Cape Aloe (Aloe ferox Mill.): A Comprehensive Reference
1. Identity, Taxonomy, and Botanical Description
Botanical and Chemical Names
Aloe ferox Mill. (synonym: A. candelabrum A. Berger), placed in the family Xanthorrhoeaceae (previously Asphodelaceae, Aloaceae, or Liliaceae s.l.), is commonly known as the bitter aloe or Cape aloe β also called khala, umhlaba, and bitteraalwyn β and is a variable species indigenous to the Cape coastal region of South Africa, occurring in Swellendam in the west and extending to the southern parts of KwaZulu-Natal in the east. The epithet ferox, meaning "ferocious," was given because of the thorny, sharp, reddish spines of the leaves.
Common names include: bitter aloe, red aloe (English); bitteraalwyn, bergaalwyn (Afrikaans); iNhlaba (Zulu); iKhala (Xhosa). The common name "Cape aloe" refers to the region of southern Africa where this plant is native.
Plant Morphology
A. ferox is a tall, single-stemmed aloe that can reach 2β3 meters in height, with dull green leaves arranged in a rosette and a large candelabra-like flower-head carrying many flowers in colors ranging from yellowy-orange to bright red. The rigid, evergreen, lance-shaped leaves are blue-green in color with small, brown, spine-like prickles along their margins and both upper and lower surfaces, and can reach 2β3 feet long and 6 inches wide. The bitter aloe will reach 2β3 metres in height, with old leaves remaining after they have dried, forming a "petticoat" on the stem.
Distinction from Aloe vera
In A. ferox, the bitter aloin is found just under the skin and can easily be separated from the gel-like inner leaf; in A. vera, aloin is found throughout the leaf and is extracted through a chemical process. Aloe ferox has been found to contain a 28% higher level of aloin and 36% more amino acids than Aloe vera.
Commercial Status and Trade
Based on annual reports from South Africa, the total export of A. ferox was 4,549 tonnes between 1981 and 1994, with the highest amounts exported to Germany, Japan, Argentina, and Italy. Industrial processing of A. ferox gel started in the early 1990s when an aloe factory was established in Albertinia. The harvesting and processing has been historically centered in the Eastern and Western Cape where A. ferox occurs most abundantly. Most of the raw materials are still wild-harvested (wild-crafted), but small plantations have been established in recent years, mainly for ease of harvesting and to allow for irrigation during periods of drought.
Aloe ferox is included in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) as of 2020, and obtained a non-detriment finding from the Scientific Authority of South Africa in 2019, indicating that harvest and international trade in Aloe ferox poses a low to moderate risk to the population in the wild.
2. Plant Parts Used and Common Preparations
Two distinct plant fractions of Aloe ferox are commercially exploited, each with different chemical compositions and applications.
- The leaf exudate (latex / bitter sap): Aloe ferox is commonly used in the production of purgative medication such as "aloe bitters" or "aloe lump." Aloe bitters are made from the dark sap of the aloe β the sap between the leaf epidermis and the gel of the leaf β which is extracted and dried to make a dark crystalline substance. The sap is collected and dried by traditional methods to produce a dark brown solid substance known as aloe bitters or Cape Aloes.
- The inner leaf gel: The juicy gel has reputed wound-healing properties, while the latex (bitter yellow layer containing the aloin) is said to have laxative properties.
To harvest ("tap") A. ferox, leaves are cut by hand using a specialized cutting tool. Tapping can only be done in warm, dry conditions. During drought periods, tapping is not done as it harms the plant. Once cut, leaves are stacked carefully in a circle with the cut ends facing inwards, allowing the juice to drain into a plastic sheet laid between them. The sap takes between 30 minutes and 5 hours to drain completely.
In addition to tonic drinks, numerous hair and skin care products are manufactured from A. ferox, including ointments, soaps, shampoos, skin moisturizing creams, sun-tan lotions, and wound-healing preparations.
The official pharmacopoeial herbal drug (as defined by the ESCOP monograph Aloe capensis) consists of the concentrated and dried juice of the leaves of various species of aloe, mainly Aloe ferox Miller and its hybrids.
The bulk of the harvested aloe products are exported, primarily to Europe and North America, in the form of sap, lumps, or powder, although some processing takes place locally.
