HerbalEGram: Volume 15, Issue 9, September 2018

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 American Botanical Council (ABC) would like to acknowledge ABC Chief Science Officer Stefan Gafner, PhD, for his contributions to this project. The original article on sweet potato was published in November 2016.

By Hannah Bauman,^a Jenny Perez,^b and Mikala Sessa^c

^a HerbalGram Associate Editor

^b ABC Education Coordinator

^c ABC Dietetics Intern (Texas State University, 2013)

Overview

The sweet potato (Ipomoea batatas, Convolvulaceae) is a trailing, herbaceous perennial in the morning glory family.^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. There are more than 400 sweet potato varieties, and most have yellow-brown or copper-colored skins with bright orange or yellow-red flesh.³ Orange-fleshed sweet potatoes (OFSPs) are the most common varieties consumed, but white, cream, yellow, pink, and deep purple varieties also exist. The sweet potato plant has alternate, heart-shaped leaves and produces funnel-shaped white, pink, or rose-violet flowers that appear in clusters in the leaf axils as the plant matures.⁴

Taxonomic confusion can arise over the common name “yam” that often is given to sweet potatoes in the market. Botanically speaking, true yams belong to the genus Dioscorea (Dioscoreaceae) and are much less common in the United States.⁵ In general, foods marketed as “yam” in the United States should be assumed to be sweet potatoes. The common potato (Solanum tuberosum, Solanaceae) belongs to a third distinct plant family.

Historical and Commercial Uses

The sweet potato has been part of human diets for more than 10,000 years, according to findings in prehistoric Peruvian caves.⁵ Sweet potatoes are one of the oldest domesticated crops, and were a staple crop prior to the introduction of cereal grains.^6,7 Brought to Europe by Portuguese explorers in the 15th century, cultivation of this resilient food crop spread to Africa, India, southern Asia, and Indonesia by the 16th century. Due to the transatlantic slave trade and European colonization in North America, sweet potatoes became a cultural food staple in the southern United States.

Because of its nutrient density, sweet potato is an important part of child nutrition programs in developing countries, and agricultural scientists are working to improve the nutritional content of sweet potato even further.⁸ Despite sweet potato’s relatively low protein content, this nutrient-dense tuber is an important source of protein for a large portion of the world’s population. In Papua New Guinea, sweet potatoes account for 60-90% of the population’s daily energy requirements.⁹

While relatively unused in Western diets, sweet potato leaves (SPLs) are eaten as a vegetable in African and Asian countries, and they have comparable nutritional quality to that of other green leafy vegetables.¹⁰ The leaves are a good source of protein, vitamin C, beta-carotene, and lutein.^7,10 Both the tubers and leaves are used around the world, especially in Asia, as livestock fodder to enhance protein and amino acid intake and improve overall health.¹⁰

In 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 (e.g., diarrhea and nausea), fever, asthma, and burns.^5,11,12 The vines and leaves of the sweet potato plant are both astringent and demulcent and have been used as traditional medicine to treat uterine bleeding and control hyperglycemia.¹³ Topically, sweet potato leaves were used to treat mastitis, burns, abscesses, and sprains.^3,13,14 In Japan, the white-skinned sweet potato has been used to treat high blood pressure, anemia, and diabetes.⁶ Native American tribes used sweet potato to treat thirst and weight loss associated with diabetes.

In traditional Chinese medicine (TCM), sweet potatoes are said to benefit dry and inflammatory conditions and strengthen the spleen and pancreas.¹⁴ According to TCM principles, each organ of the body is “in charge” of specific wellness functions. Therefore, by strengthening the spleen and pancreas, sweet potatoes are thought to help support healthy digestion, weight, and blood production.¹⁵ The tuber has been used as a galactagogue to increase milk production in nursing mothers, and tubers are still used in TCM to treat constipation and bleeding of the stomach or intestines. Some Taiwanese immigrants in the United States reportedly grow sweet potato in their home gardens and use the leaf and vine as traditional remedies for anemia and indigestion, as an astringent, and to rid the body of toxins.^2,16 In TCM, sweet potatoes also are believed to have chelating, or metal-binding, properties.¹⁴

