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 (TSU)
in San Marcos and the University of Texas at Austin (UT) through the American
Botanical Council’s (ABC’s) Dietetic Internship Program, led by ABC Education
Coordinator Jenny Perez.
We would like to acknowledge Jenny 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 Mikala Sessab
a HerbalGram Associate Editor
b ABC Dietetics
Intern (TSU, 2013)
Overview
The sweet potato (Ipomoea batatas, Convolvulaceae) is a
trailing, herbaceous perennial.1,2 It is indigenous to Central and South
America and grows best in subtropical climates, spreading along the ground and
producing oblong, tuberous roots. The flesh of the tuber comes in multiple
colors: orange is the most common, but white, cream, yellow, pink, or deep
purple varieties also exist. The sweet potato plant has heart-shaped leaves and
produces beautiful white, pink, or purple flowers as the plant matures.
Taxonomic confusion
can arise over the common name of “yam” that often is given to sweet potatoes
in the market. Botanically speaking, true yams belong to the genus Dioscorea (family Dioscoreaceae) and are
much less common in the United States.3 In general, foods marked as “yam”
in the United States should be assumed to be sweet potatoes. For reference, the
common potato is Solanum tuberosum
(Solanaceae), and belongs to a third distinct plant family than both sweet
potatoes and yams.
Currently, sweet
potatoes are produced in more than 100 countries. Domestication of the plant
has led to its wide cultivation throughout Mexico, Central America, and the
Caribbean. China now produces the most sweet potatoes, followed by Africa,
Central America, South America, and the United States.4 The sweet
potato tubers as well as the young leaves from the vines are consumed as food.1,2
While the tubers are in season in November and December, sweet potatoes
generally can be found year-round in supermarkets.
Phytochemicals
and Constituents
The sweet potato tuber
is a nutrient-dense food that provides a high amount of beta-carotene, which is
converted to vitamin A in the body.5,6 Vitamin A plays a vital role
in developing and maintaining healthy vision. In addition, sweet potatoes
contain the minerals potassium, calcium, iron, magnesium, zinc, and
phosphorous. They are rich in dietary fiber and complex carbohydrates.
Orange-fleshed sweet potatoes contain the highest levels of beta-carotene,
while purple-fleshed sweet potatoes provide valuable anthocyanins, which are
the antioxidant compounds that give berries and other foods their red, blue,
and purple hues.
Sweet potato leaves
and tubers are a good source of vitamin B6, which supports the
prevention of cardiovascular disease by protecting the blood vessels and
arteries. The human body cannot synthesize B vitamins, and they must be obtained
from external sources via the diet.
Because of its
nutrient density, sweet potatoes are incorporated into child malnutrition
programs in developing countries, and agricultural scientists are working to
improve the nutritional content of sweet potatoes even further.7
Chronic malnutrition afflicts 25% of all children under five years old
worldwide.
The antioxidant and
phenolic compounds present in sweet potatoes provide additional health
benefits. Chlorogenic, isochlorogenic, and caffeic acids have antioxidant
properties and mild blood pressure-lowering effects.8 Caffeic acid in
particular shows immunomodulatory, antimicrobial, and anti-inflammatory activities.9
When coupled with the intake of protein, the antioxidants from sweet potato are
more bioavailable.
The sweet potato leaves contain
anthocyanins, phytosterols, essential amino acids, flavonoids, caffeic acid,
and alpha-linolenic acid.5,6,10
Historical
and Commercial Uses
Sweet potatoes are one
of the oldest domesticated crops, with evidence of their consumption dating
back 10,000 years, according to findings in prehistoric Peruvian caves.3
European colonizers brought sweet potatoes to Europe from Central and South
America in the 15th century, and cultivation spread to Africa, India, southern
Asia, and Indonesia by the 16th century. Due to African slavery and European
colonization in North America, sweet potatoes became a cultural food staple in
the southern United States. Sweet potato tubers and vines are used around the
world as livestock fodder, especially in Asia.
In certain places where
sweet potatoes are endemic, such as the Amazonian region, both the leaves and
the tubers are used in folk medicine for their tonic properties, and for an
array of conditions including tumors in the mouth or throat, stomach-related
issues like diarrhea and nausea, fever, asthma, and burns.3,11 The
vines and leaves of the sweet potato plant have reported uses as an aphrodisiac
and laxative, and they were used to treat diabetes, uterine bleeding, mastitis,
burns and abscesses, and sprains.3,10,12
According to traditional
Chinese medicine (TCM) practice, sweet potatoes are said to benefit dry and
inflammatory conditions in the body and strengthen the spleen and pancreas.10
In TCM, each organ of the body is “in charge” of specific wellness functions;
therefore, by strengthening the spleen and pancreas, sweet potatoes help
support healthy digestion, weight, and blood production.13 The tuber
has been used as a galactagogue to increase a nursing mother’s milk supply. Based
on traditional use, Taiwanese immigrants in the United States are growing sweet
potato vines in their home gardens as a remedy for indigestion, as an
astringent, and as a treatment to reportedly rid the body of toxins.14
Phytochelatins (glutathione oligomers) within sweet potato bind to heavy metals
in the body in a process called chelation, which can serve to remove small
amounts of heavy metals ingested in the food supply from the body. Currently,
sweet potato tubers are still used in situations involving constipation or stomach
and intestinal bleeding.
