What is rhubarb

Rhubarb (Rheum rhabarbarum) is a species of plant in the family Polygonaceae. It is a herbaceous perennial growing from short, thick rhizomes. It produces large poisonous leaves that are somewhat triangular, with long fleshy edible stalks and small flowers grouped in large compound leafy greenish-white to rose-red inflorescences. Rhubarb is grown primarily for its fleshy stalks, technically known as petioles. The stalks are widely used as preserves and are also eaten raw, without problems.

In culinary use, fresh raw leaf stalks (petioles) are crisp (similar to celery, although they do not share the same family) with a strong, tart taste ¹. Most commonly, rhubarb stalks are cooked with sugar and used in pies, crumbles and other desserts. A number of varieties have been domesticated for human consumption, most of which are recognised as Rheum x hybridum by the Royal Horticultural Society.

For cooking, rhubarb stalks are often cut into small pieces and stewed (boiled in water) with added sugar, until soft. Little water is added, as rhubarb stalks already contain a great deal of water. Rhubarb should be processed and stored in containers which are unaffected by residual acid content, such as glass or stainless steel. Spices such as cinnamon, nutmeg, and ginger are sometimes added. Stewed rhubarb or rhubarb sauce, like applesauce, is usually eaten cold. Pectin, or sugar with pectin, can be added to the mixture to make jams.

A similar preparation, thickened with cornstarch or flour, is used as filling for rhubarb pie, tarts, and crumbles, leading to the nickname “pie plant”.

In recent times rhubarb has often been paired with strawberries to make strawberry-rhubarb pie.

Rhubarb contains anthraquinones including rhein, and emodin and their glycosides (e.g. glucorhein), which impart cathartic and laxative properties. It is hence useful as a cathartic in case of constipation.

Figure 1. Rhubarb

Is rhubarb poisonous

Rhubarb leaves contain poisonous substances, including oxalic acid, which is a nephrotoxic (damaging or destructive to the kidneys) and corrosive acid that is present in many plants ². Humans have been poisoned after ingesting the leaves, a particular problem during World War I when the leaves were mistakenly recommended as a food source in Britain ³. The toxic rhubarb leaves have been used in flavoring extracts, after the oxalic acid is removed by treatment with precipitated chalk. Oxalic acid can also be found in the stalks of rhubarb, but the levels are too low to cause any bodily harm ⁴.

Poisonous ingredients include ⁵:

• Oxalic acid
• Anthraquinone glycosides (possible)

The LD50 (median lethal dose) for pure oxalic acid in rats is about 375 mg/kg body weight ⁶, or about 25 grams for a 65-kilogram (143 lb) human. Other sources give a much higher oral LDLo (lowest published lethal dose) of 600 mg/kg. While the oxalic acid content of rhubarb leaves can vary, a typical value is about 0.5%, so a rather unlikely 5 kg (for a 70 kg human) of the extremely sour leaves would have to be consumed to reach the LD50 of oxalic acid. Cooking the leaves with baking soda can make them more poisonous by producing soluble oxalates ⁷. However, the leaves are believed to also contain an additional, unidentified toxin ⁸, which might be an anthraquinone glycoside (also known as senna glycosides).

Rhubarb leaves poisoning

Rhubarb leaves poisoning occurs when someone eats pieces of leaves from the rhubarb plant.

Symptoms may include ⁹:

• Breathing difficulty
• Burning in the mouth
• Burning in the throat
• Coma
• Diarrhea
• Eye pain
• Kidney stones
• Nausea and vomiting
• Red-colored urine
• Seizures
• Stomach pain
• Weakness

Outlook (Prognosis) of rhubarb leaves poisoning

How well you do depends on the amount of poison swallowed and how quickly treatment is received. The faster you get medical help, the better the chance for recovery.

Symptoms last for 1 to 3 days and may require a hospital stay.

Serious poisonings can result in kidney failure. Deaths have been reported, but are rare.

DO NOT touch or eat any plant with which you are not familiar. Wash your hands after working in the garden or walking in the woods.

