HerbalEGram: Volume 16, Issue 4, April 2019

Editor’s Note: Each month, HerbalEGram highlights a conventional food and briefly explores its history, traditional uses, nutritional profile, and modern medicinal research. We also feature a nutritious recipe for an easy-to-prepare dish with each article to encourage readers to experience the extensive benefits of these whole foods. With this series, we hope our readers will gain a new appreciation for the foods they see at the supermarket and frequently include in their diets.

The basic materials for this series were compiled by dietetic interns from Texas State University in San Marcos and the University of Texas at Austin through the American Botanical Council’s (ABC’s) Dietetic Internship Program, led by ABC Education Coordinator Jenny Perez. We would like to acknowledge Perez and ABC Chief Science Officer Stefan Gafner, PhD, for their contributions to this project.

By Hannah Bauman^a and Jennifer Wible^b

^a HerbalGram Associate Editor

^b ABC Dietetics Intern (Texas State, 2018)

Overview

Garden rhubarb (Rheum rhabarbarum) is an herbaceous perennial that belongs to the Polygonaceae, or buckwheat, family.¹ It has large, triangular leaves that grow up to a foot across with petioles, or stalks, that grow from short, thick rhizomes.^1,2 The stalks tend to grow in shades of deep red, light pink, and green, and are ready for harvest in mid- to late spring.^3,4 Native to Mongolia, India, China, and other Asian countries, rhubarb species grow best in cool, temperate, and subtropical climates with mild summers.^1,5 The Rheum genus includes more than 60 known species. The stalks of both R. rhabarbarum and R. rhaponticum (false rhubarb) are commonly used in cooking, while the rhizome or root of Chinese rhubarb (R. officinale) and Turkey rhubarb (R. palmatum) have a long history of medicinal use that precedes the use of the stalk as food.⁴

Phytochemicals and Constituents

Plants in the genus Rheum contain a variety of beneficial compounds, including stilbenes, flavonoids, essential oils, tannins, organic acids, and anthranoids (e.g., anthraquinones, anthrones, and bianthrones).⁵ Anthraquinones are pigmented compounds that give rhubarb stalks their color. The most common anthraquinones in Rheum species are glycosides of aloe-emodin and emodin (orange), chrysophanol (yellow), physcion (red-orange), and rhein (red).⁶ These compounds and their derivatives also give rhubarb its laxative effect.⁷ In addition, they have been shown to inhibit the growth and proliferation of cancer cells in vitro.^5,8

Rhein has been found to have many potential benefits. Due to its anti-inflammatory and antioxidant properties, rhein has been investigated for use in treating diabetic nephropathy, the leading cause of end-stage renal failure.⁹ Rhein also has been found to suppress hyperglycemia specifically by improving insulin secretion from pancreatic β-cells.

Stilbenes are phytochemicals associated with estrogenic activity.⁵ Several stilbene derivatives are present in rhubarb, such as rhaponticin and deoxyrhaponticin, which have been studied in human clinical trials and animal models for their ability to stimulate ovulation and inhibit lactation.

Rhubarb stalks contain catechins, gallic acid, and tannins that are responsible for rhubarb’s astringent properties.⁵ Flavonoids such as quercetin, rutin, isoquercitrin, and anthocyanins are most concentrated in the flowers and immature fruits of the plant. These flavonoids contribute to rhubarb’s high antioxidant content, which is similar to that of kale (Brassica oleracea, Brassicaceae) and spinach (Spinacia oleracea, Chenopodiaceae).¹⁰ Anthocyanin-rich foods like rhubarb can support urinary tract health, fight bacteria, and improve memory function.³

The essential or volatile oil components present in rhubarb are primarily terpenoids, eugenol, and methyl heptyl ketone.⁵ Organic acids such as oxalic, palmitic, acetic, malic, citric, and formic acids are present in all parts of the plant and give rhubarb its characteristic sour taste. Oxalic acid is present primarily in the form of calcium oxalate. The concentrations of organic acids in rhubarb stalks can reach 1.8% of the fresh weight, with malic acid (up to 1.8%), oxalic acid (up to 0.3%), and citric acid (up to 0.2%) as the main components.¹¹ Rhubarb leaves are toxic to humans due to the higher levels of oxalic acid (approximately 0.5%).¹² Consuming small amounts of leaves may cause nausea and mild gastrointestinal irritation, while toxicity from larger amounts may lead to renal damage, hypocalcemia, and electrolyte imbalance. However, the rhubarb stalks have a much lower oxalic acid content.

In a 2010 study, various cooking methods used for rhubarb stalks were analyzed to determine how each method impacts the bioavailability of bioactive components.¹³ Common preparations included blanching, stewing, and baking. After cooking, polyphenol levels were higher than in raw rhubarb. The highest levels were found after slow cooking for 30 minutes and baking for 20 minutes.