Phytochemicals and Constituents

The sweet potato tuber is a nutrient-dense food that provides important vitamins and minerals.¹² A source of complex carbohydrates comparable to other starchy crops like rice (Oryza sativa, Poaceae), corn (Zea mays, Poaceae), and potatoes, sweet potato tubers deliver more dietary fiber per serving.⁷ The dietary fiber in sweet potato is predominantly hemicellulose and cellulose, which protect against colon cancer and heart disease.² Sweet potato tubers contain a wide variety of macro- and micro-nutrients, including antioxidant vitamins C and E, moderate amounts of B vitamins (B₁, B₂, B₅, B₆, and folic acid, which work as metabolic cofactors), and an array of minerals essential for enzyme, protein, and carbohydrate metabolism, including potassium, calcium, iron, copper, magnesium, zinc, and phosphorus.^2,9,12

Different varieties of sweet potato are grown worldwide, and they are characterized by variations in flesh color and phytochemicals.² OFSPs contain the highest levels 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, skin, and immune function. Purple-fleshed sweet potatoes (PFSPs) provide higher amounts of anthocyanins in comparison to OFSPs and have been investigated for their anti-inflammatory and cytotoxic properties. The anthocyanins in PFSPs are glycosides of cyanidin and peonidin, which are thought to protect the digestive tract by binding to heavy metals like cadmium, arsenic, and mercury for more efficient removal.^10,12 Sweet potatoes also contain important detoxifying enzymes like catalase and zinc/copper superoxide dismutase.⁶

Although the anthocyanin and beta-carotene content provide the majority of sweet potato’s antioxidant benefits, quercetin and caffeic acid derivatives such as chlorogenic, isochlorogenic, and other caffeoylquinic acids also have antioxidant properties in addition to mild blood pressure-lowering effects.^8,12 Caffeic acid derivatives in particular have been shown to have immunomodulatory, antimicrobial, and anti-inflammatory activities.⁹

Vitamin A deficiency (VAD) affects more than 230 million children worldwide.⁹ A study conducted in 2005 found that in South Africa, 64% of children between ages 1 and 9.⁷ Lack of regular access to animal sources of vitamin A creates an increased reliance on acquiring vitamin A via beta-carotene from plant sources such as leafy greens and orange-colored roots and fruits. Research indicates that daily consumption of green leafy vegetables or sweet potatoes improves vitamin A stores in populations at risk of VAD.⁹ Studies from Africa indicate that every 3.5 ounces of OFSPs contain 35% of the daily requirement of vitamin A.¹² South Africa is using OFSP cultivation in school and community garden projects as a way to alleviate VAD locally.⁷

The SPL is also nutritious and provides twice as much protein as the sweet potato tuber, with more antioxidant polyphenols per serving than many other vegetables.^2,7,9 In addition to polyphenols, SPL contains anthocyanins, phytosterols, galactolipids, carotenoids (especially lutein), caffeic acid, and α-linolenic acid.^5,10,14,17 SPLs are best blanched for no more than 60 seconds to retain flavonoids and to reduce levels of antinutrients such as phytic acid, tannic acid, and oxalic acid.^6,17 Research indicates microwave blanching SPLs reduces the concentrations of trypsin and chemotrypsin inhibitors, improving protein digestibility when consumed.¹⁷

Modern Research

Different cultivars of sweet potato adapt differently to their environmental conditions, which directly affect their nutrient and phytochemical contents.² In general, OFSP has high beta-carotene content and the ability to alleviate vitamin A deficiency, the white-skinned sweet potato is associated with antidiabetic effects, and PFSP can modulate immune response and significantly reduce inflammation.²

Type 2 diabetes mellitus (DM2) is a metabolic disorder characterized by high blood glucose (blood sugar) resulting from the body’s inability to regulate insulin.¹⁷ When glucose isn’t properly metabolized, damage can occur to many organs, including the heart, blood vessels, kidneys, nerves, and eyes.⁶ In patients with DM2, adiponectin (a glucose metabolism-regulating hormone) levels are significantly lower, resulting in poorly regulated insulin metabolism and reduced insulin sensitivity.² In vivo studies have demonstrated the potential of sweet potato to help maintain blood glucose levels and lower insulin resistance through its ability to increase blood levels of adiponectin.^2,6

The relative glycemic load of the sweet potato tuber is lower than that of the common potato, making sweet potato a healthy, low-fat choice for diabetic patients.¹⁸ 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 levels.¹⁹ 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.

