I had already decided to write about potatoes this week before receiving an email from a reader of my glycemic index column in last Sunday’s Seattle Times, stating that I should not have said that “most vegetables” were lower-glycemic, because, in his words: “The fact is that there is almost no food, other than pure corn syrup, that is more dangerous to a diabetic than a potato in almost any form.”

I certainly don’t recommend downing pure corn syrup (or table sugar, for that matter), but potatoes…dangerous? Well, that fact, in fact, is a myth. A myth born out of oversimplification. The American Diabetes Association even includes potatoes in its list of diabetes myths. Never mind that I did say “most vegetables.”
It’s true that potatoes contain strings of glucose molecules (that’s what starch is made of, and potatoes are starchy vegetables.) However, it’s also true that there are many types of potatoes, and that not all potatoes have the same glycemic index or glycemic load (i.e., not all types of potatoes affect blood sugar to the same degree). Specifically, the GI and GL vary greatly among the species of potatoes consumed in North America.
It’s also true that how much a potato affects blood sugar depends on how it is cooked. Potatoes that have been cooked and cooled, then either eaten cold or reheated, have less of an effect on blood sugar than potatoes eaten freshly cooked and hot. Why? Because when starch is cooled, it retrogrades. Retrograded starch takes on a new, irregular structure that is harder to digest. This resistant starch delays digestion and absorption, lowering the glycemic index of that potato.
And let’s not forget that potatoes are a great source of potassium, vitamin C, dietary fiber, phytochemicals and other assorted vitamins and minerals. They are an affordable source of nutrition, and healthful when not overconsumed in fried form (i.e., potato chips, french fries, hash browns). 
A research review published in last month’s Annals of Medicine (PubMed ID: 23855880) echoed these nutritional benefits and concluded that potatoes “have been shown to have favorable impacts on several measures of cardiometabolic health in animals and humans, including lowering BP, improving lipid profiles, and decreasing markers of inflammation. Data are sparse regarding the impact of potato consumption on the postprandial glycemic response, especially when potatoes are consumed with other foods.” That last bit, the concept of mixed meals, is very important when considering the overall effect of foods on blood sugar.
When studies have found an association between potato intake and type 2 diabetes, teasing out of the details generally indicates that the potatoes were primarily fried and the association was found among obese individuals with the highest levels of potato consumption. This suggests that french fries and fried hash browns are not the healthiest ways to partake of potatoes (no surprise there), and overconsumption of potatoes and other starchy carbohydrates may not be ideal when someone has other cardiometabolic risk factors (obesity is often, but not always, associated with insulin resistance).
Since we’re on the subject, I would caution against overconsumption of starchy carbohydrates, generally. I personally enjoy potatoes, but not as often as I once did. That’s because, at some point, it occurred to me that starchy carbohydrates are really easy to overeat. My theory is that this is because they go down easy, they tend to be associated with comfort foods (mashed potatoes, mac and cheese, etc.) and are often prepared with high-fat ingredients. I don’t eat pasta as often as I once did, either, and I think I enjoy both pasta and potatoes more because they play smaller roles in my very varied diet. Moderation in action.

To learn even more about this topic, check out the article “Do Potatoes Cause Diabetes?” by Jeff Novick, MS, RD.