HerbalEGram: Volume 14, Issue 6, June 2017 |
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, ABC Special Projects Director Gayle Engels, and ABC Chief Science Officer Stefan Gafner, PhD, for their contributions to this project.
By Hannah Baumana and Taylor Moyerb
a HerbalGram Associate Editor
b ABC Dietetics Intern (Texas State, 2016)
Overview
Avocado (Persea americana) is a member of the Lauraceae family, an economically important plant family that includes cinnamon (Cinnamomum spp.), bay (Laurus nobilis), and sassafras (Sassafras albidum). The avocado tree is a perennial that grows up to 80 feet (24.4 meters) tall and bears fruit after five to ten years of growth.1 Once the tree reaches the fruiting stage, it can produce more than 100 avocados in a season. The tree produces smooth, dark green leaves and monoecious (contains both male and female organs) yellow-green flowers.2 Born from the ovary of the flower, the avocado is considered a large drupe
 and has one of the highest oil content of all known fruits, second only to olives (Olea europaea, Oleaceae).2,3
Avocado trees are endemic to tropical climates on the American continents, and they have been cultivated in other tropical and subtropical regions worldwide since the 19th century.3,4 Avocado trees were first planted in Florida in 1833 and then in California in 1856.5 Of the eight ecotypes or horticultural races of the avocado tree, the three that are most commonly used for food production include the Mexican (P. americana var. drymifolia), West Indian (P. americana var. americana), and Guatemalan (P. nubigena var. nubigena and P. nubigena var. guatemalensis) avocados. Most commercial varieties currently available are hybrids of these varietals.
The maturity season varies depending on the cultivar and can occur throughout the year. Unlike many fruits, avocados do not become fully ripe until one week after harvest.2 To delay ripening, which is necessary for transport around the world, avocados are refrigerated or coated in a thin wax to increase the fruit’s shelf life.6
Phytochemicals and Constituents
Inside the rough exterior of the avocado is a creamy blend of nutrients and secondary metabolites. The avocado is considered a medium-dense energy fruit, but, unlike most fruits, most of its caloric content comes from its abundance of monounsaturated fats.6 Monounsaturated fatty acids (MUFAs) are compounds that have been shown to promote heart health and decrease the risk of cardiovascular disease. One Hass avocado (a cultivar originally grown in southern California) has levels of MUFAs equivalent to two tablespoons of olive oil.7 Only about 14% of the avocado’s total fat content comes from saturated fat, a less healthy type of fat. As the fruit ripens, the saturated fat content decreases and the MUFA content increases.8 Its high fat and caloric content allows the avocado to mediate fat-soluble nutrient absorption and promote satiety.
Avocado also contains low amounts of carbohydrates, which makes it a good food choice for people with diabetes.8 The fruit also offers a significant amount of both insoluble and soluble fiber, which promote digestive health. Avocado is also notable for its potassium content, as it provides 60% more than an equal serving of banana (Musa spp., Musaceae).9 Potassium maintains electrolyte balance, which is important for the electrical balance of the heart (i.e., a steady, healthy heart rate), and regulates blood pressure through the modulation of liquid retention in the body.8 Potassium intake helps maintain heart health and muscle health. Avocados also provide significant amounts of B vitamins, including folate, and antioxidant vitamins C and E.3
The pulp of the avocado contains several bioactive compounds, predominantly free radical scavenging carotenoids, including lutein, zeaxanthin, and alpha- and beta-carotene.3 The xanthophylls lutein and
 zeaxanthin are highly concentrated in avocados and give the flesh its color. These bioactive compounds are fat soluble and help promote vascular health.8 By decreasing the amount of oxidized low-density lipoproteins (LDLs) in the body, xanthophylls slow the progression of damage to blood vessels. Lutein and zeaxanthin are associated with the decreased progression of macular degeneration, cataracts, and cartilage deterioration that occur with age. Carotenoids have also been found to protect the skin from oxidation and inflammation related to ultraviolet radiation.
Phenolic compounds, which have been shown to reduce oxidation, inflammation, and platelet aggregation, are also prevalent in avocados.8 Phytosterols are other significant bioactive compounds in the fruit, and reportedly can decrease the risk of coronary heart disease.6 The American Heart Association recommends that people consume two grams of sterols and stanols per day to promote heart health; half a Hass avocado (a 68-gram serving) contains 57 milligrams of phytosterols, while most other fruits contain about three milligrams per average serving.8 Mimicking the molecular structure of cholesterol, phytosterols inhibit cholesterol absorption and decrease cholesterol synthesis. This results in lowered total cholesterol levels in the body.6 Beta-sitosterol, the most prominent phytosterol in avocados (57 mg/75 g), suppresses the production of carcinogenic compounds, strengthens the immune system, and has been studied for its ability to lower cholesterol and alleviate symptoms associated with benign prostatic hyperplasia.6,10
Glutathione, a tripeptide involved in detoxification pathways and the reduction of oxidative stress and cancer risk, is also present in avocado.6 Avocados contain a higher amount of glutathione per average serving compared to comparable average servings of other fruits. Researchers are currently investigating glutathione and phytosterols to determine their potential in breast and throat cancer prevention.8
Avocado seeds have stronger antioxidant activity than the pulp. This has been attributed to the higher levels of phenolic compounds found in the seed. Major phenolic compounds in the seed include catechin, epicatechin, and leucoanthocyanidins.3 Additional phytochemicals abundant in avocado seeds include triterpenes, furanoic acids, proanthocyanidins, and a variety of polyphenols.
