylcholine, histamine, and 5-hydroxytryptamine (2) not formic acid as is frequently reported (3). The symptoms of exposure in humans consists of initial pain, swelling and redness followed in some cases by numbness and itching (2). I have personally had blood blisters and later peeling skin as a result of numerous stings to the same area (my thumb after harvesting). There is one documented case of massive exposure to stinging hairs that led to the death of several hunting dogs (4).

* Cystoliths:
Cystoliths are crystals typically made from calcium carbonate found as epidermal outgrowths in older leaves (see photo below). Various sources cite that eating older leaves will lead to a risk of kidney damage due to the presence of calcium carbonate crystals. However, I have found no evidence of any toxicity due to nettle cystoliths. It also doesn’t make biological sense as large crystals would be too large to be taken up into the bloodstream and therefore would never make it to the kidneys to be filtered out and cause damage. I suspect some of the confusion comes from the fact that the word cystolith is also a medical term used to describe urinary caliculi or kidney stones. Depending on the size and shape of cystoliths I could imagine gastrointestinal irritation being an issue, if the crystals were sharp and jagged for example. Otherwise I see no reason to continue to list this as a concern for consumption of stinging nettle.

* Oxalic acid:
Oxalic acid found in nettle and many other common foods (e.g. spinach) is frequently described as increasing one’s risk of calcium oxalate kidney stones. Oxalic acid can increase the risk of kidney stones, however, to put this in perspective oxalic acid is also the major metabolic end product of ascorbic acid (vitamin C) oxidation. A dose of 500mg vitamin C has been shown to modestly increase urinary excretion of oxalic acid (5).
Free oxalic acid absorbs more readily from the gastrointestinal tract than when it is bound up with calcium as calcium oxalate. In rats the ingestion of calcium prevents the absorption of oxalic acid in a dose dependent manner (6). Nettle itself has been shown to contain as much as 169mg calcium per 100g of dried plant material (7). I would not consider eating nettles a risk for people with normal kidney function. Combining calcium rich foods with foods high in oxalic acid would certainly be a way to mitigate any risks. Conversely consuming oxalic acid containing foods away from calcium supplements, where low calcium is a health concern, would also be prudent. Oxalic acid has been shown to decrease calcium absorption in rats (8).

*Nitrates:
Nitrates are nitrogen containing compounds that recently have been a source of controversy. Nitrate from plants is converted to nitrite and then to NO (nitrous oxide), which has been shown to reduce blood pressure (one of the reasons behind beet juice supplementation). However, nitrate and nitrite can also be converted to carcinogenic (cancer-inducing) nitrosamines in the absence of vitamin C. The highest dietary sources of nitrates are vegetables, cured meat and water contaminated with human or animal waste. Vegetables have high concentrations of vitamin C, so should not pose any risk of exposure to nitrosamines (9).
The other significant concern is that ingested nitrates can cause methemoglobinemia in infants less than 3 months of age. Methemoglobin is created when ferrous iron in hemoglobin is oxidized to ferric iron in the presence of nitrite in the bloodstream. Unlike adults, infants lack the ability to prevent this reaction from occurring at a dangerous rate. The result is that infants have low blood oxygen and present with symptoms such as blue or brown mucous membranes, irritability and rapid breathing. It should be noted that solid foods are only recommended after 4-6 months, not in infants less than 3 months of age. Despite recommendations, poisoning from vegetables containing nitrates has been documented in infants, but the main source of poisoning is contaminated well water (10).

Cystolith from Ficus elastica. Public domain.

References

  1. Uluata, S., & Ozdemir, N. (2012). Antioxidant Activities and Oxidative Stabilities of Some Unconventional Oilseeds. Journal of the American Oil Chemists’ Society, 89(4), 551–559. doi:10.1007/s11746-011-1955-0
  2. Mitchell, J. C., Rook, A. 1979. Botanical dermatology. Greenglass Ltd, Vancouver, B.C., Canada. 787 pp.
    https://www.cbif.gc.ca/eng/species-bank/canadian-poisonous-plants-information-system/all-plants-common-name/stinging-nettle/?id=1370403267214
  3. Thurston, E., & Lersten, N. (1969). The Morphology and Toxicology of Plant Stinging Hairs. Botanical Review,35(4), 393-412. Retrieved from http://www.jstor.org/stable/4353782
  4. Anon. 1982. Stinging nettle (Urtica sp.) and dogs. Vet. Hum. Toxicol., 24: 247.
  5. Robitaille, L., Mamer, O. A., Miller, W. H., Jr, Levine, M., Assouline, S., Melnychuk, D., … Hoffer, L. J. (2009). Oxalic acid excretion after intravenous ascorbic acid administration. Metabolism: clinical and experimental, 58(2), 263–269. doi:10.1016/j.metabol.2008.09.023
  6. Morozumi, M., Hossain, R.Z., Yamakawa, K. et al. (2006), Gastrointestinal oxalic acid absorption in calcium treated rats. Urol Res. 34: 168. https://doi.org/10.1007/s00240-006-0035-7
  7. Adhikari, B. M., Bajracharya, A., & Shrestha, A. K. (2015). Comparison of nutritional properties of Stinging nettle (Urtica dioica) flour with wheat and barley flours. Food science & nutrition, 4(1), 119–124. doi:10.1002/fsn3.259
  8. Connie M. Weaver, Berdine R. Martin, Jacqueline S. Ebner, Catherine A. Krueger, Oxalic Acid Decreases Calcium Absorption in Rats, The Journal of Nutrition, Volume 117, Issue 11, November 1987, Pages 1903–1906, https://doi.org/10.1093/jn/117.11.1903
  9. Brandt, K. (2016). Vegetables and Fruit in the Prevention of Chronic Age-Related Diseases: Molecular Basis of Nutrition ad Aging. Academic Press, Chapter 50, 707-722. https://doi.org/10.1016/B978-0-12-801816-3.00050-9
  10. Frank R. Greer, Michael Shannon. Infant Methemoglobinemia: The Role of Dietary Nitrate in Food and Water. Pediatrics Sep 2005, 116 (3) 784-786; DOI: 10.1542/peds.2005-1497
Nettle Tomato Soup
Tagged on: