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Sweet potato leaves: properties and synergistic interactions that promote health and prevent disease

Melissa Johnson, Ralphenia D Pace
DOI: http://dx.doi.org/10.1111/j.1753-4887.2010.00320.x 604-615 First published online: 1 October 2010

Abstract

Sweet potato (Ipomoea batatas) leaves provide a dietary source of vitamins, minerals, antioxidants, dietary fiber, and essential fatty acids. Bioactive compounds contained in this vegetable play a role in health promotion by improving immune function, reducing oxidative stress and free radical damage, reducing cardiovascular disease risk, and suppressing cancer cell growth. Currently, sweet potato leaves are consumed primarily in the islands of the Pacific Ocean and in Asian and African countries; limited consumption occurs in the United States. This comprehensive review assesses research examining the nutritional characteristics and bioactive compounds within sweet potato leaves that contribute to health promotion and chronic disease prevention. Research has affirmed the potential cardioprotective and chemopreventive advantages of consuming sweet potato leaves, thus indicating that increased consumption of this vegetable should be advocated. Since reducing the prevalence of chronic diseases is of public health concern, promoting the consumption of sweet potato leaves warrants further and more intensive research investigation.

  • antioxidants
  • bioactive compounds
  • cancer
  • cardiovascular disease
  • sweet potato leaves

INTRODUCTION

Fruits and vegetables function as dietary reservoirs of fiber, micronutrients (e.g., vitamins and minerals), phytochemicals, essential fatty acids, and other functional compounds with antioxidant activities such as carotenoids, flavonoids, and polyphenols, all of which contribute to significant decreases in the risks associated with chronic diseases.15 It has been suggested that polyphenols occupy a prominent position within the class of antioxidant compounds that confer the healthful benefits associated with fruit and vegetable consumption.6 Research has demonstrated that dietary patterns characterized by high fruit and vegetable intakes are inversely related to risks for cardiovascular diseases (CVDs) and cancer.7,8 Synergistic relationships between and among the functional and bioactive components of fruits and vegetables facilitate mechanisms that counteract the oxidative and inflammatory processes that underlie the development and progression of chronic diseases.911 The phytochemicals contained in fruits and vegetables also possess antioxidant activities, which have been shown to play roles in CVD prevention as well as cancer and inflammation inhibition. According to Issa et al.11 phytochemicals function as preventive and therapeutic agents by exhibiting antioxidant activities, modulating certain cell signaling inflammatory pathways and regulating the expression of phase I and phase II enzymes. These enzymes protect cells by facilitating the metabolism (i.e., initial modification, conjugation with charged antioxidant species such as glutathione), and excretion of exogenous or foreign chemicals within cells.

The dietary flavonoids quercetin, luteolin, and genistein have been shown to function as potent antioxidants with free radical-scavenging abilities against hydrogen peroxide (H2O2) and inhibition of O2- generation.12 These flavonoids were able to reduce oxidative damage to DNA as well as effectively inhibit lipid peroxidation in rat livers. These findings contribute to the existing body of evidence indicating that flavonoids function as dietary anticarcinogenic agents by promoting the protection of cellular constituents from oxidative damage and peroxidation. Other research has confirmed the ability of dietary antioxidants to reduce cellular oxidative damage to DNA and cellular lipid peroxidation, as well as to quench free radicals.1316 Although research conducted by Moller et al.17 failed to demonstrate a significant association between the consumption of fruits and vegetables and oxidative DNA damage among healthy individuals, there appears to be a consensus regarding the beneficial effects of fruit and vegetable consumption in the promotion of overall health and well-being. While there have been gradual increases in fruit and vegetable intake among Americans, intakes of dark green, leafy, and cruciferous vegetable are below recommended levels.18 Therefore, the continued advocacy of increased fruit and vegetable consumption, particularly of those rich in antioxidant compounds, is consistent with the Healthy People 2010 objectives; individuals are encouraged to adapt more healthful food consumption patterns in order to reduce the risks associated with chronic disease.19

