Potato Primer

by Bill Willis, PhD on February 1, 2013

February 2013: Potato Primer

Most of us who are deadly serious about training and nutrition are not doubt “meat and potatoes” people, at least at some point. As a great high-fiber source of complex carbs, potatoes are a staple in any serious athlete’s nutrition plan, and are one of the most versatile and convenient carb sources in the pantry. While many people already make the distinction between sweet potatoes and other varieties- some will opt for lower GI sweet potatoes while dieting for a show, while consuming higher GI varieties at other times- there are several factors that affect the glycemic index and digestibility of potatoes. Recent research suggests that the glycemic index of potatoes can be manipulated, which we can exploit to hone our nutrition to a razor’s edge. After reading this article you will have a better appreciation for this versatile food, and will be able to pick the right potato and right preparation method to dial in that diet no matter what your goals may be.

Some potato history

For potatoes, the path to vegetable rock star status began around 12,000 years ago. As early humans migrated across the Bering Straits to the Americas 16,000 years ago, there were already around 235 different species of wild potatoes, naturally growing across a wide range of territory from South America through Central America and the American Southwest (1).

Many of these early wild potatoes were poisonous, however, and those that were edible tasted awful. The arrival of the potato as we know it today was actually pretty lucky, considering that it has been estimated that fewer than 10 or so out of hundreds of wild potatoes actually played a role in the evolution of the cultivated potato of present day.

Edible potatoes were first cultivated in South America, where the process that led to the potatoes we know of today was set in motion around 8000 years ago, by the pre-Inca people of the Andes mountains. Potatoes were then brought to Europe during the Spanish Conquest at the end of the 16th century and spread from there, becoming a staple food of Europe’s rural/working families during the 19th century. Later the potato was introduced to Africa, India, China, Australia, and New Zealand, and is now literally consumed all over the world (1).

Sweet potato confusion

During the Spanish conquests, Spanish explorers found many new plants in the Caribbean, one of them a tuberous root, called “batatas” by the local Indians. These plants were fast growing and the roots (what we now call sweet potatoes) could be stored for months, making them ideal for prolonged storage on ships during voyages. Christopher Columbus once described the root as “resembling a yam but tasting like a chestnut”. The Spanish also considered these “batatas” to be an aphrodisiac, which led to them being one of the first New World foods that were adopted in Europe. An English explorer during that era named John Hawkins was the first Englishman to write about the newly discovered plant, which he encountered in the Caribbean in 1565. Hawkins mistakenly interpreted the native pronunciation as “patatas”, later changing the spelling to “potatoes”. These “potatoes” that were discovered in the Caribbean were actually sweet potatoes (Ipomoea batatas), which are completely unrelated to the common potato. When Europeans later came into contact with real potatoes (Solanum tuberosum) several years later, they called them potatoes as well. From that point on, Ipomoea batatas and Solanum tuberosum plants were respectively identified as “sweet potatoes” and “potatoes”, forever linking these botanically unrelated plants under the same name. Because the regular potato was also credited with the same perceived aphrodisiac properties of the sweet potato at the time, the common potato rapidly spread after it first arrived in Europe in the late 16th century (1).

Potato nutrition

Potatoes are gluten-free

For those who are gluten-sensitive or have full blown Celiac disease, gluten free sources of complex carbohydrates can be hard to come by. What is Celiac disease? People with Celiac disease- or those who carry one of a few identified genes for gluten sensitivity are sensitive to a glutamine-rich protein peptide found in wheat called gliadin. When gliadin is ingested, an enzyme called transglutaminase 2 (TG2) enzymatically catalyzes a deamidation reaction, converting certain glutamine residues to glutamic acid. While this simple substitution of amino acids doesn’t sound like a big deal, it wreaks immunological havoc in those who are gluten sensitive. Antibodies are generated against deamidated gliadin, TG2, and other “self antigens”, causing a ton of inflammation and ultimately damaging the intestinal cilia, which sets off a negative cycle of poor nutrient absorption, more inflammation and more intestinal damage. It turns out that it is also possible to be gluten sensitive in the absence of any known genotypes for gluten sensitivity or Celiac disease. Although most will be able to consume gluten with no issues, those with a mild/ undiagnosed sensitivity will tend to do better with little to no gluten in their diets. For those who fall into this category, potato starch is an awesome substitute for grain based complex carbs.

Potatoes are a great source of vitamin C, potassium, and vitamin B6

A medium potato can provide up to 27mg of vitamin C, which is around 45% of the daily value. This water soluble vitamin is a great antioxidant and is also important for collagen production, which is needed for everything from healthy gums to repairing the soft-tissue that constantly gets thrashed from all-out training sessions. Vitamin C also assists in the absorption of iron. While potatoes don’t pack as much vitamin C punch as citrus fruits, this can be a good thing; too many antioxidants around the time you train can potentially negate the insulin-sensitizing effects of exercise (2), so a pre-workout potato could provide just enough vitamin C without reducing insulin sensitivity.

