Sweet potatoes

Sweet potatoes don't have to take a long time to prepare. Cutting them into 1/2-inch slices and Healthy Steaming them for just 7 minutes not only brings out their great flavor but helps to maximize their nutritional value. And you can add cinnamon, nutmeg, and/or cloves for extra flavor and nutrition. For more on The Healthiest Way of Cooking Sweet Potatoes see the How to Enjoy section below.

This chart graphically details the %DV that a serving of Sweet potatoes provides for each of the nutrients of which it is a good, very good, or excellent source according to our Food Rating System. Additional information about the amount of these nutrients provided by Sweet potatoes can be found in the Food Rating System Chart. A link that takes you to the In-Depth Nutritional Profile for Sweet potatoes, featuring information over 80 nutrients, can be found under the Food Rating System Chart.

• Health Benefits
• Description
• History
• How to Select and Store
• Tips for Preparing and Cooking
• How to Enjoy
• Individual Concerns
• Nutritional Profile
• References

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 PotatoBotanical FamilyBotanical GenusBotanical SpeciesCommon U.S. PotatoSolanaceaeSolanumS. tuberosumSweet PotatoConvolvulaceaeIpomoeaI. batatasYamDioscoreaceaeDioscoreaD. 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-fleshedWhite-fleshedBeuregardeAgenaBatasCalifornia GoldJapanese PurpleBrazilianCarolina RubyKorean PurpleDingessCovingtonMokuauHaymanEvangelineOkinawanO'HenryGold NuggetPurpleUyentan

*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.

How to Enjoy

A Few Quick Serving Ideas

• Purée cooked sweet potatoes with bananas, maple syrup and cinnamon. Top with chopped walnuts. The fat content of the walnuts will help you get optimal absorption of the beta-carotene in the sweet potatoes.
• Steam cubed sweet potatoes, tofu, and broccoli. Mix in raisins and serve hot or cold with a curried vinaigrette dressing. Once again, the oil in the vinaigrette will help you improve the bioavailability of the sweet potatoes' beta-carotene.
• Baked sweet potatoes are delicious even when served cold and therefore make a great food to pack in to-go lunches.

WHFoods Recipes That Feature Sweet Potatoes

• Indian Style Lamb with Sweet Potatoes
• Black Bean Burrito, Indian Style
• Braised Kidney Beans & Sweet Potato
• Curried Mustard Greens & Garbanzo Beans with Sweet Potatoes
• 7-Minute Sweet Potatoes
• Golden Spinach and Sweet Potato Healthy Sauté
• Healthy Mashed Sweet Potatoes
• Yams with Ginger and Cinnamon

Individual Concerns

Sweet Potatoes and Oxalates

Sweet potatoes are among a small number of foods that contain measurable amounts of oxalates, naturally occurring substances found in plants, animals, and human beings. While sweet potatoes are not nearly as high in oxalates as foods like spinach, beet greens, or rhubarb that contain hundreds of milligrams of oxalates per 1/2 cup, sweet potatoes still average about 30-40 milligrams of oxalates per half cup, and that amount is enough to classify them as high in oxalates in most food rating systems. ("High" usually starts with at least 10 milligrams of oxalates per 3.5 ounce—100 gram—serving, and then jumps up in levels where at least 100 milligrams is considering "very high." To be counted as "low" in oxalates, a food typically needs to contain less than 5 milligrams per 3.5-ounce serving.) When oxalates become too concentrated in body fluids, they can crystallize and cause health problems. For this reason, individuals with already existing and untreated kidney or gallbladder problems may want to avoid eating sweet potatoes. Laboratory studies have shown that oxalates may also interfere with absorption of calcium from the body. Yet, in every peer-reviewed research study we've seen, the ability of oxalates to lower calcium absorption is relatively small and definitely does not outweigh the ability of oxalate-containing foods to contribute calcium to the meal plan. If your digestive tract is healthy, and you do a good job of chewing and relaxing while you enjoy your meals, you will get significant benefits—including absorption of calcium—from calcium-rich foods plant foods that also contain oxalic acid. Ordinarily, a healthcare practitioner would not discourage a person focused on ensuring that they are meeting their calcium requirements from eating these nutrient-rich foods because of their oxalate content. For more on this subject, please see "Can you tell me what oxalates are and in which foods they can be found?"

Nutritional Profile

The orange-flesh sweet potatoes are exceedingly rich in beta-carotene. The purple-flesh varieties are outstanding sources of anthocyanins, especially peonidins and cyanidins. Both types of sweet potatoes are rich in unique phytonutrients, including polysaccharide-related molecules called batatins and batatosides. Sweet potatoes also include storage proteins called sporamins that have unique antioxidant properties. Sweet potatoes are an excellent source of vitamin A (in the form of beta-carotene). They are also a very good source of vitamin C, manganese, copper, pantothenic acid, and vitamin B6. Additionally, they are a good source of potassium, dietary fiber, niacin, vitamin B1, vitamin B2, and phosphorus.

For an in-depth nutritional profile click here: Sweet potato.

In-Depth Nutritional Profile

In addition to the nutrients highlighted in our ratings chart, an in-depth nutritional profile for Sweet potatoes is also available. This profile includes information on a full array of nutrients, including carbohydrates, sugar, soluble and insoluble fiber, sodium, vitamins, minerals, fatty acids, amino acids and more.

Introduction to Food Rating System Chart

In order to better help you identify foods that feature a high concentration of nutrients for the calories they contain, we created a Food Rating System. This system allows us to highlight the foods that are especially rich in particular nutrients. The following chart shows the nutrients for which this food is either an excellent, very good, or good source (below the chart you will find a table that explains these qualifications). If a nutrient is not listed in the chart, it does not necessarily mean that the food doesn't contain it. It simply means that the nutrient is not provided in a sufficient amount or concentration to meet our rating criteria. (To view this food's in-depth nutritional profile that includes values for dozens of nutrients - not just the ones rated as excellent, very good, or good - please use the link below the chart.) To read this chart accurately, you'll need to glance up in the top left corner where you will find the name of the food and the serving size we used to calculate the food's nutrient composition. This serving size will tell you how much of the food you need to eat to obtain the amount of nutrients found in the chart. Now, returning to the chart itself, you can look next to the nutrient name in order to find the nutrient amount it offers, the percent Daily Value (DV%) that this amount represents, the nutrient density that we calculated for this food and nutrient, and the rating we established in our rating system. For most of our nutrient ratings, we adopted the government standards for food labeling that are found in the U.S. Food and Drug Administration's "Reference Values for Nutrition Labeling." Read more background information and details of our rating system.

In-Depth Nutritional Profile for Sweet potatoes

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