Contents
What is sweet potato
Sweet potato (Ipomoea batatas) is a member of the Convolvulaceae family; it is a dicotyledonous perennial plant found in the tropical and subtropical belts 1). Most varieties are edible, and all parts of the plants – shoots, leaves, vine and tubers – are consumed 2). The edible tuberous root is long and tapered, with a smooth skin whose color ranges between yellow, orange, red, brown, purple, and beige. Its flesh ranges from beige through white, red, pink, violet, yellow, orange, and purple. Sweet potato cultivars with white or pale yellow flesh are less sweet and moist than those with red, pink or orange flesh 3). The large, sweet-tasting tuberous roots, young leaves and shoots are common market vegetables. Although the soft, orange sweet potato is often called a “yam” in parts of North America, the sweet potato is botanically very distinct from a genuine yam (Dioscorea), which is native to Africa and Asia and belongs to the monocot family Dioscoreaceae 4). A true yam is a starchy edible root of the Dioscorea genus, and is generally imported to America from the Caribbean. It is rough and scaly and very low in beta carotene 5).
The centre of origin of sweet potato is Central America, but the crop is widely grown in many tropical and subtropical countries. Sweet potatoes are ranked seventh in world staple food production (expressed on a dry matter basis), after wheat, maize, rice, potato, barley and cassava. The crop is particularly important in South-East Asia, Oceania and Latin America. China still accounts for over 90% of total production; the other major sweet potato producing countries in Asia are: Indonesia, India, Japan, Vietnam, The Philippines and The Republic of Korea. Rwanda and Uganda are Africa’s largest producing countries. Sweet potato production in Latin America and The Caribbean is relatively small.
Figure 1. Sweet potato plant
Sweet potato is rich in nutrients and ranked highest in nutritional value amongst vegetables available in the United States of America 6). Among the important nutrients found in tubers are monosaccharides 7), complex carbohydrates 8), dietary fiber, beta-carotene (a source of vitamin A) 9), vitamin C, vitamin B6, anthocyanins 10), and minerals 11). The leaves of sweet potato are rich in protein, fibre, fat, vitamins, and minerals 12). Linoleic and alpha-linolenic acids 13), galactolipids 14), and bioactive substances (e.g. dietary antioxidants, including anthocyanins 15), polyphenols 16), flavonoids 17), and caffeic acid derivatives 18) are also present. These compounds have been extensively investigated for their role in health promotion in many countries.
Sweet potato nutrition facts
Besides simple starches, raw sweet potatoes are rich in complex carbohydrates, are a good source of potassium, dietary fiber and is a rich source of beta-carotene (a provitamin A carotenoid). In a 100 gram amount, raw sweet potato provides 88 calories and is a rich source of beta-carotene (a provitamin A carotenoid) that meets 100% of women and 80% of men Recommended Dietary Allowance 19). Vitamin A deficiency can erode the immune system and contribute to malnutrition, most seriously affecting pregnant women and young children in low-income countries. Somalia, like many developing countries, has a high prevalence of Vitamin A deficiency and malnutrition among its population. The use of orange-fleshed sweet potatoes – containing high levels of Vitamin A precursor – as animal fodder and food for humans has been successfully tested in southern Somalia 20).
While having moderate contents of other micronutrients, including vitamin B5, vitamin B6 and manganese (Table 1. Sweet potato nutrition facts) 21).
Sweet potato is 79% water, 20% carbohydrates, 1.6% protein, 3% dietary fiber and contains negligible amount of fat.
When cooked by baking, small variable changes in micronutrient density occur to include a higher content of vitamin C at 24% of the Daily Value per 100 g serving 22), 23).
The Center for Science in the Public Interest ranked the nutritional value of sweet potatoes as highest among several other foods 24).
Sweet potato cultivars with dark orange flesh have more beta-carotene than those with light-colored flesh, and their increased cultivation is being encouraged in Africa where vitamin A deficiency is a serious health problem. A 2012 study of 10,000 households in Uganda found that children eating beta-carotene enriched sweet potatoes suffered less vitamin A deficiency than those not consuming as much beta-carotene 25).