Rhubarb nutrition

Table 1. Rhubarb (raw) nutrition facts

Nutrient	Unit	Value per 100 g	stalk 51 g
Approximates
Water	g	93.61	47.74
Energy	kcal	21	11
Energy	kJ	88	45
Protein	g	0.9	0.46
Total lipid (fat)	g	0.2	0.1
Ash	g	0.76	0.39
Carbohydrate, by difference	g	4.54	2.32
Fiber, total dietary	g	1.8	0.9
Sugars, total	g	1.1	0.56
Minerals
Calcium, Ca	mg	86	44
Iron, Fe	mg	0.22	0.11
Magnesium, Mg	mg	12	6
Phosphorus, P	mg	14	7
Potassium, K	mg	288	147
Sodium, Na	mg	4	2
Zinc, Zn	mg	0.1	0.05
Copper, Cu	mg	0.021	0.011
Manganese, Mn	mg	0.196	0.1
Selenium, Se	µg	1.1	0.6
Vitamins
Vitamin C, total ascorbic acid	mg	8	4.1
Thiamin	mg	0.02	0.01
Riboflavin	mg	0.03	0.015
Niacin	mg	0.3	0.153
Pantothenic acid	mg	0.085	0.043
Vitamin B-6	mg	0.024	0.012
Folate, total	µg	7	4
Folic acid	µg	0	0
Folate, food	µg	7	4
Folate, DFE	µg	7	4
Choline, total	mg	6.1	3.1
Vitamin B-12	µg	0	0
Vitamin B-12, added	µg	0	0
Vitamin A, RAE	µg	5	3
Retinol	µg	0	0
Carotene, beta	µg	61	31
Carotene, alpha	µg	0	0
Cryptoxanthin, beta	µg	0	0
Vitamin A, IU	IU	102	52
Lycopene	µg	0	0
Lutein + zeaxanthin	µg	170	87
Vitamin E (alpha-tocopherol)	mg	0.27	0.14
Vitamin E, added	mg	0	0
Vitamin D (D2 + D3)	µg	0	0
Vitamin D	IU	0	0
Vitamin K (phylloquinone)	µg	29.3	14.9
Lipids
Fatty acids, total saturated	g	0.053	0.027
04:00:00	g	0	0
06:00:00	g	0	0
08:00:00	g	0	0
10:00:00	g	0	0
12:00:00	g	0	0
14:00:00	g	0.001	0.001
16:00:00	g	0.046	0.023
18:00:00	g	0.004	0.002
Fatty acids, total monounsaturated	g	0.039	0.02
16:1 undifferentiated	g	0.001	0.001
18:1 undifferentiated	g	0.037	0.019
20:01:00	g	0	0
22:1 undifferentiated	g	0	0
Fatty acids, total polyunsaturated	g	0.099	0.05
18:2 undifferentiated	g	0.099	0.05
18:3 undifferentiated	g	0	0
18:04:00	g	0	0
20:4 undifferentiated	g	0	0
20:5 n-3 (EPA)	g	0	0
22:5 n-3 (DPA)	g	0	0
22:6 n-3 (DHA)	g	0	0
Fatty acids, total trans	g	0	0
Cholesterol	mg	0	0
Other
Alcohol, ethyl	g	0	0
Caffeine	mg	0	0
Theobromine	mg	0	0
Flavan-3-ols
(+)-Catechin	mg	2.2	1.1
(-)-Epigallocatechin	mg	0	0
(-)-Epicatechin	mg	0.5	0.3
(-)-Epicatechin 3-gallate	mg	0.6	0.3
(-)-Epigallocatechin 3-gallate	mg	0	0
(+)-Gallocatechin	mg	0	0
Isoflavones
Daidzein	mg	0	0
Genistein	mg	0	0
Total isoflavones	mg	0	0
Proanthocyanidin
Proanthocyanidin dimers	mg	1.7	0.9
Proanthocyanidin trimers	mg	1.8	0.9
Proanthocyanidin 4-6mers	mg	3.4	1.7
Proanthocyanidin 7-10mers	mg	1.9	1
Proanthocyanidin polymers (>10mers)	mg	79	40.3

[Source ¹⁰]

Rhubarb health benefits

There are several active constituents known about Rhubarb, including anthraquinone derivatives, such as emodin, chrysophanol, rhein, physcion, and their glycoside compounds, and stilbene derivatives such as piceatannol, resveratrol, and their glycoside derivatives ¹¹. Besides, there are several isolated complex compounds (e.g., torachrysone-8-O-β-D-glucopyranoside, sulphated emodin glucoside, and piceatannol-4′-O-β-D-(6′′-O-p-coumaroyl)-glucopyranoside) ¹¹.