3. Traditional and Historical Use
Indigenous African Traditions
The first known people to inhabit the Western Cape centuries ago were the San, who were later called Bushmen by European settlers. Aloe depictions are found in San rock art β three such depictions have been discovered β communicating the important role aloe played in their daily practice.
Aloe bitters β the dried, crystallized dark sap β are used in the Eastern and Western Cape provinces of South Africa to alleviate symptoms of arthritis, but are mainly used to treat constipation. The dark sap and gel of Aloe ferox are used by the Xhosa in the treatment of dermal wounds, believed to stimulate wound closure as well as provide antimicrobial and anti-inflammatory activity.
The amaXhosa use the plant to heal wounds, including those from circumcision, and they also burn dried leaves to repel mosquitoes or grind it up for snuff. The amaMpondo people use aloe juice and water as a body wash.
Aloe ferox was also used by Xhosa mothers during the weaning process: the bitterness of the sap was applied to the nipples to discourage infants from breastfeeding.
Slices of the leaves were placed in the drinking water of livestock to provide immune-boosting properties.
The plant has been used since ancient times as a generic chemopreventive and anti-tumor remedy in folk medicine, and it has a well-documented history of use as a laxative.
Cape Dutch and British Traditions
The herbal traditions associated with Aloe ferox span British, Cape Dutch, Cape Herbal Medicine, and Khoi & San traditions; plant parts used include the leaf gel, leaf latex, leaves, and roots.
Pharmacopoeial and Formal Recognition
The ESCOP herbal monograph for Cape Aloes (Aloe capensis) selects and summarizes scientific studies and textbooks regarding efficacy, dosage, and safety to support the therapeutic uses of cape aloes. The therapeutic indication recognized in the ESCOP monograph is short-term use for occasional constipation. The European Medicines Agency (EMA) also issued a Community Herbal Monograph on aloe species, mainly Aloe ferox Miller and its hybrids, formalizing its status as a traditional herbal medicinal product.
4. Key Constituents and Active Compounds
Overview of Phytochemical Classes
Aloe vera and Aloe ferox contain vast phytochemical classes including anthraquinones, chromones, anthrones, phenolic compounds, flavonoids, tannins, steroids, and alkaloids, which contribute to their different pharmacological activities. These phytochemicals include polysaccharides, flavonoids, carbohydrates, coumarins, tannins, chromones, alkaloids, anthraquinones, organic compounds, pyrones, phytosterols, anthrones, sterols, vitamins, proteins, and mineral constituents.
Major Leaf Exudate Constituents
Commercial aloe bitters contains four major constituents: aloin A, aloin B, aloesin, and aloeresin A. The overall composition of major compounds in A. ferox leaf exudate is remarkably invariable despite wide natural variation; aloeresin A, aloesin, and aloin (both epimers A and B) contribute between 70% and 97% of the total dry weight, in a ratio of approximately 4:3:2, respectively.
Aloin A and B (or barbaloin) are isolated from aloe bitters by selective extraction and are principally used as a purgative. Aloesin is also isolated from aloe bitters and is used as a skin-lightening and sunscreen agent.
Anthraquinones
Some of the anthraquinones present in the leaf are aloin, aloe-emodin, aloesaponarin, chrysophanol, and isoxanthorin. Bioactive compounds including aloin, aloe-emodin, and chrysophanol have been isolated from A. ferox extracts.
The concentration of aloin and aloe-emodin determines the antioxidant property: emodin acts as an antioxidant at high concentrations and as a prooxidant at lower concentrations, while aloin acts as both a prooxidant and an antioxidant at various levels.
Chromones
Aloeresin, a chromone, is described as a potent antioxidant, with oxygen radical absorbance capacity reported to be 33 and 299 times higher than green tea and grape seed extracts, respectively.
Other Phytoconstituents
Additional constituents include aloctin A, aloctin B, aloeresin, aloesin, flavonoids, flavonols, glucomannans, glycoproteins, mannans, phenols, polysaccharides, proanthocyanidins, saponins, tannins, and alkaloids.
The variation in concentration of these chemical constituents is based on the plant part used, extraction process, solvent, stage of growth, and plant source.
