Sweet Potatoes
Contents
What’s New and Beneficial about Sweet Potatoes
- Orange-fleshed sweet potatoes may be one of nature’s unsurpassed sources of beta-carotene. Several recent studies have shown the superior ability of sweet potatoes to raise our blood levels of vitamin A. This benefit may be particularly true for children. In several studies from Africa, sweet potatoes were found to contain between 100-1,600 micrograms (RAE) of vitamin A in every 3.5 ounces—enough, on average, to meet 35% of all vitamin A needs, and in many cases enough to meet over 90% of vitamin A needs (from this single food alone).
- Sweet potatoes are not always orange-fleshed on the inside but can also be a spectacular purple color. Sometimes it’s impossible to tell from the skin of sweet potato just how rich in purple tones its inside will be. That’s because scientists have now identified the exact genes in sweet potatoes (IbMYB1 andIbMYB2) that get activated to produce the purple anthocyanin pigments responsible for the rich purple tones of the flesh. The purple-fleshed sweet potato anthocyanins—primarily peonidins and cyanidins—have important antioxidant properties and anti-inflammatory properties. Particularly when passing through our digestive tract, they may be able to lower the potential health risk posed by heavy metals and oxygen radicals. For more details on purple-fleshed and orange-fleshed sweet potatoes, please see our Description section below.
- It can be helpful to include some fat in your sweet potato-containing meals if you want to enjoy the full beta-carotene benefits of this root vegetable. Recent research has shown that a minimum of 3-5 grams of fat per meal significantly increases our uptake of beta-carotene from sweet potatoes. Of course, this minimal amount of fat can be very easy to include. In our Healthy Mashed Sweet Potatoes recipe, for example, we include 1 tablespoon of extra virgin olive oil, and with just this one tablespoon, each of our 4 servings for this delicious recipe provides 3.5 grams of fat.
- Some nutritional benefits from sweet potatoes simply may be easier to achieve if you use steaming or boiling as your cooking method. Recent studies show excellent preservation of sweet potato anthocyanins with steaming, and several studies comparing boiling to roasting have shown better blood sugar effects (including the achievement of a lower glycemic index, or GI value) with boiling. The impact of steaming is particularly interesting, since only two minutes of steaming have been show to deactivate peroxidase enzymes that might otherwise be able to break down anthocyanins found in the sweet potato. In fact, with these peroxidase enzymes deactivated, natural anthocyanin extracts from sweet potato used for food coloring may be even more stable than synthetic food colorings. This benefit isn’t limited to the food’s appearance since the anthocyanins have great health benefits as antioxidants and anti-inflammatory nutrients.
- Most dry beans and tubers have their own unique storage proteins. Soybeans have glycinins, potatoes have patatins, yams have dioscorins, and corn has zeins. While researchers have long been aware of sporamins—storage proteins in sweet potato—only recently has research shown some of their unique antioxidant properties. The potential health benefits of the sweet potato sporamins in helping prevent oxidative damage to our cells should not be surprising since sweet potatoes produce sporamins whenever subjected to physical damage to help promote healing
Health Benefits
How sweet it is for your health to eat sweet potatoes! Not only do they taste like dessert, but they provide some surprising health benefits. Many people think about sweet potatoes as being nothing more than plain old potatoes that can tweak our taste buds with some extra flavor. Yet cutting-edge research on sweet potatoes tells us that nothing could be further from the truth as they have so many unique nutritional benefits to offer!
One difficulty in describing the health benefits of sweet potatoes is knowing where to begin. There are a surprising number of nutrient categories responsible for the health benefits of this underappreciated tuber. Among these categories are antioxidants, anti-inflammatory nutrients, and blood sugar-regulating nutrients. Each category brings with it valuable health benefits.
Antioxidant Nutrients in Sweet Potato
Sweet potatoes contain a wealth of orange-hued carotenoid pigments. In countries throughout Africa, in India and in the Caribbean, sweet potatoes have been shown to be a highly effective way of providing school age children with sizable amounts of their daily vitamin A. In some studies, sweet potatoes have been shown to be a better source of bioavailable beta-carotene than green leafy vegetables. Because sweet potatoes are available in many countries on a virtual year-round basis, their ability to provide us with a key antioxidant like beta-carotene makes them a standout antioxidant food.