Historical and Commercial Uses
Archaeological records that date to 8000 BCE reveal that the avocado tree is one of the oldest known food plants in Mexico.3 Evidence suggests that people native to the area used avocados as both a food and a medicine.11 Avocado is mentioned in a 16th-century codex, Historia General de las Cosas de Nueva España, tr
 anscribed by Franciscan friar Bernardino de Sahagun from observing and interviewing the indigenous Aztec people.12 Known in Nahuatl, the native language of the Aztecs, as auacatl or ahuacaquahuitl, avocado leaves, seed, seed oil, and fruit pulp were used to treat a variety of ailments. The leaves were used in tea as a cough and cold medicine.13Avocado leaf tea was also used to relieve diarrhea, enhance menstrual flow, and treat hypertension. The leaves were also applied topically to heal bruising.
Avocado seed oil was used as an astringent to treat sores and remove scars.12 The avocado seed was also powdered and applied to infected teeth and used topically to alleviate dandruff and treat arthritis. The seed oil was applied to skin eruptions.3 The Aztecs used avocado to enhance fertility, while the Maya considered the fruit as an aphrodisiac.11 The fruit was also consumed to prevent certain so-called “cultural diseases,” such as being given the “evil eye.” The avocado is a dietary staple in Guatemala, Mexico, and neighboring countries.3
In Nigeria, avocado fruit pulp is used as a supplement to the diet to manage hypertension, and the avocado seed is ground and consumed to treat dysentery and whitlows, a painful lesion on the tip of the finger caused by the herpes simplex virus.3 Eating the fruit was said to remove infection, body aches, and inflammation.11
Modern Research
Modern research on avocado covers a variety of ailments and explores the uses of different parts of the plant. Some research has been conducted on the health benefits of the fruit, but much of it focuses on the seed and its oil. In vitro and animal studies have demonstrated that avocado seed has cytotoxic and anti-tumor, antimicrobial, dermatological, and antidiabetic properties.3 Although there are promising results from many in vitro and animal studies, there is a lack of sufficient human clinical trials on the efficacy of avocado seed extracts.
Cholesterol and Cardiovascular Health
Though avocado fruit is high in fat and calories, it can be consumed in moderate amounts as part of a healthy diet due to the type of fat it contains. A 2005 study aimed to determine how avocado consumption can affect blood vessel function, serum lipid concentrations, and weight loss in overweight and obese individuals. Fifty-five patients were placed on one of two energy-restricted diets: one diet included 200 grams of avocado fruit per day, while the other did not include any avocado.14 After six weeks, researchers found that body mass and body fat percentage dropped in both groups at similar rates, suggesting that the addition of avocado to the diet does not promote weight gain. Fat levels in the blood were also similar in both groups after the diet took place. This signifies that the fat in avocados does not increase cholesterol or triglyceride levels in the blood.
A similar study published in 2015 investigated the effects of avocado’s MUFA content on cholesterol levels.7Forty-five overweight or obese patients were placed on one of three diets in an attempt to lower their cholesterol levels for a five-week period: a low-fat diet with no added avocado, a moderate-fat diet with no added avocado, and a moderate-fat diet that included the consumption of one whole fruit per day. Blood tests were performed on patients before the diet began and at the end of the study period. While all three groups had a dramatic decrease in their total cholesterol levels, the avocado group had the most significant improvements from baseline. The low-fat, no-avocado diet produced the least desirable results; patients in this group experienced an average increase in triglyceride levels of 17.6%. Researchers also found that avocado intake not only decreases LDL cholesterol levels, but also increases high-density lipoprotein (HDL) levels, which is the desired outcome in controlling cholesterol levels in the blood.8 These results suggest that a moderate-fat diet including avocados improves serum fat levels, which decreases the risk of vascular damage and heart disease.14
Researchers also are investigating the use of avocado seed to manage hypertension. Avocado seed flour was shown to significantly reduce total cholesterol and LDL cholesterol in mice after six days.3 Animal models have also shown a dose-dependent reduction in LDL cholesterol, triglycerides, and total cholesterol when hypertensive rats were given aqueous avocado seed extracts for four weeks. Another animal model demonstrated a dose-dependent reduction in blood pressure, cholesterol, glucose, and sodium levels after five weeks.
