If the sweet potato is not very present in Western cuisines, it has always been in those of all tropical countries. Native to Central America (where it is called “Camoe”), sweet potato, despite its name and although it is a tuber, has nothing to do with potatoes. It is distinguished by its elongated shape (sometimes it looks like a big carrot), whose colors, both the skin and the flesh, vary from beige/brown to orange, red or violet. It is an integral part of southern cuisine.
History of sweet potato
It has long been believed that sweet potato came from India, and some still claim it. This plant probably grew on the Indian sub-continent before the sixteenth century. However, archaeological excavations carried out in Peruvian sites, where the earliest vestiges date back to 8 000 years before our era, indicate it’s native to South America. It is not known whether they are cultivated varieties, but if were the case, sweet potato would probably be the first plant to have been domesticated in the new world, or even on the planet. The wild ancestor of this species has never been found, although other species of the genus Ipomoea have been found.
The sweet potato was introduced by the Spaniards and the Portuguese in Europe, Asia, Africa and Australia after the conquest of the New World. On the other hand, it was introduced in Oceania long before the discovery of America, perhaps around 1 500 BCE. It would have travelled westward by borrowing the South American and then Polynesian boats to settle gradually on all the Pacific Islands, where it has long been part of the basic diet. Another hypothesis is that its seeds may have been disseminated by birds, including the Golden plover of Polynesia, reputed to be an occasional visitor to the western coast of South America. Other researchers believe the capsules containing the seeds would have been driven by marine currents and would have settled along the fertile shores of the Pacific Islands.
Today, sweet potato is grown in all tropical countries, where it’s an important food resource. In many places, livestock are also fed with it.
Sweet potato health profile
The benefits of sweet potato
Cancer. The results of an epidemiological study indicate men who have a typically “South American” diet, consisting of sweet potatoes, dry beans, okra (or okra) and rice, are less likely to be affected by prostate cancer.
A study conducted in India indicated the consumption of Brassica vegetables and sweet potatoes was associated with lesser risk of gallbladder cancer.
In Japan, researchers found a significant decrease in the risk of breast cancer among premenopausal women correlated with increased consumption of several vegetables, including potatoes and sweet potatoes.
In Japanese, root-type foods containing a lot of starch, such as sweet potato, potatoes and taro, would be associated with a lower risk of kidney cancer. As for the sweet potato leaves, they could offer protection against lung cancer.
Anthocyanins extracted from the sweet purple potato would reduce the incidence of colorectal cancer when added to the diet of animals. In vitro, sweet potato or its leaves have proven to be effective in preventing the production of genetic mutations (mutation of a gene may lead in some cases to cancer development) and the growth of cancerous cells. However, other studies will need to be done to determine whether these experimental results can be applied to the regular consumption of sweet potatoes.
Cardiovascular diseases. The sweet potato, thanks to the phenolic compounds and the anthocyanins it contains, could prevent and decrease the oxidation of “bad” cholesterol (LDL), a risk factor of cardiovascular diseases. The anthocyanins found in the sweet purple potato would also decrease the progression of the atherosclerosis.
Sweet potato leaves could also exert a protective effect on the inner wall of the blood vessels. The results of an in vitro study show that the leaf extracts lead to the relaxation of blood vessels, particularly those of aorta. However, more studies are needed before concluding a cardioprotective effect among humans.
Hepatic function. One study showed that the consumption of a drink made from sweet purple potatoes improved liver function among men at risk of hepatitis and decreased hepatic enzyme levels (these were indicators of lesions or liver disease). Some studies among animals indicated that extracts of anthocyanins from the purple sweet potato had a protective effect on the liver, for example by protecting it from damage caused by high doses of acetaminophen (e.g. Tylenol).
Diabetes. According to 3 studies, an extract of a type of white-skinned sweet potato (grown mainly in South America and Japan) could reduce insulin resistance and thus improve blood glucose control, both among animals and among people with type 2 diabetes. However, it’s still too early to conclude an antidiabetic effect of sweet potatoes, especially since they contain a significant amount of carbohydrates, a nutrient diabetics must control their diet.
Immune system. It has been shown in vitro that the “antidiabetic” ingredient in white-skinned sweet potatoes stimulated the immune response. In addition, a polysaccharide extracted from the sweet potato would exert beneficial effects on the animal’s immune system, among other things by increasing the proliferation of lymphocytes and the phagocytic function, two defense systems of the body. As the mechanisms play a role in the immune response are complex, further research will be needed to better document these effects.
Cognitive impairment. Some pigments contained in the sweet purple potato would slow down the deterioration of cognitive function and some memory disorders in mice. Further studies will be needed to confirm these results among humans.
Where are the antioxidants?
The skin of the sweet purple potato contains more anthocyanins than the flesh. Moreover, the more the color of the flesh of this potato is dark, the more it contains anthocyanins.
As for sweet potato leaves, in addition to being rich in phenolic compounds, they also contain anthocyanin. Note that the leaves of the plant are suitable for consumption.
In summary, choosing sweet potatoes of dark color, consuming them with their skins and tasting the leaves makes it easy to add antioxidant compounds to your diet.
What does the sweet potato contain?
