A couple of people who read last week’s column on dried beans, peas, and other pulses were tripped up by the fact that they contain low-glycemic carbohydrates, and one of them buttonholed me at a party over the weekend. “Beans are so starchy!” she said. “And I really try to avoid carbs. What makes beans good and potatoes bad?”
OK, let’s begin at the beginning: What are carbohydrates? The Mayo Clinic is clear and concise: “Carbohydrates, or carbs, are a type of nutrient. The three basic forms are sugars, starches, and fiber. When you eat or drink something with carbs, your body breaks down the sugars and starches into a type of sugar called glucose, the main source of energy for cells. Fiber passes through your body undigested.”
Glucose, for its part, is regulated by the hormones insulin and glucagon, which together work to move it in and out of your blood, cells, and liver. “This process helps keep your body fueled and ensures a natural balance in blood glucose,” the Mayo website continues. “Different types of carbohydrates have properties that affect how quickly your body digests them and how quickly glucose enters your bloodstream.”
Which brings us back to beans versus potatoes.
One way of classifying carbohydrates is with the glycemic index, developed in 1981 by David Jenkins, a nutrition professor at the University of Toronto. In his system, a number is given to carb-containing foods based on how much that food increases blood sugar. The theory is that low-glycemic foods help prevent weight gain and chronic diseases such as diabetes and cardiovascular disease because their carbs are assimilated by the body more slowly—if at all.
Glycemic index values only give you part of the picture, however, because they don’t reflect how much you must eat of a particular food to consume the standard level of digestible carbs. Enter the so-called glycemic load—a numerical value that indicates the change in blood glucose levels when you eat a typical serving.
The lower a food’s GI and GL, the less it affects blood sugar and insulin levels. You can find helpful, if not exhaustive, databases of the glycemic index and glycemic load values of various foods online, including those published by Harvard University and the University of Sydney. A plain baked russet potato, for instance, has a GI value of 111 and a GL value of 33 for a 150-gram serving—and thus a greater effect on blood sugar than navy beans, with their GI value of 31 and GL value of 9 for 150 grams. Lentils come in a tad lower, with a GI of 29 and a GL of 5.
Although the thinking that high-glycemic foods are bad and low-glycemic foods are good has become orthodoxy to much of the diet-conscious public, nutrition is never that simple, and scientific evidence is mixed. In one five-week randomized controlled-feeding study published in JAMA in 2014, for example, researchers followed 163 subjects who were overweight and had high blood pressure, putting them at greater risk of cardiovascular disease and type 2 diabetes.
The upshot? The team, led by Harvard professor Frank M. Sacks, concluded that diets with a low glycemic index of dietary carbohydrate, compared with a high glycemic index of dietary carbohydrate, did not result in improvements in insulin sensitivity, lipid levels, or systolic blood pressure—and in a generally healthy diet, using a glycemic index to select foods may not improve cardiovascular risk factors or insulin resistance.
In other words, if you are in good health and eat a diet rich in whole foods, there’s no need to fret about the blood-sugar impact of potatoes versus legumes.
I cannot tell you how happy that makes me, because I’m fed up with the anti-potato crowd. For all its unpretentious familiarity, the potato is a virtual über-tuber—a rich source of vitamins (including B and C), minerals (calcium, iron, phosphorus, potassium), protein, complex carbohydrates, and dietary fiber. That old adage about the nutrition being only in the potato skin is a Snopes-worthy myth, by the way; the flesh contains significant nutrients as well.
The humble spud has nourished humans for some 8,000 years, going back to when it was first domesticated in Peru. That’s why I was gobsmacked when I recently read in The Washington Post that, according to a January 2016 study published in the British Medical Journal, eating just one cup of potatoes (baked, boiled, mashed, or french fried) a week can increase a pregnant woman’s risk for gestational diabetes by 20 percent. Five or more servings a week ups the risk 50 percent. I understand why a daily fix of fries or butter-saturated mash might not be a wise idea if you’re eating for two, but what the heck?
As it turned out, that piece also raised an eyebrow or two at the watchdog publication Health News Review. “Gestational diabetes is nothing to laugh at. It puts both the mother and child at risk during delivery, and it’s linked to Type 2 diabetes later on,” wrote Kathlyn Stone. “But an observational study based on biannual dietary questionnaires designed to track disease outcomes and lifestyle factors—not diet during pregnancy—doesn’t hold water. Especially when the study didn’t track weight gain which is a major culprit in gestational diabetes and Type 2 diabetes. Never mind the fact that this type of study can’t prove cause and effect, so it’s just not accurate to state, as the Post story does, that eating potatoes ‘raises risk’ for the condition.”
The takeaway? Be an equal-opportunity carbohydrate consumer, but stick to whole foods. A vegan friend recently introduced me to the joys of a baked potato stuffed with Indian-spiced lentils or chickpeas, and a generous dollop of three-bean chili makes a great topping too. Paired with a crunchy green salad, it makes a hearty meal, and if you’re toying with the idea of giving up animal protein for Lent, work it into your repertoire. I guarantee you won’t miss the meat.