The nutritional value of the sweet potato (Ipomoea batatas) has been emphasized as its antioxidant and phenolic compounds exert improved human health and nutrition benefits.2025 In addition, the sweet potato is a robust crop harvested throughout the year and is characterized by flesh colors ranging from white to deep orange to purple. Although the roots of the sweet potato are nutritious and commonly consumed, the leaves (greens, tops, or tips) are of nutritional value and are also consumed by humans. Sweet potato leaves are considered an indigenous, tropical, leafy vegetable, predominantly in African and Asian countries, functioning as a rich source of protein, essential amino acids, antioxidants, B vitamins, minerals, and dietary fiber. 26 Although underutilized for consumption and a small component of the typical Western diet, both purple and green sweet potato leaves possess a significant amount of phytochemicals and other bioactive compounds. These leaves have antioxidant and free radical scavenging activities that may circumvent the adverse effects of the typical Western diet.25,27 Sweet potato leaves are commonly grown and consumed less by African Americans, particularly in the southeastern region of the United States.28,29 Thus, encouraging consumption of sweet potato leaves by African Americans and other racial groups may be useful for preventing cardiovascular diseases and other chronic diseases. Islam30 indicates the potential of sweet potato leaves for improving health as well as serving as a functional food ingredient to enhance the antioxidant capacity of commonly consumed food products.

NUTRIENT AND NON-NUTRITIVE COMPOSITION OF SWEET POTATO LEAVES

Although the protein, fiber, fat, vitamin, and mineral contents of sweet potato leaves vary according to cultivar and production methods, the nutritional quality of sweet potato leaves is comparable to that of other green, leafy vegetables.31 Ishida et al.32 found sweet potato leaves to be a significant source of protein and to furthermore exhibit a relatively high amino acid score. Sweet potato leaves were found to be a particularly rich source of linoleic and α-linolenic acids by Almazan and Adeyeye.33 Georgia Jet sweet potato leaves were found to contain 14.2 ± 0.28 and 33.5 ± 1.74% of linoleic and α-linolenic acid, respectively, with extraction by hexane; TU-82-155 sweet potato leaves were found to contain 13.7 ± 0.24 and 28.5 ± 0.57% of linoleic and α-linolenic acids, respectively, with extraction by hexane. Upon examination of three sweet potato leaf cultivars (i.e., J6-66, W-308, and N-31), Daniels found that the fatty acid and phytosterol contents varied according to cultivar and harvest date (i.e., day 78, 108, and 140) (unpublished data). Among the different cultivars, α-linolenic acid was the fatty acid present in the greatest concentrations, with values ranging from 0.8 mg/g for the N-31 cultivar to nearly 1.2 mg/g for the W-308 cultivar. Significant differences in α-linolenic acid concentrations were observed based on the collection date, with a nearly two-fold increase and peak at collection date 2, followed by a significant threefold decrease at collection date 3. No significant differences in linoleic acid concentrations per cultivar or collection date were present. β-sitosterol contents increased with time, with the third collection date exhibiting a significantly higher β-sitosterol content compared to collection dates one and two. The W-308 cultivar contained the greatest amount of β-sitosterol (1.37 mg/g) compared to the J6-66 (1.28 mg/g) and N-31 (0.77 mg/g) cultivars.

Pace et al. 3436 found that the proximate composition and nutrient content of sweet potato leaf tips varied according to cultivar, harvesting practices, and processing methods. The mineral contents of the sweet potato leaves analyzed were comparable to recommendations set forth by the Food and Nutrition Board of the Institute of Medicine.37 Younger Jewel sweet potato leaves (i.e., initial 10-cm portion from the tip, freeze-dried sample) compared to older leaves (i.e., second 10-cm portion from the tip, freeze-dried sample) generally contained less calcium (836 ± 42 mg/100 g versus 1,144 ± 43 mg/100 g) and iron (10.88 ± 0.41 mg/100 g versus 13.37 ± 0.73 mg/100 g). Conversely, older sweet potato leaves had lower zinc content (2.48 ± 0.13 mg/100 g versus 2.88 ± 0.15 mg/100 g) than younger leaves.34 The proximate composition (% dry weight) of Carver and Jewel cultivars of sweet potato leaves were similar throughout different harvest dates, crop years, and processing. Overall, gradual decreases in crude protein, fat, and fiber were observed during the different harvesting periods. Canned Carver sweet potato leaves had greater amounts of crude fat compared to Jewel sweet potato leaves during harvest periods 1 (7.4% versus 6.2%), 2 (7.1% versus 6.4%), and 3 (7.5% versus 6.4%). The highest amounts of crude protein in both cultivars were observed at harvest period 1 (28.2%, Carver; 25.9%, Jewel in fresh samples).35 Generally, sweet potato leaves harvested earlier with fewer topping frequencies exhibited greater nutrient content. Increased topping frequencies were associated with decreased β-carotene and zinc concentrations. Conversely, increased calcium concentrations were associated with increased topping frequencies. As expected, variability in nutrient levels were observed in different cultivars of sweet potato leaves.36 Research examining the effect of harvesting on the chemical composition and net yield of sweet potato leaves indicated that harvesting after 4 months resulted in lower sweet potato leaf yields.38 Further, depending upon the harvesting interval, the crude protein content of sweet potato leaves ranged anywhere from 26% to 30%.