Yet beta-carotene only begins to tell the story of sweet potato antioxidants. Particularly in purple-fleshed sweet potato, antioxidant anthocyanin pigments are abundant. Cyanidins and peonidins are concentrated in the starchy core of part of purple-fleshed sweet potatoes, and these antioxidant nutrients may be even more concentrated in the flesh than in the skin. That’s sweet potatoes have genes (IbMYB1 and IbMYB2) that are specialized for the production of anthocyanin pigments in the fleshy part of the tuber. Ordinary, we have to rely on the skins of foods for this same level of anthocyanin antioxidants. But not in the case of sweet potatoes! Extracts from the highly pigmented and colorful purple-fleshed and purple-skinned sweet potatoes have been shown in research studies to increased the activity of two key antioxidant enzymes—copper/zinc superoxide dismutase (Cu/Zn-SOD) and catalase (CAT).
Recent research has shown that particularly when passing through our digestive tract, sweet potato cyanidins and peonidins and other color-related phytonutrients may be able to lower the potential health risk posed by heavy metals and oxygen radicals. That risk reduction might be important not only for individuals at risk of digestive tract problems but for all persons wanting to reduce the potential risk posed by the presence of heavy metal residues (like small amounts of mercury or cadmium or arsenic) in their diet.
Storage proteins in sweet potato also have important antioxidant properties. These storage proteins—called sporamins—get produced by sweet potato plants whenever the plants are subjected to physical damage. Their ability to help the plants heal from this damage is significantly related to their role as antioxidants. Especially when sweet potato is being digested inside of our gastrointestinal tract, we may get some of these same antioxidant benefits.
Anti-Inflammatory Nutrients in Sweet Potato
Anthocyanin and other color-related pigments in sweet potato are equally valuable for their anti-inflammatory health benefits. In the case of inflammation, scientists understand even more about the amazing properties of this tuber. In animal studies, activation of nuclear factor-kappa B (NF-kB); activation of inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2); and formation of malondialdehyde (MDA) have all be shown to get reduced following consumption of either sweet potato or its color-containing extracts. Since each of these events can play a key role in the development of unwanted inflammation, their reduction by sweet potato phytonutrients marks a clear role for this food in inflammation-related health problems. In animal studies, reduced inflammation following sweet potato consumption has been shown in brain tissue and nerve tissue throughout the body.
What’s equally fascinating about color-related sweet potato phytonutrients is their impact on fibrinogen. Fibrinogen is one of the key glycoproteins in the body that is required for successful blood clotting. With the help of a coagulation factor called thrombin, fibronogen gets converted into fibrin during the blood clotting process. Balanced amounts of fibrinogen, thrombin and fibrin are a key part of the body’s health and its ability to close off wounds and stop loss of blood. However, excess amounts of these clotting-related molecules may sometimes pose a health risk. For example, excess presence of fibrinogen and fibrin can trigger unwanted secretion of pro-inflammatory molecules (including cytokines and chemokines). In animal studies, too much fibrin in the central nervous system has been associated with breakdown of the myelin sheath that surrounds the nerves and allows them to conduct electrical signals properly. If fibrin excess can trigger unwanted inflammation in nerve tissue and increase breakdown of the myelin wrapping the nerve cells (a process that is usually referred to as demyelination), health problems like multiple sclerosis (in which there is breakdown of the myelin nerve sheath) may be lessened through reduction of excess fibrinogen and/or fibrin. In preliminary animal studies, intake of sweet potato color extracts have been shown to accomplish exactly those results: reduction of inflammation, and simultaneous reduction of fibronogen levels. We look forward to exciting new research in this area of sweet potato’s anti-inflammatory benefits.