Modern
Research
Sweet potato leaf
(SPL) is being explored for its role in disease prevention due to its high
levels of bioactive compounds, especially antioxidants. Possible applications
include prevention of cardiovascular disease, type 2 diabetes, certain cancers,
and immune-compromising conditions.6,15 A recent study showed that
just 200 grams of the purple-tinged leaves consumed over a period of 14 days reduced
the oxidation of LDL cholesterol particles, which is the basis of arterial
plaque formation.
The caffeic acid,
anthocyanoside, flavonoid, and arabinogalactan-protein contents of SPL make it
a possible antidiabetic food. In an in vitro study conducted to evaluate the ability
of purple SPL to increase glucose uptake, the extracts showed varying degrees
of antihyperglycemic activity, which the researchers attributed to the
quercetin content of the leaves.12 In addition, the relative
glycemic load of the sweet potato tuber is lower than that of the common potato,
making sweet potato a healthy choice for diabetic patients.16 The
glycemic load is calculated from a food’s glycemic index, a value assigned to
foods based on how slowly or quickly their ingestion causes increases in blood
glucose (blood sugar) levels.17 Foods with a low glycemic index
release glucose slowly, which is generally desirable for glucose control, while
foods with a high glycemic index are preferable for hypoglycemic (low blood
sugar) conditions.
Polyphenol-rich SPL
extracts have also shown antitumor effects. A study in mice showed that oral administration
of a fractionated SPL extract for six weeks inhibited growth and progression of
prostate cancer cells, inducing apoptosis (a pre-programmed “cell suicide” that
halts the spread of cancerous cells) and decreasing tumor volume.18
Researchers observed no further toxicity from the administration of the
extract. Sweet potato peels, which are usually discarded as waste, also contain
beneficial amounts of antioxidants, and an in vitro study using an alcoholic
extract of the peels showed cytotoxic activities against breast, colon, ovary,
and lung cancer cells.19
The purple tuber,
which contains higher levels of antioxidant anthocyanins and anthocyanidins in
comparison to the orange-fleshed variety, has been investigated for its
anti-inflammatory and cytotoxic properties. In a simulation of a superficial
wound, an extract of a Taiwanese varietal of purple sweet potato was found to
promote wound-healing and supported the repair process.20 In the
same study, an in vitro investigation showed that the extract induced apoptosis
of breast, gastric, and colon cancer cell lines.
Nutrient Profile21
Macronutrient
Profile: (Per 1 cup
raw sweet potato, cubed [approx. 133 g])
114 calories
2.1 g protein
26.8 g carbohydrate
0.1 g fat
Secondary
Metabolites: (Per 1
cup raw sweet potato, cubed [approx. 133 g])
Excellent
source of:
Vitamin A: 18,869 IU (377.4%
DV)
Very
good source of:
Manganese: 0.34 mg (17%
DV)
Dietary Fiber: 4 g (16%
DV)
Vitamin B6:
0.28 mg (14% DV)
Potassium: 448 mg
(12.8% DV)
Good
source of:
Magnesium: 33 mg (8.25%
DV)
Thiamin: 0.1 mg (6.67%
DV)
Phosphorus: 63 mg (6.3%
DV)
Vitamin C: 3.2 mg (5.3%
DV)
Also
provides:
Riboflavin: 0.08 mg
(4.7% DV)
Iron: 0.81 mg (4.5%DV)
Calcium: 40 mg (4% DV)
Folate: 15 mcg (3.75% DV)
Niacin: 0.74 mg (3.7%
DV)
Vitamin K: 2.4 mcg (3%
DV)
Vitamin E: 0.35 mg (2.3%
DV)
DV = Daily Value as
established by the US Food and Drug Administration, based on a 2,000-calorie
diet.
Recipe: Healthy Loaded Baked Sweet
Potatoes
Courtesy of North Carolina Sweet
Potato Commission22
Ingredients:
- 4 sweet potatoes
- 1 cup plain,
unsweetened Greek yogurt
- 2 tablespoons
freshly-squeezed lime juice
- 1/4 teaspoon salt
- 1 cup cooked or canned
black beans (drained and rinsed, if canned)
- 1/4 cup toasted pumpkin
seeds
- 2 tablespoons chopped
chives
- 2 teaspoons chili
powder
Directions:
- Heat oven to 400°F.