Juicing rhubarb stalks preserves most of the nutrients and phytochemicals while minimizing antinutrients. A study found that rhubarb juice has lower levels of oxalic acid than the raw stalks do, but it also contains no calcium. Magnesium, potassium, and sodium were still present in the juice. Catechin, procyanidins B₁ and B₂, and rutin were also present in the rhubarb juice.¹⁴

Historical and Commercial Uses

Several species of rhubarb have been used medicinally, culinarily, and ornamentally.⁵ In northern Asia, dried rhizomes of R. palmatum and R. officinale have been used medicinally as a purgative and an antibacterial for dysentery for 5,000 years.^1,12

Rhubarb was imported throughout Europe beginning with the advent of the Silk Road in the first century BCE.¹⁵ Rhubarb stalks began to be consumed as a food product in 17th-century England; however, Europeans were unable to grow rhubarb of the same medicinal quality as that from Asia due to the differing climate and the plant’s likelihood to hybridize, and they sought reliable sources for trade.^4,15 Beginning in the 1730s, the British East India Company and the Rhubarb Commission in Mongolia supplied Europe with large quantities of rhubarb root. Due to food shortages during World War I, the British government encouraged the consumption of rhubarb leaves as a substitute for spinach. This lead to cases of oxalic acid poisoning and even death. In the 1820s, rhubarb was introduced to the United States, specifically Maine and Massachusetts.⁴

Due to its tart flavor, rhubarb often is sweetened with sugar or combined with other fruits. Rhubarb is also known as “pie plant” because it often is stewed for pie fillings. Rhubarb stalks are also cooked with strawberries, apples, or ginger as a jam, added to baked goods, or fermented to make wine.

Modern Research

Anticancer Properties

Anthraquinones found in rhubarb have been investigated for their potential antitumor effects. In both in vitro and in vivo studies, the anthraquinones emodin and aloe-emodin were found to inhibit growth of various types of cancer cells and tumors.¹⁶ They also increased sensitivity to other chemotherapeutic agents. There is potential for these anthraquinones to be developed as new chemotherapeutic drugs or for use in conjunction with other drugs.

Diabetes and Metabolic Diseases

Supplementation with rhubarb stalk powder, which has a high insoluble fiber content, has been correlated with lower blood cholesterol levels in men with high cholesterol.¹⁰ However, a study on diabetic mice found that rhubarb stalk fiber supplementation did not have significant cholesterol-lowering effects. More research is necessary to evaluate rhubarb as an adjuvant treatment for high cholesterol, particularly in diabetic patients.¹⁷

Rhein from rhubarb has exhibited anti-inflammatory and antiproliferative actions, and was found to have hypoglycemic and hypolipidemic activities in animal models. In addition to reducing inflammation, rhein also improved the insulin secretory function of pancreatic β-cells in animal models, which suggests a positive effect on diabetic nephropathy, a complication of uncontrolled diabetes.⁹

Estrogenic Activity

In 2015, a randomized, single-blind, controlled trial concluded that both rhubarb root and stalk can be used as a possible treatment for the symptoms of dysmenorrhea and menopause, and are safe alternatives to hormone replacement therapy with no known adverse side effects.³

Consumer Considerations

The leaves of the rhubarb plant should not be consumed, as they contain higher concentrations of oxalates, which can be eliminated only through the renal pathways, and may cause kidney damage. The consumption of rhubarb stalk also is contraindicated for those with a history of renal disease or kidney stones due to the oxalic content still present in the stalk.^9,16

Stalks should not be harvested for the first two years, after which a single plant may produce four to 12 pounds of stalks annually. Stalks can be kept for two to four weeks in the refrigerator, blanched and stored in the freezer, or canned for long-term storage.¹⁸

Potential drug interactions with rhubarb root supplementation include an increase in the effects of blood-thinning medications (e.g., warfarin), an increase in the occurrence of adverse effects from digoxin, depletion of potassium stores if used while taking corticosteroids, and enhancing the effects of stimulant laxatives.³

Nutrient Profile¹⁹

Macronutrient Profile: (Per 1 cup diced rhubarb [approx. 122 g])

26 calories
1.1 g protein
5.5 g carbohydrates
0.2 g fat

Secondary Metabolites: (Per 1 cup diced rhubarb [approx. 122 g])

Excellent source of:

Vitamin K: 35.7 mcg (29.8% DV)

Very good source of:

Vitamin C: 9.8 mg (10.9% DV)
Manganese: 0.24 mg (10.4% DV)

Good source of:

Calcium: 105 mg (8.1% DV)
Potassium: 351 mg (7.5% DV)
Dietary Fiber: 2.2 g (7.3% DV)

Also provides:

Riboflavin: 0.04 mg (3.1% DV)
Magnesium: 15 mg (3.6% DV)
Vitamin A: 124 IU (2.5% DV)
Folate: 9 mcg (2.3% DV)
Niacin: 0.37 mg (2.3% DV)
Vitamin B₆: 0.03 mg (1.8% DV)
Iron: 0.3 mg (1.7% DV)
Thiamin: 0.02 mg (1.7% DV)
Phosphorus: 17 mg (1.4% DV)

DV = Daily Value as established by the US Food and Drug Administration, based on a 2,000-calorie diet.