Potential Improvement of Blood Sugar Regulation
Many people think about starchy root vegetables as a food group that could not possibly be helpful for controlling their blood sugar. That’s because many people realize that food starches can be converted by our digestive tract into simple sugars. If foods are especially concentrated in starch, there can often be a risk of too much simple sugar release in our digestive tract and too much pressure upon our bloodstream to uptake more sugar. (The result in this situation would be an overly quick elevation of our blood sugar level.) What’s fascinating about sweet potatoes is their ability to potentially improve blood sugar regulation—even in persons with type 2 diabetes— in spite of their glycemic index (GI) rating of medium. (Sweet potatoes are one of four WHFoods vegetables that have a GI ranking of medium. The other three vegetables are beets, corn, and leeks.) The 6.6 grams of dietary fiber in a medium sweet potato are definitely a plus in terms of blood sugar regulation, since they help steady the pace of digestion. But recent research has also shown that extracts from sweet potatoes can significantly increase blood levels of adiponectin in persons with type 2 diabetes. Adiponectin is a protein hormone produced by our fat cells, and it serves as an important modifier of insulin metabolism. Persons with poorly-regulated insulin metabolism and insulin insensitivity tend to have lower levels of adiponectin, and persons with healthier insulin metabolism tend to have higher levels. While more research on much larger groups of individuals to further evaluate and confirm these blood sugar regulating benefits, this area of health research is an especially exciting one for anyone who loves sweet potatoes but is nevertheless concerned about healthy blood sugar regulation.
Other Potential Health Benefits
One of the more intriguing nutrient groups provided by sweet potatoes—yet one of the least studied from a health standpoint—are the resin glycosides. These nutrients are sugar-related and starch-related molecules that are unusual in their arrangement of carbohydrate-related components, and also in their inclusion of some non-carbohydrate molecules. In sweet potatoes, researchers have long been aware of one group of resin glycosides called batatins (including batatin I and batatin II). But only recently have researchers discovered a related group of glycosides in sweet potato called batatosides (including batatodide III, batatoside IV, and batatoside V). In lab studies, most of these sweet potato glycosides have been shown to have antibacterial and antifungal properties. To what extent these carbohydrate-related molecules in sweet potatoes can provide us with health benefits in these same antibacterial and antifungal areas is not yet clear. But we expect to see increasing interest in sweet potato’s batatins and batatosides and their potential to support our health.
Description
Although sweet potatoes may be part of the Thanksgiving tradition, be sure to add these wonderful naturally sweet vegetables to your meals throughout the year; they are some of the most nutritious vegetables around. Sweet potatoes can be found in your local markets year-round, however they are in season in November and December.
While many people lump all potatoes into one category, potatoes can actually be quite different in terms of their taste, flavor, texture, and nutrients. Sweet potatoes are no exception! Sweet potatoes belong to an entirely different food family than either yams or the common potato that is such a large part of the U.S. diet. From a science standpoint, here’s a chart showing how different these three types of potatoes actually are.
Type of Potato Botanical Family Botanical Genus Botanical Species Common U.S. Potato Solanaceae Solanum S. tuberosum Sweet Potato Convolvulaceae Ipomoea I. batatas Yam Dioscoreaceae Dioscorea D. alata* * Worldwide, there are nearly 200 commonly eaten species of yam.
In some food families all foods belong not only to the same botanical family and genus but sometimes to the same species as well! For example, both Swiss chard and beets belong to the same botanical family and genus and species. When foods belong to the same family and genus, they are more likely to share commonality in their nutrient composition and health benefits. From this standpoint, potatoes are far more diverse, and when you choose sweet potatoes as a dietary root vegetable, you are getting a truly unique type of potato that is nutritionally different from other types.
Depending upon the variety, of which there are about 400, the skin and flesh of the sweet potato may be almost white, cream, yellow, orange, pink, or deep purple, although white/cream and yellow-orange flesh are most common. Sometimes this root vegetable will be shaped like a potato, being short and blocky with rounded ends, while other times it will be longer with tapered ends.
There are common names for most cultivars of sweet potatoes, and you may find the chart below helpful in selecting a sweet potato with the flesh color you desire based on the cultivar name.