- Line a rimmed baking
sheet with foil. With a fork, pierce sweet potatoes all over and place on
baking sheet. Place sheet in oven and bake until tender, about 45 minutes.
- While potatoes bake,
combine yogurt with lime juice and salt in a small bowl. Cover and refrigerate.
- When sweet potatoes
are done, remove sheet from oven and cool 10 minutes. With a small spoon, break
and remove the skin from tops of the potatoes. Use a fork to gently mash the
insides.
- Top each sweet potato
with 1/4 cup black beans, 1/4 cup yogurt mixture, 1 tablespoon pumpkin seeds, a
sprinkle of chives, and a dusting of chili powder.
References
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Guide. Portland, OR: Timber Press; 2006.
- Davidson A. The Oxford Companion to Food. New York,
NY: Oxford University Press; 1999.
- Mateljan G. World’s Healthiest Foods: The Force For
Change To Health-Promoting Foods and New Nutrient-Rich Cooking. 2nd ed.
Seattle, WA: World’s Healthiest Foods; 2015.
- Huntrods D. Sweet
Potato Profile. Agricultural Marketing Research Center website. April 2013.
Available at: www.agmrc.org/commodities-products/vegetables/sweet-potato-profile/.
Accessed October 19, 2016.
- Madison D. Edible: An Illustrated Guide to the World’s
Food Plants. Washington, DC: National Geographic Society; 2008.
- Johnson M, Pace R.
Sweet potato leaves: properties and synergistic interactions that promote
health and prevent disease. Nutrition
Reviews. October 2010;68(10):604-615.
- Agroecology Case
Studies. Sweet Potato to Fight Vitamin A Deficiency and Reduce Malnutrition.
Oakland, CA: Oakland Institute; Durban, South Africa: AFSA; 2015.
- Olthof M, Hollman PCH,
Katan MB. Chlorogenic acid and caffeic acid are absorbed in humans. J Nutr. January 2001;131(1):66-71.
- Magnani C, Isaac VLB,
Correa MA, Salgado HRN. Caffeic acid: a review of its potential use in
medications and cosmetics. Anal Methods.
2014;6:3203-3210.
- Pitchford P. Healing with Whole Foods. Berkeley, CA: North Atlantic Books; 1993.
- Li TSC. Vegetables and Fruits: Nutritional and Therapeutic Values. Boca Raton,
FL: CRC Press; 2008.
- Lee CL, Lee SL, Chen
CJ, et al. Characterization of secondary metabolites from purple Ipomoea batatas leaves and their effects
on glucose uptake. Molecules. 2016;21(6):745.
- Ergil MC, Ergil KV. The Pocket Atlas of Chinese Medicine.
New York, NY: Thieme; 2009.
- Jiang S, Quave CL. A
comparison of traditional food and health strategies among Taiwanese and
Chinese immigrants in Atlanta, Georgia, USA. J Ethnobiol Ethnomed. 2013;9:61.
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& Lovers of Natural Foods. White River Junction, VT: Chelsea Green
Publishing Company; 1996.
- Atkinson FS,
Foster-Powell K, Brand-Miller JC. International tables of glycemic index and
glycemic load values: 2008. Diabetes Care.
December 2008;31(12):2281-2283.
- Glycemic index and
glycemic load for 100+ foods. Harvard Health Publications: Harvard Medical
School. August 27, 2015. Available at: www.health.harvard.edu/diseases-and-conditions/glycemic_index_and_glycemic_load_for_100_foods.
Accessed November 2, 2016.
- Gundala SR, Yang C,
Lakshminarayana N, et al. Polar biophenolics in sweet potato greens extract
synergize to inhibit prostate cancer cell proliferation and in vivo tumor
growth. Carcinogenesis. 2013;34(9):2039-2049.
- Oluyori AP, Shaw AK,
Olatunji GA, et al. Sweet potato peels and cancer prevention. Nutr Cancer. September 2016;1-8.
- Sugata M, Lin CY, Shih
YC. Anti-inflammatory and anticancer activities of Taiwanese purple-fleshed sweet
potatoes (Ipomoea batatas L. Lam) extracts.
Biomed Res Int. 2015;2015:768093.
- Basic Report: 11507,
Sweet potato, raw, unprepared. United States Department of Agriculture
Agricultural Research Service website. Available at: https://ndb.nal.usda.gov/ndb/foods/show/3207.
Accessed October 20, 2016.
- Healthy Loaded Baked
Sweet Potatoes. North Carolina Sweet Potato Commission website. Available at: www.ncsweetpotatoes.com/recipe/healthy-loaded-baked-sweet-potatoes/.
Accessed October 20, 2016.
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