Sweet potato

Sweet potatoes rank as the world’s seventh most important food crop (after wheat, rice, maize, potatoes, barley and cassava) because of their versatility and adaptability, as they have relatively few pests, grow well in poor soil and require minimal input of labour, water, fertiliser or chemicals. They are major crops with potential for managing food security and climate change, as under conditions of extensive agriculture, the plant combines drought tolerance with high yields.

Sweet potatoes belong to the morning glory family (Convolvulaceae) and are only distantly related to the common potato (Solanum tuberosum). Sometimes referred to as yams, sweet potatoes are not true yams, which are of the Dioscoreaceae family.

Nutrition

Tubers (roots) contain unique anthocyanidins, phenolic acids, trypsin inhibitors with glutathione peroxidase activity (antioxidant), triterpenoid saponins and salicylic acid. The purple sweet potatoes have the highest antioxidant polyphenol and flavonoid content.

Sweet potatoes produce more edible energy per hectare per day than wheat, rice or cassava and are good sources of complex carbohydrates, fibre, protein and micronutrients — iron, zinc, calcium, manganese, potassium and phosphorus — with low fat.

Orange-fleshed sweet potato contains B complex, vitamins C, and E and is an important source of beta-carotene. The colour of the food is linked to its health benefits — lighterfleshed varieties have higher levels of phenolic compounds, yellow/orange colours have higher betacarotene, phytosterols and phenolic acids, and the purple varieties have higher levels of unique antioxidant anthocyanidins (quercetin, luteolin, myricetin).

Therapeutic uses

Sweet potato roots are antioxidant, anti-inflammatory, hepatoprotective, cardioprotective, antihypertensive, anticancer, improve memory and cognition, metabolic disorders such as diabetes and intestinal barrier function.

Liver

The anthocyanidins in sweet potatoes have been shown to protect the liver from a range of toxins, inhibiting lipid peroxidation and scavenging free radicals.

Neurology

Bioactive compounds from purple sweet potato have measurable memory-enhancing effects — improving spatial learning, memory and cognitive function in ageing mice. Neuroprotective effects were also demonstrated after ischaemic stroke and neuronal cell damage.

Cancer

Sweet potato consumption (especially purple flesh potatoes) may protect against a wide variety of cancers (due to the polysaccharides and polyphenols). Research demonstrated that breast, gastric, colon, bladder and prostate cancer growth inhibition and apoptosisinducing benefits when animals consumed sweet potatoes regularly.

Antidiabetic

The anthocyanidins and carotenoid components from purpleand orange-fleshed sweet potatoes reduce obesity, inhibit fat accumulation and restore triglyceride levels and lower cholesterol, reducing the risk of cardiovascular disease and atherosclerosis.

Sweet potato consumption is also effective (in mice) in reducing type 2 diabetes — relieving inflammation, inhibiting platelet aggregation and improving endothelial function. Chlorogenic acid (also in coffee) inhibits glucose uptake in the intestinal brush border and modifies glucose tolerance in humans.

A clinical trial was conducted on 61 type 2 diabetic patients eating bwhite sweet potatoes once a day for 12 weeks. The results confirmed the beneficial effects — improving both fasting and postprandial glucose control, decreasing HbA1c (indicating longer term glucose control) and lowering both total and LDL cholesterol levels.

Cooking

Cooking by baking, boiling, steaming or microwaving changes the nutrient density, increasing the phenolic and antioxidant content as well as the starch, and decreasing sugar and betacarotene.

References available on request.

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