Orange-fleshed* Purple-fleshed White-fleshed Beuregarde Agena Batas California Gold Japanese Purple Brazilian Carolina Ruby Korean Purple Dingess Covington Mokuau Hayman Evangeline Okinawan O’Henry Gold Nugget Purple Uyentan *Most sweet potato cultivars that contain the word “gold” or the word “red” in their title are orange-fleshed varieties.
The intensity of the sweet potato’s yellow or orange flesh color is directly correlated to its beta-carotene content. Our bodies can typically produce vitamin A from the beta-carotene in orange-fleshed sweet potato; this is why this nutrient is often referred to as “Provitamin A.”
Purple-fleshed sweet potatoes, on the other hand, are a fantastic source of anthocyanins (especially peonidins and cyanidins) and have outstanding antioxidant activity. In one study, the antioxidant activity in purple sweet potatoes was 3.2 times higher than that of a type of blueberry! An equally amazing fact about sweet potatoes is the antioxidant capacity of all their parts. Recent research has shown different genes to be at work in the flesh versus skin of the sweet potato producing different concentrations of anthocyanin antioxidants. Even the leaves of the sweet potato plant have been shown to provide important antioxidant benefits and are included in soups in many cuisines. Sweet potatoes can be grouped into two different categories depending upon the texture they have when cooked: some are firm, dry, and mealy, while others are soft and moist. In both types, the taste is starchy and sweet with different varieties having different unique tastes.
Sweet Potatoes and Yams
In the U.S., there is often much confusion between sweet potatoes and yams. They are completely different foods, belonging to different plant families. Sweet potatoes are much more highly available in the U.S. than are yams.
This confusion exists for two reasons. First, as a shopper, it is possible for you to find sweet potatoes and yams that look reasonably alike in terms of size, skin color, and flesh color. Second, government agencies have allowed these terms to be used somewhat interchangeably on labeling, so that you often cannot rely on the grocery store signs to help you determine whether you are looking at a bin full of sweet potatoes or a bin full of yams. For example, in many stores you can find bins that are labeled “Red Garnet Yams” and “Jewel Yams” and the foods in these bins are actually sweet potatoes. Here are some general practical rules that you can follow.
- In most U.S. groceries, you should assume that you are always purchasing a sweet potato, even if the sign says “yams.”Over 1 million sweet potatoes are commercially grown in the U.S. each year, while commercial production of yams in the U.S. is rare.
- Don’t use flesh color to decide whether you are getting a sweet potato or a yam. Both root vegetables come in a variety of colors. Once again, you should assume that you are getting a sweet potato regardless of flesh color.
- If you are specially seeking a true yam (from the plant genus Dioscorea), it might be helpful to visit a more internationally focused store that specializes in foods from tropical countries.
Here is a little more background about the relationship between sweet potatoes and yams.
The name “yam” was adopted from “nyami“—the Fulani (West African) word that means “to eat” and that has traditionally been used to refer to yams. Yams are native to Africa and Asia, and unlike potatoes, they have the potential to grow to a much larger size.
From a science perspective, true yam is a root vegetable belonging to the Dioscoreaceae family, which are monocotyledons (or “monocots” for short, with the prefix “mono” referring to the fact that they have only one embryonic seed leaf). Sweet potatoes belong to the Convolvulaceae or morning glory plant family, are dicotyledons (or “dicots” for short, with the prefix “di” referring to the fact that they have two embryonic seed leaves), and are known by the scientific name of Ipomoea batatas. So as you can see, these two root vegetables come from very different parts of the plant world, even though their names have become so intertwined in the U.S. marketplace.
History
Sweet potatoes are native to Central and South America and are one of the oldest vegetables known to man. They have been consumed since prehistoric times as evidenced by sweet potato relics dating back 10,000 years that have been discovered in Peruvian caves.
Christopher Columbus brought sweet potatoes to Europe after his first voyage to the New World in 1492. By the 16th century, they were brought to the Philippines by Spanish explorers and to Africa, India, Indonesia and southern Asia by the Portuguese. Around this same time, sweet potatoes began to be cultivated in the southern United States, where they still remain a staple food in the traditional cuisine.
Worldwide, about 80 million tons of sweet potatoes are grown in China each year, with Africa producing about 14 million tons, Central and South America about 2 million, and the U.S. about 1 million tons. Within the U.S., over half of all commercially grown sweet potatoes come from the southern states (especially North Carolina).
