Prickly Pear for Diabetes

By Astrid Pujari, MD

The common name, prickly pear, applies to several different species of cacti that belong to the genus Opuntia sp. These plants are characterized by their flat, fleshy pads, known as nopales in Spanish, which look like leaves but actually are modified stems. Like all cacti, most species have long spines on the surface. However, the Opuntia sp. are unique in that they also have short, fine spines called glochids, which are difficult to see but can detach and lodge in the skin, causing irritation.

Prickly pear has beautiful flowers that develop in the fall into edible fruit, known as "tuna." Both the fruit and the immature pads are traditional Mexican foods, although they now are more widely consumed—particularly in the southwest United States. Although most species of prickly pear are edible, Opuntia streptacantha, a species native to Mexico, is considered the best for consumption because of its taste. This also is the species that has been studied most extensively as a possible treatment for diabetes.

Pharmacology and Mechanism of Action

Prickly pear is high in fiber, in particular, a form of soluble fiber called pectin. Like all soluble fibers, the pectin found in prickly pear has been associated with lipid- and glucose-lowering effects because it slows carbohydrate and fat absorption from the gut.1 However, research also suggests that prickly pear may increase insulin sensitivity and hepatic cholesterol metabolism.1,2 The constituents responsible for the latter effect are unknown, although extracts of the whole broiled stem are thought to be more effective than those made from the raw stem, or from isolated components of the stem.3 Prickly pear also is a good source of vitamin C, calcium, potassium, and iron.4

Animal Studies

In animals, consumption of prickly pear prior to glucose tolerance testing has been shown to decrease both the area under the curve and the hyperglycemic peak resulting from glucose ingestion.5 Furthermore, one study of experimentally induced diabetes found that animals ingesting Opuntia sp. achieved a normal hemoglobin A1c at 15 weeks, and that treatment with both Opuntia sp. and insulin was superior to insulin alone.6

Clinical Studies

Current studies assessing the effect of prickly pear on diabetes are limited by short duration and small sample size. At present, the study of longest duration in which the effect of prickly pear on blood sugar was assessed in diabetic patients occurred over a one-week period;7 the remainder of the research has assessed the effect of Opuntia sp. on blood sugar only in the acute setting, within a three- to six-hour time period. Thus, although preliminary data suggest that opuntia may lower blood sugar in Type 2 diabetics, further research is necessary to evaluate prickly pear’s potential role in the long-term management of diabetes.

Several studies have been published demonstrating that broiled whole opuntia stems acutely lower serum glucose in patients with diabetes. One trial of 32 Type 2 diabetics found that in comparison to water or a broiled-squash control, subjects who ingested broiled whole opuntia stems had a statistically significant mean reduction in serum glucose (-16.2%) and insulin (-40.3%) at 180 minutes.8

Another study of eight people with Type 2 diabetes found that in comparison to both a water control and crude raw extracts of opuntia, only broiled whole stems caused a statistically significant decrease in serum glucose over three hours, with a maximum decrease of 48.3 mg/dL from basal values at 180 minutes.3 A third trial of eight Type 2 diabetics found that in comparison to a water control, 500 g of broiled opuntia stems resulted in a statistically significant 46.7 mg/dL decrease in serum glucose from basal value at 180 minutes.9

Interestingly, however, when opuntia was given to 14 healthy individuals and 14 Type 2 diabetics in another study, healthy individuals did not demonstrate a statistically significant change in serum glucose and insulin levels over a three-hour period.10 In contrast, a statistically significant reduction in serum glucose (-40.8 mg/dL) and serum insulin (-7.8 mU/mL) was noted in the diabetics in this study. These results suggest that prickly pear may work via a different mechanism than current hypoglycemic agents. All of these trials are limited by small sample size and short duration, and most of the trials on prickly pear were published by the same author. Ideally, these results should be confirmed by other investigators.

The longest study of prickly pear was a crossover, single-blind study of 14 Type 2 diabetics who withdrew their oral hypoglycemic agent and received either 30 opuntia capsules or 30 placebo capsules three times per day for one week.7 Serum glucose, triglycerides, and cholesterol increased from baseline while patients were on the placebo, but remained stable in those taking opuntia. However, the overall quality of this study was poor, with concerns including the lack of double-blinding, small sample size, and short duration. Furthermore, this formulation required patients to take a large number of capsules, which would make this method of administration impractical over a long period of time.

Prickly pear is thought to decrease cholesterol via its high soluble fiber content. One study, conducted in 24 otherwise healthy, hyperlipidemic males on a step I diet, found that replacing 625 kJ with 250 g of prickly pear decreased total cholesterol by 12%, LDL by 15%, and triglycerides by 11% over eight weeks.1 Interestingly, the authors also noted that subjects demonstrated improved insulin sensitivity, which could not be accounted for by the increase in dietary soluble fiber. Again, this study is limited by its small sample size, short duration, and lack of blinding.

Prickly pear also may be beneficial simply as a food for diabetic patients. The glycemic index for prickly pear in comparison to white bread was 10 in one study,11 suggesting that substituting prickly pear for carbohydrates with a higher glycemic index in the diabetic menu may help to control postprandial spikes in blood sugar.