Musculoskeletal and Integumentary Systems
Recent research has also tried to identify whether avocado/soybean (Glycine max, Fabaceae) unsaponifiables (ASUs) can help manage osteoarthritis (OA). Unsaponifiables are a byproduct of oils used in soap production. A 2015 study found that a daily dose of 300 mg of ASUs for three months demonstrated improvement in pain, stiffness, and physical function, and also improved the structure of the collagen and cartilage in the joints.15 In patients taking the ASU supplements, cartilage breakdown and inflammatory cytokines were inhibited. ASUs also inhibited cholesterol absorption and synthesis, which is significant because LDL levels tend to be high in OA patients. Several studies of varying quality have been conducted on the effects of ASUs on OA, and many have produced positive results. While ASUs are used as a drug in some countries, they are regulated as a dietary ingredient in the United States.
Topical application of avocado oil appears to promote collagen synthesis and aid in wound healing and other skin disorders. In an animal model, the topical application of avocado oil shortened wound-healing time when compared to petroleum jelly.16 When combined with vitamin B12, avocado seed oil was shown to be as effective as calcipotriol (a synthetic derivative of a form of vitamin D) cream in treating plaque psoriasis.17 While the effectiveness of calcipotriol cream decreased after four weeks, the efficacy of the avocado oil/B12 preparation lasted the duration of the 12-week trial. These results indicate that the avocado oil/B12 cream may be suitable as a long-term topical therapy for patients with plaque psoriasis.
Avocado seed may promote the growth of keratinocytes (the cells that comprise most of the outermost layer of skin), which makes it an ingredient of interest in the treatment of skin disorders, including psoriasis and sunburn. In vitro studies of keratinocytes exposed to UVB radiation have shown that pre-treatment and post-treatment with an avocado seed extract improved cell viability, reduced the number of sunburned cells, enhanced DNA repair, and reduced the secretion of pro-inflammatory mediators.3 Avocado seed’s ability to promote the growth of keratinocytes is being further researched as a possible alternative topical therapy for psoriasis. Conventional psoriasis treatments have significant adverse side effects including hepatotoxicity (from use of methotrexate), nephrotoxicity (from use of cyclosporine), teratogenicity (from use of oral retinoids), and even cancer (from use of cyclosporine and psoralen and ultraviolet A [PUVA] radiation).18
The unsaponifiable fatty acids (UFAs) from avocado seed are used in skin care products and cosmetics to improve skin quality and elasticity, moisture retention, stretch marks, keratosis (benign skin growths that can occur with age), hypo- and hyperpigmentation, and redness of the skin.3
Cytotoxic Activity
Avocado seed extract was found to induce apoptosis (normal, preprogrammed cell death) in human breast cancer cells, and avocado peel extract, which has a higher flavonoid and phenolic compound content, produced even more potent results.3 Persin, a compound isolated from avocado leaves, also has demonstrated the ability to induce apoptosis in human breast cancer cells.4
Antimicrobial Activity
An ethanolic extract of avocado seed was found to inhibit the growth of Salmonella enteritidis, Citrobacter freundii, Pseudomonas aeruginosa, Enterobacter aerogenes, Staphylococcus aureus, and Bacillus subtilis in vitro. Antifungal activity was also evident against Candida species, Cryptococcus neoformans, and Malassezia pachydermatis.3 In vivo research is needed to confirm these results.
Antidiabetic Activity
Ethanolic extracts of avocado seed have been shown to reduce blood glucose by 47-55% in animal models, while aqueous seed extracts have been shown to reduce glucose concentrations by 73-78% in diabetic rats and lower glucose by 35-39% in non-diabetic rats.3 Avocado seed extract appears to protect pancreatic islet cells and contain substances that mimic insulin, stimulate the production of insulin by β-cells, and enhance glucose utilization.
Nutrient Profile19
Macronutrient Profile: (Per 1 cup cubed fruit [approx. 150 grams])
240 calories
3 g protein 12.8 g carbohydrate 22 g fat
Secondary Metabolites: (Per 1 cup cubed fruit [approx. 150 grams])
Excellent source of:
Dietary Fiber: 10.1 g (40.4% DV)
Vitamin K: 31.5 mcg (39.4% DV) Folate: 122 mcg (30.5% DV) Vitamin C: 15 mg (25% DV) Potassium: 728 mg (20.8% DV) Vitamin B6: 0.4 mg (20% DV)
Very good source of:
Vitamin E: 3.1 mg (15.5% DV)
Niacin: 2.6 mg (13% DV) Riboflavin: 0.2 mg (11.8% DV) Magnesium: 44 mg (11% DV) Manganese: 0.2 mg (10% DV)
Good source of:
Phosphorus: 78 mg (7.8% DV)
Thiamin: 0.1 mg (6.7% DV)
Also provides:
Iron: 0.8 mg (4.4% DV)
Vitamin A: 219 IU (4.38% DV) Calcium: 18 mg (1.8% DV)
DV = Daily Value as established by the US Food and Drug Administration, based on a 2,000-calorie diet.
Recipe: Avocado-Mango Salsa
Courtesy of Hass Avocado Board20
Ingredients:
Directions:
Photo credits:
Top: ©2017 Steven Foster
Middle: http://www.kjokkenutstyr.net
Bottom: Image from the Florentine Codex, Fr. Bernardino de Sahagun. 16th century.
References
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Monday, 17 July 2017
Food as Medicine: Avocado (Persea americana, Lauraceae)