Sibiya found no significant differences in dry matter, zinc, calcium, and total dietary fiber content of sweet potato leaves when comparing topping to non-topping harvesting practices, although the iron content was significantly different (unpublished data). In comparison to the stems-petioles, sweet potato leaves yielded the highest mineral content. In agreement with previously mentioned research, observed yearly variations were attributed to variations in soil, climate, and time of harvest. These findings confirm earlier research indicating the effects of harvesting practices on the nutrient content of sweet potato leaves.39

In addition to harvesting and topping frequencies, the nutrient content of sweet potato leaves may be affected by traditional processing practices such as sun drying, blanching, cooking, and ventilated-container storage systems. Conventional blanching and cooking of sweet potato leaves may significantly decrease the total carotenoid concentrations contained in this vegetable.40 Other researchers found that the blanching of green sweet potato leaves for 60 seconds resulted in increased flavonoid retention, with levels similar to those in fresh leaves.27 Blanching of sweet potato leaves has also resulted in decreased levels of anti-nutritional factors such as phytic acid, tannic acid, and, to a lesser extent, oxalic acid.41 In addition to the conventional blanching methods, microwave blanching methods are able to significantly reduce trypsin and chymotrypsin inhibitor activities.42 Conventional blanching of sweet potato leaves facilitated greater retention of nutrients in comparison to microwave blanching (unpublished data). Although traditional blanching methods resulted in higher ascorbic acid and thiamin levels, microwave blanching of sweet potato leaves yielded higher riboflavin levels. Even though harvesting and processing practices may affect the concentration of certain nutrients contained in the sweet potato leaf, the leaf's overall integrity as a rich source of dietary nutrients remains intact. The nutritional composition of sweet potato leaves highlights its potential as a functional dark green, leafy vegetable that may help individuals meet their nutritional needs when consumed as part of the diet.43

In comparison with the stems and stalks, the leaves of the sweet potato have the highest amount of soluble dietary fiber. Ishida et al.32 found the soluble dietary fiber content of sweet potato leaves to average around 6%, whereas others have reported crude fiber levels of over 7%.44 Almazan and Zhou45 found total dietary fiber content (% fresh weight) of sweet potato leaves to range from 9.37% to 19.15%; Mosha et al.46 found the dietary fiber content of sweet potato leaves to average around 38% (dry weight basis). The dietary fiber content of sweet potato leaves is similar to those of other vegetables characterized as having high dietary fiber content. The relatively high dietary fiber content of sweet potato leaves suggests that consumption may impart benefits such as reduced risk for certain diseases and lower cholesterol and glucose levels, as observed when consuming other foods with similar fiber content.

Chemical composition of sweet potato leaves

Galactolipids are critical components of plant wall membranes and may offer a significant contribution to the n-3 polyunsaturated essential fatty acid composition of plants. Recently, Napolitano et al.47 identified novel galactolipids in sweet potato leaves. The monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) fractions present in the nonpolar component of galactolipids within the sweet potato leaf are characterized by the presence of specific fatty acyl groups attached to the sn-1, sn-2, or sn-3 positions of the glycerol backbone. Five novel galactolipids were identified with α-linolenic acid occupying the R1 or R2 positions. The amounts of MGDG compounds were as follows: 3.01 ± 0.31 mg/g (C16:0 at R1 and C18:3 [cis n-3] at R2); 6.89 ± 0.57 mg/g (C18:3 [cis n-3] at R1 and C19:2 at R2); 0.71 ± 0.06 mg/g (C19:2 at R1 and C18:3 [cis n-3] at R2); 11.92 ± 0.69 mg/g (C20:1 [trans n-9] at R1 and C18:3 [cis n-3] at R2); 0.17 ± 0.01 mg/g (C21:0 at R1 and C18:3 [cis n-3] at R2).

Two novel galactolipids were identified with α-linolenic acid occupying the R1 or R2 positions. The amounts of DGDG compounds were as follows: 2.21 ± 0.21 mg/g (C19:2 at R1 and C18:3 [cis n-3] at R2); 3.25 ± 0.33 mg/g (C20:1 [trans n-9] at R1 and C18:3 [cis n-3] at R2).