Adverse Effects

Hypoglycemia may be possible in diabetics using nopal, particularly in combination with other agents. Monitoring serum glucose is advisable. Appropriate precautions should be taken when handling and preparing the cactus, since dermal exposure to the glochids has been associated with skin reactions.

Dose and Preparation

There have been several studies evaluating the effect of different doses, preparations, and duration of effect for prickly pear on serum glucose levels in Type 2 diabetics. Broiled whole prickly pear stems were superior to raw preparations or dehydrated extract in two separate studies.3,12 Furthermore, there appears to be a significant dose response: At 100 g, the maximal decrease in serum glucose was 2.3 mg/dL from baseline in comparison to the 46.7 mg/dL drop noted with 500 g of broiled whole prickly pear stems.9 The peak hypoglycemic effect with a 500 g dose is thought to occur at three hours, while the duration of action is approximately six hours.13 In general, the immature stems are the best for consumption, as mature stems are often tough and fibrous.

Traditionally, both the fruit and stems have been used for diabetes, though research has primarily focused on the hypoglycemic action of the stems of Opuntia streptacantha. They are available in specialty stores, particularly those carrying traditional Mexican fare. Most studies use a dose of 500 g of broiled stem three times per day, taken with meals. Bulk powders or capsules are not thought to be as effective.

The ripe fruit of the plant can be eaten fresh or cooked. If eaten fresh, the glochids must be removed, either by peeling the fruit carefully or by passing it through an open flame. The fruit also can be made into a juice, by simmering it for about half an hour and pressing it through a strainer. Juice can be bought ready made as well, although because the taste is otherwise tart, the juice often contains added sugar, limiting its appropriateness for diabetics. Traditionally, one glass of juice is recommended three times per day with meals.

Conclusion

The whole broiled stems of prickly pear have been shown to decrease blood sugar over a three-hour time period in Type 2 diabetics. It remains unclear whether prickly pear is beneficial for the long-term management of blood sugar in Type 2 diabetics. However, prickly pear may be beneficial when substituted for other carbohydrates as part of a diabetic meal plan because of its low glycemic index.

Recommendation

Prickly pear should not be recommended as a substitute for standard hypoglycemic agents, but ultimately it may prove to be a useful adjunct to conventional care for people with Type 2 diabetes. It may be useful when substituted for other carbohydrates in a diabetic meal plan because of its low glycemic index. Traditionally used as a food for hundreds of years, it appears to be safe. Careful monitoring of serum glucose is advisable if this food is introduced to diabetic patients on hypoglycemic agents.

Dr. Pujari is Clinical Educator for the Internal Medicine Residency Program at Virginia Mason Medical Center in Seattle, WA.

References

1. Wolfram RM, et al. Effect of prickly pear (Opuntia robusta) on glucose and lipid metabolism in non-diabetics with hyperlipidemia. Wien Klin Wochenschr 2002;114:840-846.

2. Fernandez ML, et al. Prickly pear (Opuntia sp.) pectin alters hepatic cholesterol metabolism without affecting cholesterol absorption in guinea pigs fed a hypercholesterolemic diet. J Nutr 1994;124:817-824.

3. Frati Munari AC, et al. Hypoglycemic action of Opuntia streptacantha Lemaire: Study using raw extracts. Arch Invest Med (Mex) 1989;20:321-325.

4. Shapiro K. Natural products used for diabetes. J Am Pharm Assoc 2002;42:217-226.

5. Roman-Ramos R, et al. Anti-hyperglycemic effect of some edible plants. J Ethnopharmacol 1995;48:25-32.

6. Trejo-Gonzalez A, et al. A purified extract from prickly pear cactus (Opuntia fuliginosa) controls experimentally induced diabetes in rats. J Ethnopharmacol 1996; 55:27-33.

7. Frati Munari AC, et al. Evaluation of nopal capsules in diabetes mellitus. Gac Med Mex 1992;128:431-436.

8. Frati Munari AC, P et al. Hypoglycemic effect of Opuntia streptacantha Lemaire in NIDDM. Diabetes Care 1988;11:63-66.

9. Frati-Munari AC, et al. Hypoglycemic action of different doses of nopal (Opuntia streptacantha Lemaire) in patients with type II diabetes mellitus. Arch Invest Med (Mex) 1989;20:197-201.

10. Frati AC, et al. Influence of nopal intake upon fasting glycemia in type II diabetics and healthy subjects. Arch Invest Med (Mex) 1991;22:51-56.

11. Frati-Munari AC, et al. The glycemic index of some foods common in Mexico. Gac Med Mex 1991;127:163-170; discussion 170-171.

12. Frati-Munari AC, et al. Effect of a dehydrated extract of nopal (Opuntia ficus indica Mill.) on blood glucose. Arch Invest Med (Mex) 1989;20:211-216.

13. Frati-Munari AC, et al. Duration of the hypoglycemic action of Opuntia streptacantha Lem. Arch Invest Med (Mex) 1989;20:297-300.