Potatoes are an excellent source of potassium, for which it is estimated that less than 3% of Americans get enough of on a regular basis. As one of the most concentrated and cost-effective dietary sources of potassium, potatoes rank highest in potassium content among the 20 top-selling fruits and vegetables.

There are several benefits to getting potassium right. Many people with hypertension do not process sodium normally, which contributes to high blood pressure. Increasing potassium can negate the negative effects of dietary sodium in these people (3). Diets high in potassium rich fruits and vegetables may also help maintain lean body mass (4) and bone density (5) as we age.

Potatoes are also a great source for vitamin B6, a water soluble vitamin that plays an important role in carb and protein metabolism. B6 aids in the synthesis of nonessential amino acids, and is also an important cofactor for several key enzymes involved in energy metabolism. B6 is also required for the synthesis of hemoglobin, the oxygen-carrying component of red blood cells.

Potatoes are a good source of dietary fiber and resistant starch

One medium potato with the skin packs two grams of fiber. Of course, we all know the benefits of dietary fiber including slower gastric emptying, improved blood lipid profiles and greater satiety. Potatoes also contain a special type of fiber called resistant starch, however, which is resistant to digestion in the small intestine. Resistant starch is found naturally in legumes, bananas (especially the under-ripe, green ones), and potatoes as well as some unprocessed whole grains. Resistant starch is fermented in the large intestine into a prebiotic fiber, which helps to stimulate the growth of healthy bacteria in the intestinal tract. Resistant starch has several health-promoting effects. By increasing the production of short-chain fatty acids (6-8), resistant starch helps to protect the colon from harmful microorganisms (6-8) which may even help to prevent cancer (9).

In addition to promoting colon health, the systemic effects of resistant starch include better glucose tolerance, increased insulin sensitivity, reduced plasma triglycerides, and overall increases in satiety.

Potato buying and storage tips

When buying potatoes, look for clean, smooth, and firm-textured potatoes with not cuts, bruises, or discoloration. Potatoes will keep the longest when stored in a well-ventilated space at a temperature of 45-55F, where they can be stored for months. Keep in mind that colder temperatures (such as in a refrigerator) cause the starch in potatoes to naturally start converting to sugar, giving the potatoes a sweet taste, and making them discolored when they are cooked. If refrigerated, allowing the potato to gradually warm to room temperature before cooking can reduce the degree of discoloration. Keep potatoes out of direct sunlight, and avoid placing potatoes in areas that get hot, such as beneath the sink, beside large appliances, or in areas that receive lots of bright sunlight such as countertops near a window. Storage in perforated paper or plastic bags is the best way to extend shelf life. (Usually this is how they are packaged at the store when you buy them.) Also avoid washing potatoes before storing, because the dampness can promote quicker spoiling.

A note on “green potatoes” or sprouting potatoes…

If you store potatoes long enough before eating them, the skin begins to turn green, and sprouts form on the surface. The green color is actually caused by the build-up of a chemical called solanine, which is a natural reaction to light exposure. Solanine produces a bitter taste if eaten, and if consumed in large amounts can even be toxic, so keep potatoes out of direct sunlight and eat them before they turn green. If you do find some green coloration on the surface, the potato can still be eaten; just cut the green portions of the skin away before cooking and eating.

Sprouts are a sign that the potato is trying to grow (aren’t we all?). Early sprouting is promoted by exposure to light, and also ethylene gas, which is naturally released by potatoes in storage. Keeping potatoes in a dark, well ventilated space can minimize sprouting. Also keep in mind that ethylene is a ripening factor for other fruits and vegetables. Citrus fruits and apples tend to release a ton of this, so it’s best not to store potatoes near apples or oranges to avoid premature sprouting. It’s ok to eat a sprouting potato; just cut the sprouts away before cooking and eating.

Potato types

Russet potatoes: are the most widely used type of potato in the US. These potatoes have a brown skin and white flesh. Russets are best for baking, which results in a light and fluffy texture. Depending on how they were prepared, Russets tend to be higher on the glycemic index scale.

Red potatoes: have a red skin and a white flesh. One of the favored uses of red potatoes is in soups and stews, because they hold up extremely well after boiling. The flesh tends to be moist and waxy so they stay firm throughout the cooking process, while russets tend to fall apart. Red potatoes are slightly sweet and always tender, and also tend to be lower glycemic index, closer to that of a sweet potato.

White potatoes: are an all purpose potato with a white flesh and white (sometimes also light tan) skin. Best for mashing, these potatoes are slightly dense and creamy, with a subtly sweet flavor. They tend to have thin- even delicate skin which adds just the right amount of texture for a mashed potato dish without the need for peeling. White potatoes tend to have an intermediate glycemic index, lower than Russet potatoes but higher than sweet potatoes, depending on the variety.

Yellow potatoes: Yellow varieties of potatoes have been well-known throughout Europe for some time, but are quickly gaining popularity here in the US. (Yukon Gold is a popular variety.) Yellows have a golden skin and golden flesh. Yellow potatoes are versatile but also especially good for grilling, where the crispy skin enhances the dense and almost buttery texture of the flesh. Yellow potatoes tend to have an intermediate glycemic index; higher than red potatoes, but toward the lower range of the whites.