This observation of the dominant presence of C18:3 fatty acids within the galactolipids means sweet potato leaves may be classified as an “18:3 plant.” The relative abundance of the omega-3 polyunsaturated fatty acid α-linolenic acid within the leaves of the sweet potato indicates this green, leafy vegetable has potential as a source of this fatty acid. Christensen48 indicates that the biological activity of galactolipids, in enhancing anti-inflammatory and anticarcinogenic processes, is attributed to their possible role in human health promotion.

Bioactive components of sweet potato leaves

Sweet potato leaves contain a vast array of dietary antioxidants, including anthocyanins, polyphenols, flavonoids, and caffeic acid derivatives. These bioactive components demonstrate, to one degree or another, an ability to reduce oxidative stress, which appears to be the fundamental and common risk factor for chronic diseases. Islam et al.49 reported on the identification and characterization of 15 anthocyanins exhibiting both antioxidative and antimutagenic qualities in sweet potato leaves. In comparison to the other components of Beauregard, Covington, and Hernandez sweet potato cultivars, the leaves were found by Truong et al.22 to have the greatest total phenolic acid content compared to the roots. No significant differences in leaf total phenolic content (mg chorogenic acid per 100 g fresh weight) among Beauregard (1223.6), Covington (1224.8) and Hernandez (1298.1) cultivars were observed. All of the sweet potato leaf cultivars examined demonstrated a high level of radical scavenging activity in comparison to the other parts of the plant. These findings are consistent with research confirming that sweet potato leaves display one of the highest total antioxidant capacities and total polyphenol contents among green leafy vegetables consumed by the Fijian people.50

Research conducted by Sanchez-Moreno et al.51 suggested that dietary polyphenols exhibit greater antioxidant activities and inhibition of LDL oxidation compared to other antioxidant compounds. In addition, polyphenols exhibit oxygen radical scavenging capabilities and inhibition of lipid peroxidation. Results of research examining the polyphenol content and antioxidant activity of sweet potato leaves are in agreement with these findings.52 Both green and purple sweet potato leaves contained an appreciable flavonoid content and high-level free radical scavenging activity. The average total flavonoid (flavonols and flavones) content of green and purple sweet potato leaves were determined to be 185.01 ± 2.47 mg/kg and 426.82 ± 8.86 mg/kg, respectively. 27 Both quercetin and myricetin were present in green and purple sweet potato leaves (143.78 ± 5.04 mg/kg and 38.88 ± 1.79 mg/kg, respectively, for green leaves and 266.86 ± 4.37 mg/kg and 155.87 ± 3.60, respectively, for purple leaves). The flavone luteolin was found only in purple sweet potato leaves, whereas the flavone apigenin was present only in green sweet potato leaves.

Sweet potato leaves contain a variety of caffeic acid derivatives, which are correlated with the total polyphenolic content of the specific genotype. 53 Caffeoylquinic acid derivatives (CQA) isolated from sweet potato leaves exhibited antimutagenic, radical scavenging, and antioxidative characteristics. Mono-, di-, and tricaffeoylquinic acid derivatives isolated from sweet potato leaves exhibit increasing levels of antimutagenic activities as the number of caffeoyl groups increase. 54 Research suggests that the major phenolic compounds in sweet potato leaves are 3,5-di-O-caffeoylquinic acid and 4,5-di-O-caffeoylquinic acid.55 The major bioactive compounds in green and purple sweet potato leaves are presented in Table 1.

View this table:
Table 1

Health-promoting and disease-preventing bioactive compounds in purple and green sweet potato leaves.