5. Mechanisms of Action
Laxative Mechanism
The purgative effects of the dark sap are attributed to the action of reducing water reabsorption into the intestine and by directly stimulating the smooth muscle of the gut. The mechanism involves bacterial conversion of aloin to aloe-emodin in the large intestine, which stimulates peristalsis and inhibits water and electrolyte reabsorption, producing softer, more frequent stools.
Aloe dried juice contains aloe-emodin-9-anthrone, which acts specifically on the colon and has laxative properties. The active metabolite aloe-emodin-9-anthrone is generated through bacterial cleavage of the parent glycosides in the colon.
Anti-inflammatory Mechanism
Similar to A. vera, A. ferox is well endowed with anti-inflammatory compounds. Aloe species have been reported to have pharmacological activities including anti-inflammatory, immunomodulatory, antibacterial, antifungal, antiviral, antiproliferative, antidiabetic, laxative, wound healing, moisturizing, anti-aging, and skin protection.
Antidiabetic Mechanism (Preclinical)
Aloesin and aloesinol of A. ferox have been shown to decrease blood sugar levels in mice with an increase in adiponectin levels in vitro and a decrease in plasma insulin levels in vivo. Some studies on rats showed antidiabetic activity through potential inhibition of Ξ±-glucosidase, with little effect on Ξ±-amylase inhibition in in vitro assays.
Anticancer Mechanism (Preclinical)
The dichloromethane extract of cape aloe caused a growth inhibitory effect in Ehrlich ascites tumor cells, a decrease in DNA synthesis, and an accumulation of cells in the G1 phase. Aloe-emodin given to mice in which tumor cells had been injected inhibited growth of malignant tumors; other animal data also suggest that components of aloe inhibit tumor growth and improve survival, and various in vitro assays demonstrated anticarcinogenic activity of aloe-emodin.
Drug Permeability Enhancement
The ability of Cape aloes to enhance the transport of poorly permeable drugs has also been documented in research literature.
6. Scientific Evidence by Area of Use
6.1 Constipation and Laxation
Evidence Strength: Moderate (animal and mechanistic studies; limited controlled human trials specific to A. ferox; traditional use recognized by EMA and ESCOP).
One preclinical study evaluated the effects of oral administration of A. ferox resin at doses of 10, 25, 50, 100, and 200 mg/kg on intestinal transit in mice. The hydroxyanthracene derivatives present in the resin were identified as aloin (Rf 0.35), comprising 33.5% of hydroxyanthracene derivatives expressed as aloin. The resin extract at 50, 100, and 200 mg/kg increased gastrointestinal motility at a 30-minute interval to 93.5%, 91.8%, and 93.8%, respectively, compared to the control group at 46.5%.
A further study evaluated the efficacy of the aqueous leaf extract of A. ferox against loperamide-induced constipation in Wistar rats, with constipation induced by oral administration of loperamide (3 mg/kg body weight), and the constipated rats treated with 50, 100, and 200 mg/kg body weight/day of the extract for 7 days, during which feeding characteristics, body weight, fecal properties, and gastrointestinal transit ratio were monitored. The extract increased intestinal motility, which in turn enhanced colonic peristalsis in the rats, with the possible mechanism being the enhancement of fluid release, thereby increasing intestinal secretion.
The ESCOP monograph mentions a study in 6 healthy volunteers in which aloes were administered orally for 7 days, and aloe-emodin was detected as a metabolite in the plasma only sporadically and at maximum concentrations of less than 2 ng/ml.
The EMA concluded that short-term use of aloe species containing hydroxyanthracene derivatives in cases of occasional constipation can be regarded as safe, and identified a safe maximum daily dosage of 30 mg hydroxyanthraquinones.
The FDA banned aloe latex in over-the-counter laxative products in 2002, requiring manufacturers to remove it from OTC formulations due to insufficient safety and efficacy data meeting modern regulatory standards. This regulatory action applies to all aloe latex-derived OTC laxatives, including those from A. ferox.
6.2 Obesity, Lipid Profile, Blood Pressure, and Glycemic Control
Evidence Strength: Preliminary (one small randomized controlled clinical trial; additional animal data only).
One CONSORT-reported randomized controlled clinical study enrolled 20 Romanian persons with obesity treated with diet and Aloe ferox-based supplements, and 20 matched controls treated with diet and placebo. The treatment consisted of 2 capsules/day (Aloe ferox 460 mg) for 2 weeks, followed by a 2-week break, repeated 3 times. Blood pressure and anthropometric parameters including BMI, total cholesterol, and abdominal circumference, as well as biochemical parameters including fasting blood glucose (FBG), uric acid, and lipid profile, were evaluated at baseline and after 3 months.