Table 1. Sweet potato (raw with skin) nutrition facts
Nutrient | Unit | sweetpotato, 5\” long 130 g | Value per 100 g | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Approximates | |||||||||||||||||||
| Water | g | 102.78 | 79.06 | ||||||||||||||||
| Energy | kcal | 114 | 88 | ||||||||||||||||
| Protein | g | 2.09 | 1.61 | ||||||||||||||||
| Total lipid (fat) | g | 0.07 | 0.05 | ||||||||||||||||
| Carbohydrate, by difference | g | 26.75 | 20.58 | ||||||||||||||||
| Fiber, total dietary | g | 4.0 | 3.1 | ||||||||||||||||
| Sugars, total | g | 5.56 | 4.28 | ||||||||||||||||
| Minerals | |||||||||||||||||||
| Calcium, Ca | mg | 40 | 31 | ||||||||||||||||
| Iron, Fe | mg | 0.81 | 0.62 | ||||||||||||||||
| Magnesium, Mg | mg | 32 | 25 | ||||||||||||||||
| Phosphorus, P | mg | 62 | 48 | ||||||||||||||||
| Potassium, K | mg | 448 | 345 | ||||||||||||||||
| Sodium, Na | mg | 73 | 56 | ||||||||||||||||
| Zinc, Zn | mg | 0.40 | 0.31 | ||||||||||||||||
| Vitamins | |||||||||||||||||||
| Vitamin C, total ascorbic acid | mg | 3.2 | 2.5 | ||||||||||||||||
| Thiamin | mg | 0.000 | 0.000 | ||||||||||||||||
| Riboflavin | mg | 0.081 | 0.062 | ||||||||||||||||
| Niacin | mg | 0.741 | 0.570 | ||||||||||||||||
| Vitamin B-6 | mg | 0.278 | 0.214 | ||||||||||||||||
| Folate, DFE | µg | 16 | 12 | ||||||||||||||||
| Vitamin B-12 | µg | 0.00 | 0.00 | ||||||||||||||||
| Vitamin A, RAE | µg | 942 | 725 | ||||||||||||||||
| Vitamin A, IU | IU | 18870 | 14515 | ||||||||||||||||
| Vitamin D (D2 + D3) | µg | 0.0 | 0.0 | ||||||||||||||||
| Vitamin D | IU | 0 | 0 | ||||||||||||||||
| Vitamin K (phylloquinone) | µg | 2.3 | 1.8 | ||||||||||||||||
| Lipids | |||||||||||||||||||
| Fatty acids, total saturated | g | 0.026 | 0.020 | ||||||||||||||||
| Fatty acids, total monounsaturated | g | 0.000 | 0.000 | ||||||||||||||||
| Fatty acids, total polyunsaturated | g | 0.013 | 0.010 | ||||||||||||||||
| Fatty acids, total trans | g | 0.000 | 0.000 | ||||||||||||||||
| Cholesterol | mg | 0 | 0 | ||||||||||||||||
| Other | |||||||||||||||||||
| Caffeine | mg | 0 | 0 | ||||||||||||||||
Benefits of sweet potato
Sweet potato (Ipomoea batatas) ranks as the seventh most important crop worldwide. Sweet potato is a plant found in the tropical and subtropical belts and is one of the most nutritious tropical and subtropical vegetables.
Purple sweet potato, a special sweet potato cultivar, has been extensively investigated because large amounts of anthocyanin accumulate in its tuberous roots. Anthocyanin is well known for its free radical-scavenging activity and beneficial effects on human health. Its biosynthetic pathway has been well characterized in model plants. Although large-scale systematic studies have been performed to identify the anthocyanin proteins present in sweet potato, information on the regulation of anthocyanin synthesis in sweet potato is insufficient 27).
As well as being popular in cooking in countries in Asia-Pacific, Africa and North America, sweet potato is also used in traditional medicine for the treatment of diabetes mellitus. Research in animal and human models suggests a possible role of sweet potato in glycaemic control 28).
Sweet potato for type 2 diabetes mellitus
Diabetes mellitus is a metabolic disorder resulting from a defect in insulin secretion, insulin action, or both. A consequence of this is chronic hyperglycaemia (that is elevated levels of plasma glucose) with disturbances in carbohydrate, fat and protein metabolism. Long-term complications of diabetes mellitus include retinopathy, nephropathy and neuropathy (i.e. problems with the eyes, kidneys and peripheral nerves). Diabetes mellitus also increases the risk of cardiovascular disease.
Type 2 diabetes mellitus is a global public health issue 29), 30). The increase in numbers of people with type 2 diabetes mellitus across the age spectrum is of concern. Given the progressive nature of the disease, and the multiple pathophysiological (disease) abnormalities associated with it, accelerated ageing is suspected. This is supported by evidence both at molecular and functional levels 31).