In 2015, researchers found that when a concentrated form of the chemical physcion (also reported to be called parietin) – which gives rhubarb stems their color – was added to leukaemia cells in the lab, half of them died within two days ¹². A modified form of physcion was also able to reduce tumor growth in mice injected with human cancer cells. This laboratory and animal study ¹² looked at the role of a protein called 6-phosphogluconate dehydrogenase (6PGD) in cancer cells. This protein is involved in a pathway that helps give cancer cells the energy and building blocks they need to divide rapidly and create new cancer cells, and so form tumors. Adding physcion to human leukaemia cells, lung cancer, or head and neck cancer cells grown in the lab, or human leukaemia cells taken directly from a patient, stopped them dividing as much as they normally would. At the highest concentration tested, physcion caused about half of the leukaemia cells taken directly from a patient to die over 24 to 48 hours. Physcion did not have this effect on normal human cells in the lab. The study found that a chemical found in rhubarb called physcion, and related chemicals, are able to reduce cancer cell growth in the lab and in mice. They do this by blocking a protein called 6PGD ¹². Overall, these findings open another avenue for investigating potential cancer treatments. Much more research will be needed to make sure these chemicals are effective and safe enough to progress to testing in humans. We will need to wait to see the results of these studies before we know whether these chemicals could become anti-cancer drugs in the future.

References

1. Hood, Karen Jean Matsko. 2011. Rhubarb Delights Cookbook: a Collection of Rhubarb Recipes. Spokane Valley, WA: Whispering Pine Press International, Inc. pp. 20, 22. ISBN 9781930948006.
2. http://www.cbif.gc.ca/eng/species-bank/canadian-poisonous-plants-information-system/all-plants-common-name/canadian-poisonous-plants-information-system-rhubarb-common-name/?id=1370403267192
3. Robb, H. F. (1919). “Death from rhubarb leaves due to oxalic acid poisoning”. J. Am. Med. Assoc. 73: 627–628. https://jamanetwork.com/journals/jama/article-abstract/221917
4. Cooper, M. R., Johnson, A. W. 1984. Poisonous plants in Britain and their effects on animals and man. Her Majesty’s Stationery Office, London, England. 305 pp.
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6. http://www.rhubarbinfo.com/poison
7. Everist, Selwyn L., Poisonous Plants of Australia. Angus and Robertson, Melbourne, 1974, p. 583
8. Rhubarb leaves poisoning. Medline Plus. https://medlineplus.gov/ency/article/002876.htm
9. Stegelmeier BL, Field R, Panter KE, et al. Selected poisonous plants affecting animal and human health. In: Haschek WAM, Rousseaux CG, Wallig MA, eds. Haschek and Rousseaux’s Handbook of Toxicologic Pathology. 3rd ed. Philadelphia, PA: Elsevier; 2013:chap 40.
10. United States Department of Agriculture Agricultural Research Service. National Nutrient Database for Standard Reference Release 28. https://ndb.nal.usda.gov/ndb/search/list
11. Rokaya M. B., Münzbergová Z., Timsina B., Bhattarai K. R. Rheum australe D. Don: a review of its botany, ethnobotany, phytochemistry and pharmacology. Journal of Ethnopharmacology. 2012;141(3):761–774. doi: 10.1016/j.jep.2012.03.048. https://www.ncbi.nlm.nih.gov/pubmed/22504148
12. Lin R, Elf S, Shan C, et al. 6-Phosphogluconate dehydrogenase links oxidative PPP, lipogenesis and tumour growth by inhibiting LKB1–AMPK signalling. Nature Cell Biology 17, 1484–1496 (2015). https://www.nature.com/articles/ncb3255