Purple sweet potato leavesGreen sweet potato leavesFunctionReferences
PolyphenolsPolyphenolsAntioxidative; free radical scavenging; improved immune response; decreased lipid peroxidation; decreased DNA oxidation; inhibition of LDL oxidationHuang et al. (2007)29; Chang et al. (2007)70; Chang et al. (2007)75; Chen et al. (2005)62; Chen et al. (2008)61; Salleh et al. (2002)80
AnthocyaninsAnthocyaninsChemopreventive; anti-inflammatoryHou (2003)81; Karlsen et al. (2007)82
Caffeic acid derivativesCaffeic acid derivativesAntioxidative; antimutagenic; chemopreventive; antidiabeticYoshimoto et al. (2002)54; Kurata et al. (2007)79; Islam et al. (2009)25; Jung et al. (2006)83
QuercetinQuercetinAntihypertensive; chemopreventiveEdwards et al. (2007)84; Caltagirone et al. (2000)85
KaempferolAntioxidative; chemopreventivePark et al. (2006)86; Luo et al. (2009)87
MyricetinMyricetinChemopreventive; antidiabeticKnekt et al. (2002)4
FisetinAnti-inflammatory; chemopreventiveLu et al. (2005)88; Geraets et al. (2009)89; Lim and Park (2009)90
MorinChemopreventive; anti-inflammatoryKawabata et al. (1999)91; Galvez et al. (2001)92
IsorhamnetinCardioprotectiveSanchez et al. (2007)93
LuteolinAnti-inflammatory; chemopreventiveJu et al. (2007)94; Lim et al. (2007)95; Jang et al. (2008)96
ApigeninChemopreventiveLiu et al. (2005)97; Van Dross et al. (2003)98; Caltagirone (2000)85
Mono- and di- galactosyldiacylglycerolAnti-inflammatoryLenti et al. (2009)99
CarotenoidsCarotenoidsCardioprotective; chemopreventiveRao and Rao (2007)100
Dietary fiberDietary fiberChemopreventive; antidiabetic; cardioprotective; increased fecal excretion of bile acidsInnami et al. (1998)73; Pereira et al. (2004)101; Weickert and Pfeiffer (2008)102; Dahm et al.103
PhytochemicalsPhytochemicalsAntioxidative; antimutagenic; vasorelaxationIslam et al. (2009)25; Runnie et al. (2004)77
Omega-3 fatty acidsOmega-3 fatty acidsCardioprotective; anti-inflammatoryMassaro et al. (2009)104

SWEET POTATO LEAVES IN HEALTH AND DISEASE

Sweet potato leaves consumption

Bioactive compounds within fruits and vegetables are critical for protecting cellular components from oxidative damage contributing to disease pathogenesis.56 When incorporated into the diets of animals, sweet potato leaves have potential mechanisms to improve dietary protein and amino acid intake, as well as improve growth performance.57,58 Earlier studies indicated that sweet potato leaves as a daily component of the New Guinean diet contributed a significant amount of nitrogen to the value of protein in the diet.59 It was also estimated that sweet potato leaves contribute nearly one-third of the overall antioxidant activity contained in vegetables consumed as part of the typical Taiwanese diet (unpublished data). Consumption of the traditional Hawaiian diet, which contains sweet potato leaves, as well as other foods containing dietary fiber, complex carbohydrates, and a low fat content, reduced the risks associated with cardiovascular disease.60 Researchers at Tuskegee University have demonstrated the consumption of sweet potato leaves affects serum lipid profiles in both humans and animals, and thus shows potential for reducing the risks associated with the development of cardiovascular disease (unpublished data).

Although sweet potato leaves have been consumed for their medicinal and health-promoting properties in various countries throughout the world, this vegetable remains relatively unexploited in the typical Western diet. Consequently, very few in vivo and in vitro research studies examining the effect of sweet potato leaf consumption on health have been conducted. The polyphenolic and other antioxidant compounds contained in sweet potato leaves can thus contribute to daily dietary allowances for optimal health. The major nutrients and bioactive components within sweet potato leaves are believed to act in synergy to decrease the risks associated with certain diseases (Figure 1).

Figure 1

Synergistic effects of the major nutritional and bioactive compounds within sweet potato leaves in disease prevention and health promotion.

Sweet potato leaves and oxidative stress

Chen et al.61,62 found that consumption of 200 g of purple sweet potato leaves per day over the course of a 2-week period affected plasma α-tocopherol, erythrocyte glutathione, and plasma total antioxidant status. Although plasma α-tocopherol levels were significantly decreased by the end of week 2 (11.4 ± 1.4 µmol/L at baseline, compared to 9.8 ± 1.4 µmol/L at day 7 and 7.8 ± 0.8 µmol/L at day 14), plasma total antioxidant status remained virtually unchanged (0.8 ± 0.1 mmol/L at baseline, compared to 0.9 ± 0.2 mmol/L at days 7 and 14). Ameho et al.63 reported similar reductions in α-tocopherol in rats fed diets supplemented with quercetin, an antioxidant found in sweet potato leaves. In contrast, erythrocyte glutathione was significantly greater at day 14 (34.5 ± 7.4 µmol/L) compared to baseline (day 0, 25.9 ± 11.7 µmol/L). The observed increase in glutathione may be attributed to the polyphenols contained in sweet potato leaves, which may facilitate the inhibition of glutathione reductase,64 as well as enhance the expression of γ-glutamylcysteine synthetase, the rate-limiting enzyme for glutathione synthesis.65,66