How to Select and Store
Choose sweet potatoes that are firm and do not have any cracks, bruises or soft spots. Avoid those that are displayed in the refrigerated section of the produce department since cold temperature negatively alters their taste.
Sweet potatoes should be stored in a cool, dark and well-ventilated place, where they will keep fresh for up to ten days. Ideally, they should be kept out of the refrigerator in a cool, dry, dark place not above 60°F /15°C, which would fit the characteristics of a root cellar. Yet since most people don’t have root cellars, we’d suggest just keeping your sweet potatoes loose (not in a plastic bag, but if desired, a brown paper bag with multiple air holes punched in it will work) and storing them in a cool, dark, and well-ventilated cupboard away from sources of excess heat (like the stove).
Tips for Preparing and Cooking
Tips for Preparing Sweet Potatoes
If you purchase organically grown sweet potatoes, you can eat the entire tuber, flesh and skin. Yet, if you buy conventionally grown ones, you should peel them before eating since sometimes the skin is treated with dye or wax; if preparing the sweet potato whole, just peel it after cooking.
As the flesh of sweet potatoes will darken upon contact with the air, you should cook them immediately after peeling and/or cutting them. If this is not possible, to prevent oxidation, keep them in a bowl covered completely with water until you are ready to cook them.
The Healthiest Way of Cooking Sweet Potatoes
Fortunately, from a nutrition standpoint, you have a number of good options for cooking sweet potatoes. While we have our own personal recommendation (namely, Healthy Steaming) here are some additional options that you may want to consider: Boiling: In several studies looking at the bioavailability of beta-carotene from sweet potatoes, boiling has been shown to be an effective cooking method. Consumption of boiled and mashed sweet potatoes has been shown to raise blood levels of vitamin A in children. When compared to roasting or baking, boiling has also been shown to have a more favorable impact on blood sugar regulation and to provide sweet potatoes with a lower glycemic index (GI) value. In one study, the average GI value for roasted sweet potato was 82, for baked sweet potato 94, and for boiled sweet potato 46.Stir-Frying: Multiple studies have shown better absorption of the beta-carotene from sweet potatoes when fat-containing foods are consumed along with the sweet potatoes. (It doesn’t take much fat for this better absorption to take place—only 3-5 grams.) What fat makes possible is the conversion of beta-carotene into a special form called micellar form. Micelles are specialized collections of molecules that allow fat-soluble substances (like beta-carotene) to move around comfortably in non-fat environments (like our water-based bloodstream). They can also make it easier for fat-soluble substances to get absorbed from our digestive tract. Among several studies that have shown the benefits of a fat-containing meal for absorption of beta-carotene from foods sweet potatoes, one study has shown that stir-frying in oil is one specific cooking technique for sweet potatoes that can enhance the bioavailability of their beta-carotene. It’s interesting to note that the sweet potato stir-fry in this study used a very low stir-frying temperature of 200°F (93°C) and that only 5 minutes of stir-frying were required to achieve the beta-carotene bioavailability benefits.
While we recognize boiling and stir-frying as viable options for cooking sweet potatoes, we recommend Healthy Steaming of sweet potatoes for maximum nutrition and flavor. Our basic logic here is simple. It’s easily possible to add a small amount of fat (like a tablespoon of extra virgin olive oil) to your sweet potato recipe after the sweet potatoes have been cooked. In that way, you will be able to avoid any heating of vegetable oils that might damage their heat-sensitive nutrients. At the same time, with the practice of steaming, you’ll be able to avoid submersion of the sweet potato in boiling water. That kind of submersion could result in the leeching of additional water-soluble nutrients from the sweet potato. Our Healthy Steaming method for sweet potatoes is quite simple: Fill the bottom of a steamer pot with 2 inches of water. While waiting for the water to come to a rapid boil, slice potatoes into 1/2-inch slices. Steam for 7 minutes and toss with our Mediterranean Dressing and top with your favorite optional ingredients. For details see, 7-Minute Sweet Potatoes.
