The Healthy (and tasty) Sweet Potato
This week we celebrate sweet potatoes while they are in the peak of their season. Although they are a traditional holiday favorite, why not enjoy the great taste and nutrient-richness of sweet potatoes throughout the year? Rich in beta-carotene, sweet potatoes provide you with powerful antioxidant protection against damage to cellular structures and DNA.
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 Potatoes
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 Potatoes
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.
Sweet Potatoes’ 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 of Sweet Potatoes
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.
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).
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.
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.
Sweet potatoes don’t have to take a long time to prepare. Cutting them into 1/2-inch slices and Quick 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.