Purple/blue potatoes: are less common in the produce isle, but are becoming more and more popular lately. Purple potatoes have a deep purple skin with a flesh that ranges from deep purple to almost white. A favorite for salads, the moist, firm flesh retains its shape while adding color to any salad. Purple potatoes tend to have a mild, nutty flavor. The purple pigment is also a great potential source of antioxidants.

Potato starch: amylose vs. amylopectin

The major carbohydrate source in all plants including potatoes is starch, which consists of two different types of carbohydrate molecules. The first is amylose, which is a linear glucose polymer. Because of its linear structure, amylose can be packed together very tightly, which limits access to digestive enzymes. As a result amylose is digested more slowly.

The second major type of carbohydrate in starch is amylopectin, which forms glucose polymers with a highly branched structure. (Glycogen, which is the form of glucose storage for humans and animals, is a more branched version of amylopectin). The branched structure of amylopectin prevents tight packing at the molecular level, providing much easier access to digestive enzymes. This causes amylopectin to be broken down and digested more rapidly than amylose. Now we can understand a big factor determining the glycemic index of any type of starch: the higher the ratio of amylose to amylopectin, the lower the glycemic index. Therefore high GI potatoes tend to have more amylopectin and less amylose, while lower GI potatoes such as sweet potatoes tend to have more amylose and less amylopectin.

Potato Glycemic Index

A basic chart listing the GIs of several common potatoes is presented below:

type	GI
Russet	98
white	70-90
sweet	50
red	48-50

Keep in mind that this chart is only a very rough guide; amylose to amylopectin ratio is not the only factor determining the glycemic index of potatoes. You may have noticed that older GI charts are rarely consistent when they list the GI for potatoes, and newer charts often make the distinction between boiled and baked potatoes. It turns out that the glycemic index of potatoes is not set in stone. The structure and digestibility of potato starch in particular is affected by cooking and processing methods (10), and the method of cooking- which can have a huge effect on glycemic index- isn’t always stated for potatoes in GI charts.

During the cooking process, starch absorbs water, causing it to swell, which permanently disrupts the crystal structure (i.e. the molecular arrangement of the starch molecules), making it more accessible to digestive enzymes such as amylase. When previously heated starch is cooled, it tends to condense, resulting in the formation of irregular chemical bonds, creating a molecular structure that is more resistant to digestion. Repeated cycles of heating and cooling form starch molecules that are more and more resistant to digestion (10). In 2005 a research group recognized that changes to the molecular structure of potato starch might have a big effect on glycemic index, so they set out to determine the effects of cooking on common potatoes (11). The researchers found that potatoes that were cooked, then cooled and consumed cold had a 40% lower glycemic index than those that were consumed right after cooking, before they were allowed to cool. Cooking and then cooling can also increase the amount of resistant starch; cooked potatoes, which contain about 7% resistant starch, increase to around 13% resistant starch after cooling. The effects of cooling after heating varies with the type of potato, but the overall trend is that pre-cooking potatoes, then consuming them cold or reheated substantially reduces glycemic index.

Wrap-up

As far as carbs are concerned, there is no one-size fits all source; it is best to choose the right tool for the job. The ability to adjust glycemic index based on the preparation method makes potatoes one of the most versatile complex carb sources available. For high GI applications such as after a workout, choose a Russet or white potato, bake it in the microwave and consume it fresh, while it is still hot. This will ensure the GI is around the 90-100 range. For lower GI applications, sweet potatoes are not the only option; the GI of any potato can be adjusted downward by as much as 40% by pre-cooking the potato, cooling it in the refrigerator overnight, and then consuming it cold. This causes the starch molecules to condense into a slightly irregular structure, slowing digestion and reducing the effective glycemic index. Because they are also great source of fiber, resistant starch, potassium, vitamin C, and vitamin B6, potatoes pack a powerful nutritional punch, and are definitely worth including in any nutrition plan.

Reference List

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Geleijnse JM, Kok FJ, Grobbee DE. Blood pressure response to changes in sodium and potassium intake: a metaregression analysis of randomised trials. J Hum Hypertens 2003;17:471-80.
Dawson-Hughes B, Harris SS, Ceglia L. Alkaline diets favor lean tissue mass in older adults. Am J Clin Nutr 2008;87:662-5.
Tucker KL, Hannan MT, Chen H, Cupples LA, Wilson PW, Kiel DP. Potassium, magnesium, and fruit and vegetable intakes are associated with greater bone mineral density in elderly men and women. Am J Clin Nutr 1999;69:727-36.
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Hylla S, Gostner A, Dusel G, Anger H, Bartram HP, Christl SU, et al. Effects of resistant starch on the colon in healthy volunteers: possible implications for cancer prevention. Am J Clin Nutr 1998;67:136-42.
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