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, transcribed 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.13
Avocado 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.7 Forty-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:
2
large ripe avocados, approximately 8 ounces each, peeled, pitted, and diced
1
ripe mango, peeled, pitted, and diced (Read more about the benefits of mango here.21)
1 cup
seeded, diced tomato (Read more about the benefits of tomato here.22)
1/4
cup chopped fresh cilantro (Read more about the benefits of cilantro here.23)
2
tablespoons minced red onion
1
tablespoon minced jalapeño pepper, seeds removed for a milder salsa
1
tablespoon freshly-squeezed lime juice
1/2
teaspoon salt
1/4
teaspoon ground black pepper
Directions:
In a
medium bowl, add all ingredients except avocado and stir to combine.
Add
avocado and toss gently to mix. Serve right away with chips or as a condiment
for fish or chicken.
Photo credits: Top: ©2017 Steven Foster Middle: http://www.kjokkenutstyr.net Bottom: Image from the Florentine Codex, Fr. Bernardino de Sahagun. 16th century.
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