The ability of sweet potato leaves to reduce cardiovascular disease risk when consumed has been demonstrated in both animals and humans. In research conducted by Yoon, hamsters consuming diets containing sweet potato leaves incorporated into lower fat (12% of calories) diets exhibited more favorable lipid profiles and weight status; the same was found in hamsters fed a higher fat diet (45% of calories) containing the same levels of sweet potato leaves. Hamsters consuming diets containing sweet potato leaves (mixed cultivars) exhibited higher antioxidant capacities, indicating the ability of antioxidants within sweet potato leaves to reduce the oxidative stress associated with CVD development. In comparison to hamsters consuming diets containing 2% sweet potato leaves to those consuming 4%, the latter displayed significantly lower plasma total cholesterol, low-density lipoprotein (LDL-C), and very-low-density lipoprotein (VLDL-C). Triglyceride levels of hamsters consuming sweet potato leaves in both normal and high-fat diets were significantly lower compared to those not consuming sweet potato leaves. High-density lipoprotein (HDL-C) levels were significantly greater among hamsters consuming diets with 2% and 4% sweet potato leaves in a high-fat diet compared to those consuming a high-fat diet devoid of sweet potato leaves. Increases in high-density cholesterol levels were observed among animals consuming diets with higher levels of sweet potato leaves (unpublished data).

The effects of sweet potato leaf tip consumption on serum lipids and cardiovascular disease risk has been examined in men and women. Daily consumption of approximately 120 g of Jewel sweet potato leaves for 14 days resulted in significant reductions in blood pressure and weight (unpublished data). The ability of sweet potato leaves consumption to affect weight status and blood pressure concomitantly demonstrates this vegetable's potential for helping to reduce the risks associated with the development of cardiovascular disease and associated comorbidities. Since it is recognized that when weight is lost,67 blood pressure also decreases, it remains to be determined whether or not the observed reductions in blood pressure are attributable to the bioactive compounds in sweet potato leaves, weight loss, or a combination of both. This research suggests that consumption of sweet potato leaves for longer than 14 days would result in greater reductions in serum lipids, blood pressure, and body weight.

Sweet potato leaves and immune response

The effects of consuming purple sweet potato leaves on the modulation and manifestation of immune response in basketball players was evaluated by Chen et al.62 The daily consumption of 200 g of purple sweet potato leaves for 2 weeks resulted in increased plasma polyphenol concentrations. Moreover, consumption of purple sweet potato leaves resulted in significant increases in proliferation responses in peripheral blood mononuclear cells, as well as the cytotoxic activity of natural killer cells. Consumption of sweet potato leaves also favorably enhanced immune response. More specifically, consumption of sweet potato leaves resulted in increased lymphocyte proliferation, interleukin (IL)-2 and IL-4. Individuals consuming diets containing purple sweet potato leaves exhibited higher cytotoxic natural killer (NK) cell activities, suggesting the ability of dietary polyphenols to deactivate free radicals, prevent lipid peroxidation, and promote increased immunity.

Sweet potato leaves and lipid peroxidation

Consumption of purple sweet potato leaves has been associated with decreased lipid peroxidation as well as reductions in LDL oxidation and reactive oxygen species production.61,68 Chen et al.61 demonstrated that the daily consumption of 200 g of purple sweet potato leaves over the course of 2 weeks was able to decrease lipid peroxidation and DNA damage. Although the plasma α-tocopherol concentrations were significantly lower in both the control and sweet potato leaves groups at the end of week 2, the average plasma α-tocopherol level was greater in those consuming purple sweet potato leaves compared to the control group. Erythrocyte glutathione concentrations were significantly increased by 33.3% following consumption of purple sweet potato leaves. In regard to oxidative stress, consumption of purple sweet potato leaves resulted in significant decreases in malondialdehyde and 4-hydroxy-2-nonenal (7.8 ± 0.3 µmol/L at day 0 versus 7.4 ± 0.2 µmol/L at day 14) and nonsignificant reductions in urinary 8-hydroxy-deoxyguanosine (8.1 ± 5.6 ng/mL at day 0 versus 6.9 ± 3.1 ng/mL at day 14). In addition, LDL oxidation lag time was significantly increased in those consuming purple sweet potato leaves (78.0 ± 12.9 min at day 0 versus 89.7 ± 16.5 at day 14).