In Japan, a dietary supplement made from a crude extract of white-skinned sweet potato called caiapo is sold as an over-the-counter antidiabetic medication.^2,20 Clinical studies that examined the efficacy of caiapo used a total of four capsules daily, each capsule containing 168-336 mg caiapo extract.⁶ In a systematic review of clinical trials that examined the use of caiapo sweet potato tablets for DM2, the most effective dose was 4 g daily. When compared with placebo, there was a significant reduction in fasting blood glucose and glycosylated hemoglobin A1c levels when caiapo dose was increased from 2 g to 4 g daily in both the short term (six weeks to two months) and longer term (three to five months).^17,20 After 12 weeks, hemoglobin A1c levels were lowered by 0.5% on average, which indicates caiapo had similar glucose-lowering effects as common diabetic medications like acarbose and nateglinide, thus indicating caiapo’s potential usefulness in managing diabetes.²⁰

Studies that examined the nutrient density and bioactive compounds in SPL suggest that regular consumption can reduce incidence of chronic disease and promote overall health. Possible applications include reduction of the risk of cardiovascular disease, DM2, certain cancers, and immune-compromising conditions.^3,10 Sweet potato leaves contain high levels of anthocyanins, lutein, and other antioxidant compounds that scavenge free radicals and modulate immune system function, caffeic acid derivatives that are anti-mutagenic and reduce cancer cell growth, and essential fatty acids that reduce inflammation and improve cardiovascular health.¹⁰ The ratio of linoleic acid to α-linolenic acid is 1:2 in SPL, which is considered advantageous to provide further cardiovascular protection from chronic inflammation and oxidative damage.¹⁰

A recent study showed that 200 g of the purple-tinged leaves consumed daily over a period of 14 days reduced the oxidation of LDL cholesterol, which causes arterial plaque formation.⁶ Daily consumption of approximately 120 g SPLs for 14 days resulted in significant reductions in blood lipids, blood pressure, and body weight.¹⁰ A separate study on basketball players who consumed 200 g purple SPLs daily for two weeks demonstrated immune-modulating effects, including a significant increase in natural killer cell activity resulting from an increase in polyphenols present in the blood after consumption.²

Consumer Considerations

Domestication of the sweet potato plant has led to its widespread cultivation as an important global food crop throughout Mexico, Central America, and the Caribbean.⁴ Currently, sweet potatoes are cultivated in more than 100 countries.²¹

In developing countries, many farmers are highly dependent on root and tuber crops, which are sustainable and easy to grow.²¹ In tropical and subtropical climates, sweet potatoes are a high-yield crop that is drought tolerant and adaptable.⁷ Sweet potato is unusual in that all parts of the plant are edible and can be harvested, as needed, over several months, providing a food reserve when major grain crops fail due to drought.^7,9 Increasing cultivation and consumption of sweet potato and sweet potato-based processed foods could significantly reduce VAD and increase global food security.⁹

Because of its nutritional qualities, the sweet potato plant was selected by the National Aeronautics and Space Administration (NASA) as one of the foods to be tested for long-term space travel and use by astronauts as a major food source.^9,22 Sweet potatoes also can be processed into value-added products, including shelf-stable bulk ingredients such as sweet potato starch or flour.⁹ The sweet potato is an important source of starch in China, Vietnam, Korea, Taiwan, and the Philippines. In these countries, sweet potato is used as a functional food ingredient for added nutrients, and it also is used in baked goods, noodles, and soups for its thickening and binding properties.⁹

The peak of sweet potato season is late fall, but tubers are available throughout the year in most grocery stores. When purchasing sweet potatoes, optimal characteristics include smooth-skinned tubers with tapered ends and no bruising or harvesting scars.²³ Sweet potatoes can be stored for a month or longer in a cool dry place or root cellar.³ The carotenoid content and bioavailability in sweet potato tubers decreases over time and with processing. Baking sweet potatoes provides the least loss in nutrients when compared to steaming or deep-frying.^9,22 The beta-carotene from OFSP appears to be more bioavailable than that from common green leafy vegetables and is better absorbed when combined with a fat source.^9,22

Nutrient Profile²⁴

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.

Image credits:

Sweet potato tubers. Image courtesy of Llez.
Sweet potato flowers. Image courtesy of H. Zell.
Sweet potato tuber. ©2018 Steven Foster.

References

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