After 3 months of Aloe ferox administration, significant differences between the study group and the control group were observed regarding BMI (P = .03), total cholesterol (P = .032), LDL cholesterol (P = .01), and fasting blood glucose (P = .018).
At the time of publication, the authors noted that there were no prior studies about the effectiveness of Aloe ferox on humans concerning obesity, lipid profile, or glycemic control, with prior work limited to animals. The small sample size (n=20 per group) and a single-center design represent significant limitations; independent replication is needed.
6.3 Wound Healing and Skin Applications
Evidence Strength: Preliminary (animal models; in vitro studies; traditional use widely documented; robust controlled human trials specifically for A. ferox are limited).
Compared with A. vera gel, the therapeutic effects of A. ferox for topical wound healing were less addressed in scientific literature. A study examined two types of whole-leaf juice prepared from A. ferox and A. arborescens, with incision wound healing investigated using both rat and rabbit models. Results indicated that the two types of whole-leaf juice preparations exhibited therapeutic properties including facilitation of the healing process, selective inhibition of microbial growth, and zero side-effects on the skin during the observation period.
The dark sap and gel of Aloe ferox are used by the Xhosa in the treatment of dermal wounds and are believed to stimulate wound closure as well as provide antimicrobial and anti-inflammatory activity.
6.4 Antimicrobial Activity
Evidence Strength: Preliminary (in vitro data only; no controlled clinical trials).
Aloe ferox has been reported to have antimicrobial activity against Candida albicans, Gardnerella vaginalis, Shigella sonnei, and N. gonorrhoeae. Dichloromethane:methanol (DCM:MeOH) extracts were found to have a minimum inhibitory concentration (MIC) of 8 mg/mL compared to 0.04 mg/mL for the ciprofloxacin control; methanol extracts of A. ferox inhibit N. gonorrhoeae at an MIC of 0.5 mg/mL. These are in vitro findings and do not establish clinical efficacy.
6.5 Antioxidant Activity
Evidence Strength: Preliminary (in vitro assays; no clinical trials).
The antioxidant properties of Aloe ferox have been determined using ORAC (Oxygen Radical Absorbance Capacity) and FRAP analysis. As noted above, aloeresin β a chromone constituent β has demonstrated notable radical-scavenging capacity in laboratory assays, though in vitro antioxidant measurements do not reliably translate to clinical benefit.
6.6 Anticancer Properties
Evidence Strength: Preclinical only (cell lines and animal models; no human clinical trials for A. ferox specifically in oncology).
In silico studies have shown that anthraquinones including chrysophanol, aloe-emodin, aloeresin, aloin A & B, and other related compounds are potential SARS-CoV-2 3CLpro protease inhibitors, though these remain purely computational findings.
Apoptosis-inducing anthraquinones in A. ferox have been proposed as the basis for anticancer activity seen in cell models. All such evidence remains preclinical; no controlled human clinical trials on A. ferox in cancer prevention or treatment have been reported.
7. Body Systems and Health Areas Associated with Cape Aloe
- Gastrointestinal system: Primary use as a stimulant laxative for occasional constipation; traditional use for digestive complaints; recognized by ESCOP and EMA.
- Metabolic/cardiovascular: Preliminary clinical evidence for effects on BMI, total cholesterol, LDL, and fasting blood glucose; very limited human data.
- Integumentary (skin): Traditional and preclinical evidence for wound healing, antimicrobial, anti-inflammatory, and skin-lightening effects (aloesin); used in cosmetic formulations.
- Musculoskeletal: Traditional use for arthritis symptom relief using the crystallized bitter sap, as documented in South African ethnobotany.
- Immune system: Preclinical immunomodulatory activity; used traditionally in livestock for immune support.
- Oncology (preclinical): In vitro and animal data on antiproliferative and pro-apoptotic activity of constituent anthraquinones.
8. Dosage Forms and Reported Dosages
The pharmacopoeial herbal drug consists of the concentrated and dried juice of the leaves of Aloe ferox Miller and its hybrids. Reported forms include dry crystalline extract ("aloe lump"), powder, capsules, liquid extracts, juices, topical gels, and cosmetic formulations.