Nagai et al. 68 found that sweet potato leaves exhibited considerable antioxidant inhibition of LDL oxidation both in vivo and in vitro. Sweet potato leaves were able to significantly prolong LDL lag time and were able to reduce indicators of lipid peroxidation and oxidative stress, namely plasma concentrations of thiobarbituric acid reactive substances. Decreases in LDL mobility in human umbilical vein endothelial cells were observed as well. Further, sweet potato leaves were able to inhibit the production of reactive oxygen species in human monocytic cells.

Sweet potato leaves and hematological parameters

In addition to affecting oxidative stress, immune response, and lipid peroxidation, consumption of sweet potato leaves demonstrated improvements in the hematological parameters in rabbits provided with increasing increments (i.e., 1 mL, 3 mL, and 5 mL) of sweet potato leaf extract every 2 weeks. 69 From week 1 through week 2, with an intake of 1 mL sweet potato leaf extract, packed cell volume in rabbits increased from 35.0% to 35.3%. Increases in packed cell volume were observed in the same rabbits at weeks 3 (39.0%, with 3 mL of sweet potato leaf extract), 4 (40.5%, with 3 mL of sweet potato leaf extract), 5 (41.8%, with 5 mL of sweet potato leaf extract), and 6 (43.0%, with 5 mL of sweet potato leaf extract). Likewise, leukocyte and platelet counts in rabbits increased significantly with increasing dietary concentrations of sweet potato leaf extract. Leukocyte count at week 1 with 1 mL of sweet potato leaf extract was 4,587.5 compared to 7,700.0 at week 6 after 5 mL of sweet potato leaf extract consumption. Platelet counts increased from week 1 to week 2 in rabbits provided with 1 mL and 3 mL of sweet potato leaf extract; platelet cell counts decreased from week 1 to week 2 after 5 mL of sweet potato leaf extract. Chang et al.70 reported increases in leukocytes, erythrocytes, and platelet cell counts after daily consumption of purple sweet potato leaves for 2 weeks. Slight decreases in monocyte, eosinophil, and basophil percentages were also observed.

Sweet potato leaves and cardiovascular diseases

Cardiovascular disease is a term used to refer to a group of chronic, degenerative diseases affecting the heart and/or blood vessels and is the leading cause of death among Americans. According to the American Heart Association, an estimated one in three Americans have one of more types of CVD, with hypertension or high blood pressure being the most prevalent.71 CVD is of public health concern as it is the leading cause of morbidity and mortality in the United States and costs billions of dollars in medical care annually. Engaging in dietary practices that promote increased consumption of antioxidant-rich fruits and vegetables may decrease the risks associated with CVD. The typical Western (i.e., American) diet is atherogenic in nature, plentiful in calories, refined and/or processed carbohydrates, total fat, saturated fat, cholesterol, and sodium, and deficient in whole-grain products, fruits, and vegetables.72

The consumption of sweet potato leaves has been associated with mediating certain physiological responses that may decrease the risks associated with CVD. Sweet potato leaves have demonstrated decreased lipid peroxidation and DNA damage,61 increased fecal excretion of bile acids,73 and regulation of blood glucose, plasma insulin levels, and lipid profiles.74 Sweet potato leaves have an approximate 1:2 ratio of linoleic to α-linolenic fatty acids,33 which may further protect the cardiovascular system from excessive inflammation and oxidative damage.

The consumption of sweet potato leaf powder has been associated with increased fecal excretion of bile acids and decreased hepatic cholesterol concentrations in rats, as reported by Innami et al.73 Rats consuming sweet potato leaf powder demonstrated significantly lower levels of liver total cholesterol and triglyceride compared to those consuming the control diet. Significant decreases in plasma total cholesterol and triglycerides were observed in non-insulin-dependent diabetic rats consuming 25, 50, and 100 mg/kg of flavones extracted from sweet potato leaves in a study conducted by Zhao et al.74

It has been suggested that the dietary fiber contained in the leaves of sweet potatoes may facilitate the increased excretion of bile acids and cholesterol, which may assist in lowering serum cholesterol levels and decrease the risks associated with the development and progression of cardiovascular diseases. Conversely, there were no significant decreases in serum total cholesterol levels following consumption of sweet potato leaves in humans, as reported by Weekly (unpublished data). No change in serum cholesterol level was observed by Chen et al.61 in individuals consuming 200 g of purple sweet potato leaves per day for 2 weeks In agreement with the results reported by Innami et al.,73 Chang et al.70,75 observed slight increases in serum cholesterol and triglycerides following consumption of sweet potato leaves.

