The better potato: How small differences become big health claims

A health-conscious shopper stands in the produce aisle with two choices. 

The white potato looks suspect. The sweet potato looks like the responsible decision. One is starchy, high glycemic, and quick to become sugar. The other is richer in fibre, slower to digest, and easier on blood sugar.

White and sweet potatoes are different. The question is whether those differences matter.

The reputation is built from real differences. White potatoes contain more of their carbohydrate as starch. Sweet potatoes contain more natural sugar and more fibre. Their vitamin and mineral profiles differ, and white potatoes can raise blood glucose more quickly when both are prepared the same way.

A nutrition chart can make the case look settled. One food has more of something desirable. The other has more of something we have learned to fear. What the chart rarely shows is how much we need and how much comes from everything else we eat.

Sweet potatoes provide about six tenths of a gram more fibre per 100 grams. Adults need 25 to 38 grams a day. Someone eating little fibre will not repair the problem by changing the color of the potato. Someone eating plenty of fibre will lose little by choosing white.

The same applies to calcium, iron, and protein. Percentages can look large even when both potatoes contribute little. A difference can be accurate and still give a false impression of importance.

Starch deserves closer attention because it sits at the center of the white potato’s reputation. White potatoes contain somewhat more starch. Sweet potatoes contain more total carbohydrate and more natural sugar. During digestion, the available starch and sugar in both largely become glucose.

The body does not receive good sweet potato glucose and bad white potato glucose.

How quickly that glucose arrives matters for someone with diabetes or insulin resistance. Glycemic index estimates the speed of the rise. Glycemic load also accounts for the amount eaten. Neither number belongs permanently to a potato because the potato changes in the kitchen.

Boiling leaves more structure intact than baking until very soft. Mashing breaks that structure apart. Cooling allows some starch to resist digestion. Frying adds concentrated fat and calories.

A mashed sweet potato does not remain protected because it is orange. A boiled white potato is not equivalent to instant mashed potatoes because both began with white flesh.

A potato eaten alone differs from one eaten with a hamburger patty and broccoli. Beef adds protein and fat. Broccoli adds fibre, water, and volume. Glycemic index cannot fully describe that dinner because it measures carbohydrate eaten by itself.

A boiled white potato can have a glycemic index near 78, while a boiled sweet potato can sit near 44. With 20 grams of digestible carbohydrate, their glycemic loads are about 15.6 and 8.8. Prepared the same way and eaten alone, the white potato creates the larger expected glucose challenge.

Cooking can create an even larger difference. Boiling one variety of sweet potato produces a glycemic index near 41. Baking the same variety raises it to about 94. With 20 grams of digestible carbohydrate, the glycemic load rises from about 8.2 to 18.8. The kitchen creates a larger change than the difference between the boiled white and sweet potatoes.

Put that potato beside the hamburger and broccoli. Most of the calculated glycemic load still comes from the potato, but protein, fat, fibre, food volume, and meal order change digestion and absorption.

Eating vegetables and protein before the carbohydrate can lower blood glucose by about 29 percent after 30 minutes, 37 percent after an hour, and 17 percent after two hours. Those numbers do not predict this exact dinner. They show how much the rest of the plate can alter the response.

White and sweet potatoes remain different. Their glycemic response can differ meaningfully. Orange sweet potatoes also provide far more beta carotene, which the body can convert to vitamin A. Most other differences shrink when compared with daily needs.

By dinner, the potato has been boiled or baked, perhaps mashed, cooled, or reheated, and placed beside everything else on the plate. Its color is only one part of what the body receives.

Seth Schneider is a health columnist for the Bonner County Daily Bee.