- Laxative use (regulatory guidance): The WHO has recommended that products containing anthraquinone glycosides should not be used continuously for longer than 1β2 weeks, and identified a safe individual dose as "corresponding to 10β30 mg hydroxyanthraquinones per day." The EMA identified a safe maximum daily dosage of 30 mg hydroxyanthraquinones for short-term use in occasional constipation.
- Clinical trial dosage (obesity/metabolic): The human CONSORT trial used 2 capsules per day of Aloe ferox 460 mg for 2 weeks, followed by a 2-week break, repeated 3 times over 3 months.
- Preclinical laxative dosage (animal): In the loperamide-induced constipation rat model, doses of 50, 100, and 200 mg/kg body weight/day of aqueous leaf extract were used for 7 days.
- International Aloe Science Council standard (for oral consumption products): The maximum allowable aloin content in aloe-derived material for oral consumption is less than 10 ppm (parts per million); for non-medical use, the recommended limit is 50 ppm or lower.
9. Safety Considerations and Drug Interactions
General Adverse Effects
Ingestion of aloe preparations is associated with diarrhea, hypokalemia, pseudomelanosis coli, kidney failure, as well as phototoxicity and hypersensitive reactions. Aloe latex, which comes from just under the plant's skin, can cause serious side effects such as acute kidney failure when consumed in large quantities.
Genotoxicity and Carcinogenicity
Aloe vera whole leaf extract showed clear evidence of carcinogenic activity in rats and was classified by the International Agency for Research on Cancer (IARC) as a possible human carcinogen (Group 2B). This classification relates to the non-decolorized whole-leaf form containing high anthraquinone levels.
However, a 2021 study using the in vivo alkaline comet assay (OECD 489) investigated the potential in vivo genotoxicity of dried Aloe ferox juice at dose levels of 500, 1000, and 2000 mg/kg/day in mice, and found that Aloe ferox showed no genotoxic activity in preparations of single cells from the colon of the treated mice.
The European Food Safety Authority (EFSA) Food Additives and Nutrient Sources (ANS) Panel concluded that "hydroxyanthracene derivatives should be regarded as genotoxic and carcinogenic unless there are specific data to the contrary, and that there is a safety concern for extracts containing hydroxyanthracene derivatives, although uncertainty persists."
Considering the possible presence of aloe-emodin and emodin in extracts, the EFSA Panel concluded that hydroxyanthracene derivatives should be considered genotoxic and carcinogenic unless specific contrary data exist, and the Panel was unable to provide advice on a daily intake that does not give rise to concerns about harmful effects to health.
A conclusion from one 2021 review stated that results of recent studies clearly demonstrate that neither aloe-emodin nor whole aloe extract is genotoxic β though this conclusion remains subject to ongoing regulatory debate.
Duration of Use
The WHO has recommended that products containing anthraquinone glycosides should not be used continuously for longer than 1β2 weeks. Prolonged use as a laxative may lead to dependency, electrolyte disturbances, and structural changes in the colon (pseudomelanosis coli).
Drug Interactions
Overuse of aloe latex may increase the risk of adverse effects from cardiac glycosides, such as digoxin, which are used for some heart problems. This interaction risk is mediated by the hypokalemia that anthraquinone-induced catharsis can produce, which sensitizes the myocardium to digitalis toxicity.
Cosmetic Use Safety
Aloe Ferox Leaf Juice and Aloe Ferox Leaf Juice Extract function primarily as skin-conditioning agents and are included in cosmetics only at low concentrations. The Cosmetic Ingredient Review (CIR) Expert Panel evaluated these ingredients and their safety profile in topical use differs materially from internal use of the latex fraction.
Cytotoxicity of Aloin
The toxicity of aloin was evaluated in human Jurkat T lymphocytes using flow cytometry and microscopy; aloin treatment resulted in decreased cell size, increased granularity, a block at the G2/M phase of the cell cycle, and loss of both membrane integrity and mitochondrial membrane potential in a dose-dependent manner, suggesting a mitochondrial-dependent pathway for aloin-induced apoptosis. These in vitro findings underscore the need for caution regarding concentrated aloin exposure.
References
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