In hamsters, consumption of sweet potato leaves, compared to other diets, resulted in more favorable biomarkers that suggest decreased disease risk (i.e., significantly lower plasma total cholesterol, triglycerides, VLDL-C, LDL-C, and higher HDL-C).76 Sweet potato leaves have also demonstrated the ability to facilitate vasorelaxation of the rat aorta.77 These findings suggest that bioactive components contained in sweet potato leaves have the ability to promote additional protection of the vascular endothelium, thereby offering protection against cardiovascular diseases.

Sweet potato leaves and cancer

Greater consumption of vegetables rich in vitamin A, such as sweet potato leaves, decreases the risks associated with the development of lung cancer. Individuals consuming upper tertile levels of sweet potato leaves exhibited reduced risk for lung cancer.78 Research has demonstrated the suppression of human cancer cell growth by the bioactive components contained in sweet potato leaves. Kurata et al.79 determined that polyphenolic compounds, caffeic acid, and di- and tricaffeoylquinic acids isolated from sweet potato leaves are able to suppress the growth of human cancer cells. The findings of this research suggest that these compounds induce the apoptotic process in cancerous cells. These findings are in agreement with the antimutagenic and anticancer properties of polyphenols contained in the sweet potato leaf, which can prevent the mutation of normal cells into cancerous cells and suppress the growth of cancer cells, respectively. It is believed that caffeic acid and caffeoylquinic acid derivatives contained in sweet potato leaves may interact with polyphenols to exert antimutagenicity effects.

CONCLUSION

Dietary patterns are one of the most modifiable risk factors for chronic, non-communicable diseases such as cardiovascular diseases, diabetes mellitus, hypertension, and certain cancers. Increased fruit and vegetable consumption has been associated with decreased disease risk, and has thus served as a target for many dietary therapies, nutritional interventions, and national health campaigns. Particularly important in regard to fruit and vegetable consumption are the nutritional composition and physiological characteristics that support reductions in oxidative stress, inflammation, endothelial dysfunction, dyslipidemia, hypertension, and hyperglycemia. Sweet potato leaves contain bioactive compounds that reduce the aforementioned characteristic risks for CVD and other chronic diseases.

Although the tuberous root of the plant is most commonly consumed in the Western part of the world, the leaves of the sweet potato are decidedly rich in nutrients and functional compounds: complex carbohydrates, protein, amino acids, soluble and insoluble dietary fiber, omega-3 fatty acids, micronutrients (i.e., vitamins and minerals), antioxidants, and other bioactive compounds. The ability of antioxidants and other bioactive compounds within sweet potato leaves to reduce the risks associated with disease is mediated by their ability to promote more favorable antioxidant status, free radical-scavenging capacities, and thwart processes involved in disease pathogenesis. Bioactive compounds contained in the sweet potato leaf have been demonstrated to be potent anti-inflammatory, cardioprotective, chemopreventive, antidiabetic, and antimutagenic agents in disease prevention. Specifically, the bioactive compounds of sweet potato leaves have been studied in humans and animals. Observed disease-prevention effects have been documented with regard to preventing oxidative stress, enhancing immune responses, and reducing lipid peroxidation.

Serving as a reservoir of antioxidant molecules and biologically active compounds that act in synergy, the sweet potato leaf may be exploited further to promote health and reduce the risks associated with disease pathogenesis. Additional research is required to provide scientific evidence regarding the synergistic interactions between the bioactive compounds in sweet potato leaves as well as their effect on gene expression and subsequent disease pathogenesis. Although this traditional medicinal plant is a relatively rare component of the typical Western diet, in the United States, sweet potatoes are grown as commercial crops and in private gardens, primarily in the southeastern region of the country. For many private farmers, this crop could be a two-in-one cash crop providing economic benefits from the sale of both the leaves and roots. Extraction of bioactive components within the leaves can produce compounds for pharmaceutical research purposes. The ability of sweet potato leaves to promote health and prevent disease emphasizes the value of this novel nutrient-dense green leafy vegetable as a powerful vehicle to improve public health and nutrition.

Acknowledgments

This work was supported by the Tuskegee University College of Agriculture, Environmental and Natural Sciences.

Declaration of interest

The authors have no relevant interests to declare.

REFERENCES

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