Some varieties of sweet potatoes, including the purple one (cultivars developed mainly in New Zealand and Japan), contain many anthocyanins, pigments giving its color. The anthocyanins are part of the flavonoid family and are known for their multiple anti-cancer, anti-inflammatory and antioxidant properties. The anthocyanins of the purple sweet potato showed an antioxidant activity in vitro superior to those of vitamin C2, as well as the one of the anthocyanins from the grape peel, red cabbage, elderberry berry and purple corn. The antioxidant power of anthocyanins from the purple sweet potato would play a major role in the beneficial effects on cardiovascular health and on the liver. Research is continuing on them.
Other researchers have noticed that several extracts of anthocyanins, including those contained in the sweet purple potato, could be useful in the prevention of diabetes. These compounds inhibit an enzyme that partly influences the elevation of blood glucose which occurs after the digestion of carbohydrates among the animal.
Finally, anthocyanins would contribute to the enhancement of cognitive functions among mice by reducing inflammation and oxidative stress in their brain.
Carotenoids are compounds with antioxidant properties, they are able to neutralize the free radicals of the body. The consumption of carotenoid-rich foods would be related to a lesser risk of suffering from several diseases, such as cancer and cardiovascular disease, although some results are controversial. The sweet potato contains large quantities of these carotenoids and beta-carotene. This is a precursor of vitamin A (i.e. the body transforms it into vitamin A according to its needs).
The bioavailability of the beta-carotene of the orange sweet potato would be much greater when the vegetable is cooked. Studies have shown daily consumption of cooked sweet potatoes increases vitamin A levels among children, and among men at risk of starvation in this vitamin. The results show the conversion of beta-carotene to vitamin A in the body is facilitated by the presence of at least 3 g of fat taken at the same meal. The sweet potato would therefore be a food of choice in the prevention of vitamin A deficiency, particularly in developing countries.
Phenolic compounds have an antioxidant activity playing a protective role in the incidence of several cardiovascular and degenerative diseases. According to one study, the sweet potato leaves would contain the most, followed by the peel and the flesh. The leaves would contain even more than some green leaves. Sweet potato puree, whether made from the whole vegetable or peeled, would contain as many phenolic compounds and would have a similar antioxidant activity.
Trypsin inhibitory protein
Trypsin inhibitory protein, contained in sweet potato, showed an in vitro antioxidant effect and could be beneficial to health. This substance could be useful in cancer prevention, particularly in the case of leukemia. However, few studies have been published on the subject and further research will be needed to confirm these results.
This protein, found in the white sweet potato, would participate in part in the antidiabetic virtues of vegetables.
Choice and conservation
Sweet potatoes are found in most grocery stores. The color of their flesh goes from white cream to purple through orange and red. From a nutritional point of view, orange or purple-fleshed varieties are preferable.
Although sweet potato originates from the tropics, researchers have selected varieties which can be grown in the north with relative success. Very decorative, the plant is also grown in flower beds. The propagation is done by cuttings. Cuttings can be obtained from a specialist or taken from one or more of the plants having been kept potted inside during the winter. It will be enough to put them in water until they have formed good roots.
Transplant seedlings as soon as they are delivered or keep them in water until it’s possible to put them in the ground, i.e. when the dangers of freezing are removed. Choose a sunny, hot and weed-free place. Shape earthy buttons to help grow tubers.
The pH should be between 5 and 6.5. Apply a biological fertilizer based on phosphate and potassium. Avoid nitrogen that may favour stems at the expense of tubers. Make sure that the irrigation is constant, but do not allow the plants to have their feet in water.
The harvest is done when the frost blackens the leaves or when the temperature falls below 10 ºC. Make the tubers mature for 1 week or 2 in a humid place where the temperature is about 27 ºC, then keep at 18 ºC in the dark.
Ecology and Environment
Less demanding for soil and more productive per hectare than cereals, sweet potato is also particularly well suited to arid climates and dry soils. Moreover, since it has rampant stalks, it protects the ground against wind erosion. In the tropical countries, unlike the large cereal crops, it is mainly made up of small producers, especially women, who often cultivate it on tiny plots of land. It allows them to feed their families at a lower cost. The plant is easy to multiply, since it’s usually enough to collect cuttings on the established plants and to prick them directly into the Earth, without any other important intervention. Energetic and nutrient rich, it is, together with the other root vegetables, an invaluable food source for the world’s most deprived.
In Africa and the Caribbean, it’s the basis of a prolific artisanal industry: the manufacture of a flour that is less expensive and richer than wheat. The peasants turn it into breads, cakes and other common consumer products which they sell in the local markets.
Considered as a safety food, it has saved millions of people from starvation, in Africa, Japan and China, as a result of ecological or human disasters (drought, armed conflict, etc.). In the semi-arid plains of eastern Africa, it bears the name of Culta Abana, literally “protecting children”. In addition, it’s currently considered to be one of the best solutions to the serious vitamin A deficiency problem in sub-saharan Africa. In these regions, millions of children under 5 years of age suffer from xerophthalmia, an ocular disease directly attributable to this deficiency and partly responsible for the high mortality rate among pregnant women. However, researchers have observed the consumption of orange-fleshed sweet potatoes, even in small quantities, eliminates the risk of vitamin A deficiency.