The blood glucose-lowering activities of sweet potato were demonstrated in animal studies. A number of bioactive compounds were isolated from the leaves 32) and also the tubers 33), 34). These compounds, together with dietary fiber, contribute to blood glucose-lowering activitiy. Besides glycemic control, sweet potato has shown anti-sclerotic activity and inhibition of glycation in test tube studies 35), 36), as well as antihypertensive 37), antioxidative 38), 39), antimutagenic 40), chemopreventive 41), and cardioprotective properties 42). Finally, there is a suggestion that sweet potato may delay amyloid formation and prevent neuronal damage in the brain of mice 43), 44), 45), 46), 47).
This review 48) of randomised controlled trials found only three studies (with a total of 140 participants) that evaluated the effects of sweet potato for type 2 diabetes mellitus compared with a fake medicine (placebo). All these trials were of very low quality. Two studies with 122 participants showed improved long-term metabolic control of blood sugar levels as measured by glycosylated haemoglobin A1c (HbA1c) which was moderately lowered by 0.3% in participants who were given 4 g sweet potato tablets a day for three to five months. The duration of treatment ranged from six weeks to five months. No study investigated diabetic complications, death from any cause, health-related quality of life, well-being, functional outcomes or costs. Adverse effects were mostly mild, and included abdominal distension and pain. There are many varieties of sweet potatoes and sweet potato preparations. More trials are needed to assess the quality of the various sweet potato preparations as well as to evaluate further the use of different varieties of sweet potato in the diet of diabetic people. In conclusion there is insufficient evidence about the use of sweet potato for type 2 diabetes mellitus 49).
What is anthocyanin ?
Anthocyanins are members of the flavonoid group of phytochemicals, a group predominant in teas, honey, wines, fruits, vegetables, nuts, olive oil, cocoa, and cereals 50). Anthocyanins occur ubiquitously in the plant kingdom and confer the bright red, blue and purple colors to fruits and vegetables such as purple sweet potato, berries, grapes, apples, purple cabbage and corn 51). Epidemiologic studies suggest that the consumption of anthocyanins lowers the risk of cardiovascular disease, diabetes, arthritis and cancer due, at least in part, to their anti-oxidant and anti-inflammatory activities 52). The daily intake of anthocyanins in residents of the United States is is estimated to be between 180 and 215 mg or about 9-fold higher than that of other dietary flavonoids 53).
In both in vitro and in vivo research trials, anthocyanins have demonstrated marked ability to reduce cancer cell proliferation and to inhibit tumor formation 54), 55), 56), 57). The capacity of anthocyanin pigments to interfere with the process of carcinogenesis seems to be linked to multiple potential mechanisms of action including inhibition of cyclooxygenase enzymes and potent antioxidant potential. Hou et al. 58) revealed that anthocyanins inhibit tumorigenesis by blocking activation of a mitogen-activated protein kinase pathway. This report provided the first indication of a molecular basis for why anthocyanins demonstrate anticarcinogenic properties. In other research, fruit extracts with significant anthocyanin concentrations proved to be effective against various stages of carcinogenesis 59), 60), 61), 62), but the individual role of anthocyanins versus other components was not determined, in part because the anthocyanins were too easily degraded during bioassays if separated from stabilizing cofactors such as other phenolic constituents 63). In vivo studies have shown that dietary anthocyanins inhibit cancers of the gastrointestinal tract and topically applied anthocyanins inhibit skin cancer. Although experimental studies have clearly demonstrated the anti-cancer activity of anthocyanins, epidemiological studies have not revealed protective effects of anthocyanin consumption on cancer risk in humans, and their antioxidant activity in humans remains questionable 64).
The roles of anthocyanin pigments as medicinal agents have been well-accepted dogma in folk medicine throughout the world, and, in fact, these pigments are linked to an amazingly broad-based range of health benefits. For example, visual acuity can be markedly improved through administration of anthocyanin pigments to animal and human subjects, and the role of these pigments in enhancing night vision or overall vision has been particularly well documented 65). Oral intake of anthocyanosides from black currants resulted in significantly improved night vision adaptation in human subjects 66), and similar benefits were gained after administration of anthocyanins from bilberries 67). Three anthocyanins from black currant stimulated regeneration of rhodopsin (a G-protein-coupled receptor localized in the retina of the eye), and formation of a regeneration intermediate was accelerated by cyanidin 3-rutinoside 68). These studies strongly suggest that enhancement of rhodopsin regeneration is at least one mechanism by which anthocyanins enhance visual acuity.
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