Image credits (top to bottom):

Rhubarb stalks. Image courtesy of Mako.
Garden rhubarb. ©2019 Steven Foster.
Rheum rhabarbarum illustration from Plantarum indigenarum et exoticarum icones ad vivum coloratae, oder, Sammlung nach der Natur gemalter Abbildungen inn- und ausländlischer Pflanzen, für Liebhaber und Beflissene der Botanik. 1788.
Chinese rhubarb. ©2019 Steven Foster.

References

1. van Wyk B-E. Food Plants of the World: An Illustrated Guide. Portland, OR: Timber Press; 2006.
2. Bratsch A, Mainville D. Specialty Crop Profile: Rhubarb. Virginia Cooperative Extension. 2009;438(110):1-7. Available at: https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/438/438-110/438-110_pdf.pdf. Accessed March 27, 2019.
3. Marcel C. Rhubarb: Evidence-Based Care Sheet. Cinahl Information Systems. October 2016:1-3.
4. Rhubarb. New World Encyclopedia. Available at: www.newworldencyclopedia.org/entry/Rhubarb. Accessed April 10, 2018.
5. Agarwal SK, Singh SS, Lakshmi V, Verma S, Kumar S. Chemistry and pharmacology of rhubarb (Rheum species) – A Review. Journal of Scientific and Industrial Research. 2001;60:1-9.
6. Krenn L, Wichtl M, Loew D. Rhei radix. In: Blaschek W, ed. Wichtl – Teedrogen und Phytopharmaka. 6th ed. Stuttgart, Germany: Wissenschaftliche Verlagsgesellschaft mbH. 2016:550-553.
7. Why Shouldn’t You Eat Rhubarb Leaves? - The Chemistry of Rhubarb. Compound Interest. April 17, 2015. Available at: www.compoundchem.com/2015/04/16/rhubarb/. Accessed April 11, 2018.
8. Zheng Q-xia, Wu H-feng, Guo J, et al. Review of rhubarbs: Chemistry and pharmacology. Chinese Herbal Medicines. 2013;5(1):9-32.
9. Zeng C-C, Liu X, Chen G-R, et al. The molecular mechanism of rhein in diabetic nephropathy. Evidence-Based Complementary and Alternative Medicine. 2014;487097.
10. Takeoka GR, Dao L, Harden L, Pantoja A, Kuhl JC. Antioxidant activity, phenolic and anthocyanin contents of various rhubarb (Rheum spp.) varieties. International Journal of Food Science and Technology. 2013;48(1):172-178.
11. Hoffmann-Bohm K, Ferstl W. Rheum. In: Hänsel R, Keller K, Rimpler H, Schneider G, eds. Hagers Handbuch der Pharmazeutischen Praxis, Band 6. Drogen P-Z. Berlin & Heidelberg, Germany: Springer-Verlag. 1994:411-439.
12. Barceloux DG. Rhubarb and oxalosis (Rheum species). Medical Toxicology of Natural Substances: Foods, Fungi, Medicinal Herbs, Toxic Plants, and Venomous Animals. 2009;55:403-411.
13. McDougall GJ, Dobson P, Jordan-Mahy N. Effects of different cooking regimes on rhubarb polyphenols. Food Chemistry. 2010;119:758-764.
14. Will F, Dietrich H. Processing and chemical composition of rhubarb (Rheum rhabarbarum) juice. LWT – Food Science and Technology. 2013;50(2):673-678.
15. Foust CM. Kremers Award Lecture: Mysteries of Rhubarb: Chinese Medicinal Rhubarb through the Ages. Pharmacy in History. 1994;36(4):155-159.
16. Huang Q, Lu G, Shen H-M, Chung MC, Ong CN. Anti-cancer properties of anthraquinones from rhubarb. Medicinal Research Reviews. 2007;27(5):609-630.
17. Cheema SK, Goel V, Basu TK, Agellon LB. Dietary rhubarb (Rheum rhaponticum) stalk fibre does not lower plasma cholesterol levels in diabetic rats. British Journal of Nutrition. 2003;89(2):201.
18. Beddes T. Rhubarb in the Garden. Utah State University Cooperative Extension: Horticulture. July 2011.
19. Basic Report: 09307, Rhubarb, raw. United States Department of Agriculture Agricultural Research Service website. National Nutrient Database for Standard Reference Legacy Release. April 2018. Available at: https://ndb.nal.usda.gov/ndb/foods/show/09307. Accessed March 27, 2019.
20. Spiced Braised Rhubarb. Saveur. March 20, 2013. Available at: www.saveur.com/article/recipes/Spiced-Braised-Rhubarb. Accessed March 27, 2019.
21. Bauman H, Hill K. Food as Medicine: Ginger (Zingiber officinale, Zingiberaceae). HerbalEGram. 2015;12(3). Available at: http://cms.herbalgram.org/heg/volume12/03March/March2015_FaM_Ginger.html. Accessed March 27, 2019.