July 2011 Issue

Bitter Melon — Fruit’s Role in Diabetes Management Is Promising But Uncertain
By Jasmin Ilkay, MPH, RD
Today’s Dietitian
Vol. 13 No. 7 P. 10

Also known as bitter gourd, karela, or balsam pear, bitter melon (Momordica charantia) is a perennial plant that grows in the tropical and subtropical regions of Asia, South America, East Africa, and the Caribbean.1 It is related to honeydew, cantaloupe, muskmelon, and casaba. While not the most attractive fruit (resembling a green cucumber with bumpy skin), bitter melon packs a healthful punch; it is an excellent source of vitamins A and C and potassium.2

Bitter melon has been used as a folk remedy for treating cancer, aiding digestion, and fighting viral infections. The fruit has also been used medicinally in Asia, Africa, India, and South America to lower blood glucose levels. Experimental and clinical research conducted worldwide has established its blood glucose-lowering action.

Today, bitter melon remains a popular and frequently studied dietary botanical used for managing diabetes. But 60 years of research on this bitter-tasting plant as an adjunctive for diabetes management has yielded uncertain results.

Mechanism of Action
Vicine, charantin, and polypeptide-P are the three known compounds present in bitter melon that are responsible for its antidiabetic properties. Together they increase glucose uptake and glycogen synthesis in the liver, muscle, and adipose tissue and also improve glucose tolerance.1 Possible mechanisms include increased insulin secretion, tissue glucose uptake, liver muscle glycogen synthesis, glucose oxidation, and decreased hepatic gluconeogenesis.3

Phytochemistry
Researchers from the Garvan Institute of Medical Research and the Shanghai Institute of Materia Medica recently isolated four glycosides from fresh bitter melon. The study authors explain that these compounds appear to activate the enzyme AMP-activated protein kinase, which has been shown to regulate GLUT4 translocation and is thought to be responsible for bitter melon’s hypoglycemic mechanism.4 In vivo studies in mice have shown a significant increase in glucose disposal and an increase in fatty acid oxidation after acute administration of the glycosides.4

Research
In Vivo Studies in Laboratory Animals
Animal studies have consistently demonstrated the hypoglycemic effects of bitter melon (as a whole plant) and its seeds, fruit pulp, and leaves.5 These studies have shown that the assorted forms of bitter melon have the potential to improve glucose tolerance, suppress postprandial hyperglycemia in rats, and enhance insulin sensitivity and lipolysis.5

Several human studies have shown similar hypoglycemic effects of bitter melon.

Human Studies
Researchers in India have been documenting bitter melon’s antidiabetic effects since the 1950s6, and their work has since sparked other researchers’ interest in investigating the fruit’s antidiabetic role.

A systematic review published in Diabetes Care in 2003 cited a handful of human studies that support bitter melon’s role in lowering blood glucose. Two placebo-controlled, short-term metabolic studies reported bitter melon fruit juice’s and extract’s acute effects on lowering blood glucose.3 Two additional uncontrolled, open-label trials cited in the review reported positive effects on glycemic control after subjects used bitter melon for seven to 11 weeks.3 Despite each of the four studies mentioned having less than 20 subjects, the authors of the review highlighted bitter melon as one of the most promising supplements for diabetes management.

Only a handful of large human clinical trials have studied bitter melon’s antidiabetic effects. The largest study evaluating the fruit’s blood glucose-lowering effect was conducted in India and published in 1999 in the Bangladesh Medical Research Council Bulletin. Researchers used an aqueous suspension of bitter melon pulp in 100 patients with type 2 diabetes. They evaluated bitter melon’s effect one hour after administration and then two hours later with a 75-g oral glucose tolerance test. After the two-hour glucose tolerance test, subjects’ average blood glucose was 222 mg/dL. This was 14% lower than the previous day’s value of 257 mg/dL. Regardless, researchers measured bitter melon’s effects after only two hours; whether there is any long-term benefit to using bitter melon is unknown.

In a randomized, double-blind, placebo-controlled study conducted in 2004, Dans et al aimed to determine whether the addition of bitter melon capsules to standard therapy could decrease hemoglobin A1c levels by 1% in three months. The 40 subjects were either newly diagnosed with type 2 diabetes or had poorly controlled type 2 diabetes with A1c levels between 7% and 9%. The researchers advised the subjects to self-administer two capsules of bitter melon extract three times per day. The results, published in the Journal of Clinical Epidemiology in 2007, revealed only a 0.217% decline in the experimental group. The mean difference in the placebo group vs. the experimental group was not statistically significant. Also, the authors relied on self-reported intake of the supplements and did not disclose the actual amount of bitter melon the subjects took. The authors concluded that bitter melon’s effectiveness is uncertain; however, the results could be used to estimate the sample size for larger trials.

Dosage and Safety
Bitter melon preparations vary widely. As a dietary supplement, bitter melon is often sold as an extract, a powder, juiced, or dried. All parts of the plant are used, either collectively or individually. Supplement companies’ average daily dosage recommendation ranges from 50 to 100 mL of juice and 200 to 900 mg of extract or powder. Concern exists about the varied preparation methods and doses because there is insufficient data to suggest a standardized treatment protocol.6 The lack of standardization may also increase the likelihood of unwanted side effects.

In general, the studies cited previously reported few side effects from the use of bitter melon. A small number of subjects reported gastrointestinal discomfort and diarrhea.6 Children should not consume the red arils, which cover the seed, as they have been reported to be toxic in this population, causing vomiting, diarrhea, and death.1 Bitter melon is also contraindicated in women who are pregnant because it may induce bleeding, contractions, and abortion.1 A few animal studies have reported hypoglycemia and hepatotoxicity.1 Bitter melon supplementation may produce an additive effect in humans when used with insulin or hypoglycemics.1

Bottom Line
Even though researchers have been studying it since the 1950s, bitter melon is far from being recommended as a standard complementary therapy for managing blood glucose levels. Larger clinical trials are required to establish a standardized treatment regimen and measure the long-term effects of bitter melon use.

Because of the aforementioned risks and need for more research, people must use bitter melon with caution. Patients with diabetes who would like to integrate bitter melon supplements into their medication regimen must consult with their physician and check their blood glucose levels regularly.

— Jasmin Ilkay, MPH, RD, is a lecturer for the human nutrition and food science department at  California State Polytechnic University in Pomona and a freelance writer.

 

References
1. Memorial Sloan-Kettering Cancer Center. Bitter melon. Available at: http://www.mskcc.org/mskcc/html/69138.cfm. Accessed January 2, 2011.

2. USDA National Nutrient Database. Balsam-pear (bitter gourd), pods, raw. Available at: http://www.nal.usda.gov/fnic/foodcomp/cgi-bin/list_nut_edit.pl

3. Yeh G, Eisenberg D, Kaptchuk T, Phillips R. Systematic review of herbs and dietary supplements for glycemic control in diabetes. Diabetes Care. 2003;26(4):1277-1294.

4. Tan MJ, Ye JM, Turner N, et al. Antidiabetic activities of triterpenoids isolated from bitter melon associated with activation of the AMPK pathway. Chem Biol. 2008;15(3):263-273.

5. Leung L, Birtwhistle R, Kotecha J, Hannah S, Cuthbertson S. Anti-diabetic and hypoglycaemic effects of Momordica charantia (bitter melon): A mini review. Br J Nutr. 2009;102(12):1703-1708.

6. Raman A, Lau C. Anti-diabetic properties and phytochemistry Momordica charantia L (Cucurbitaceae). Phytomed. 1996;2(4):349-362.

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